|Thrown when an application tries to call an abstract method.
Normally, this error is caught by the compiler. It could occur at run time only if the callee were modified
to make the method or enclosing class abstract without recompiling both caller
- This usually happens in unsigned Applets when you attempt
to do something Applets are not allowed to do, such as read a file or talk
to a server other than the one you were loaded from. You are getting slapped down by the security
sandbox. However, they also occur in other contexts you would not think would be related to security,
e.g. in RMI (Remote Method Invocation)
when you have a mangled serialised object, or if you try to register a duplicate
Oracle bug number 4809366
: The Bea JVM fails to give permission for getClassLoader
even when you ask for AllPermissions
- One common way to get in trouble is to use absolute URL (Uniform Resource Locator)
your unsigned Applet. The Applet will then work
on one website only, or locally only. Instead, use a relative
URL and convert it to
- Watch out for this one. Sometime when you test your Applets locally they work, but when you use
them on the web they fail with AccessControlException. Remember, Applets are
not supposed to read files, though you can do an AppletContext. showDocument. You can however read resources in the jar file.
|Applet not inited
||Applet not inited|
- You are using Internet Explorer which has a defective or
- Missing package statement.
- These erroneous Applets will often work with the Java Plug-in, or when run locally, but will fail
with native Javas when run on the web.
- There can also be problems with your jar file having too much or too little qualification of class
names. Your <APPLET CODE= must have nothing but the class name, without
- Make sure the case and name exactly match the name of the *.java file,
*.class file, and class name. For a class in a package this would have dots in
it, e.g. com.mindprod.mypackage.Myclass, but it would not have any directory
- Your CODEBASE= parameters must have an absolute http://-style reference to the base directory where the code is stored.
- For a local hard disk, the only thing I could get to work reliably on NT with all browsers and
AppletViewers is leaving the CODEBASE out entirely. You may find for your
platform you have to code it something like this: ///C|//MyDir/ or
- Your ARCHIVE= parameter must have a list of the jar files, separated by
commas. If you have too little or too much qualification, or if you fail to use the file naming
conventions of your server, you will be in trouble.
- You tried to do something in an unsigned Applet only signed Applets are allowed to do such as reading restricted system properties, perhaps
indirectly via Color. getColor.
|Application Failed To Start
||The application has failed to start because its side-by-side
configuration is incorrect.|
|Most likely you are using the Microsoft Express C++
compiler to create your JNI (Java Native Interface). It links to a run time library which is not present. You need to put the
necessary redistribution support DLLs (Dynamic Link Libraries)
on the path, or install the entire C++ run time library on all client
machines. The catch is other vendors may do the same thing, and you can have versioning problems, more
commonly known as DLL (Dynamic Link Library)
hell. Some compilers allow /MT or /LD static linking to create a standalone DLL
that does not require a run time, but Express 2008 will not.|
||ArithmeticException: / by zero|
|You divided by 0. If it occurred in a paint method, likely this
occurred as a result of creating a Font with zero height.|
|You have used an array index outside the allowable range. There may be several array references
x[i] in the same line. Don’t leap to conclusions about which one caused
the trouble. Arrays are indexed from 0 to x.length-1,
not 1 to x.length the way FØRTRAN programmers
are used to. When you allocate the array you specify the length something like this:
int x = new int[ 10 ];
|The rules of casting arrays and casting the elements of arrays are subtle. See the gotchas.|
||Bad installation. No jre found in the configuration file.|
|If you got that error message while trying to launch a JWS (Java Web Start)
application, it means your deployment files are corrupt. This is easy to fix. At the command line, type
javaws.exe -viewer. Then delete all the applications. Then launch your apps
|bad magic number
||Bad magic number|
- The first four bytes of a class file are supposed to say CAFEBABE in
hex. They don’t.
- Most likely the problem is you uploaded your class file to your Web server using
FTP (File Transfer Protocol)
ASCII (American Standard Code for Information Interchange)
mode instead of BINARY mode which mangled the class file.
Oracle bug number 4151665
: Sometimes when a class is not found it gets reported as corrupt instead.
- Novices get this error when they try to run a program without compiling it first e.g
- If the class file seems ok, consider the possibility some other class file on the classpath is the
one the ClassLoader is looking at.
- See ClassFormatError
||Bad major version number|
|Most likely your class files use a more recent version of Java than the Java runtime you are using. This
most commonly happens with Applets in Internet Explorer which
supports only an ancient version of Java.|
||Applet works fine run as application, but shows a blank screen without any error message
when run as an Applet. There is not even an Applet loading message.|
|Check the <applet code=com.mindprod.mypackage.Myclass.class tag. It is spelled code
|BoxLayout can’t be shared.
||BoxLayout can’t be shared.|
|You wrote something like:
( new BoxLayout( this,
BoxLayout. PAGE_AXIS ) );
JFrame.contentPane.setLayout ( new BoxLayout(
contentPane, BoxLayout. PAGE_AXIS ) );
||Caused by: java.lang.RuntimeException: Broken Pipe!|
|Your pipe connection stopped working. You may have run out of scratch disk space. You may
have clogged RAM (Random Access Memory)
by failing to release resources. Your compressed datastream may be corrupt. The process reading a pipe may
have closed it, so you can’t write more data to it.|
|can’t create virtual machine
||Could not create the Java virtual machine|
|You wrote java -xxxx.jar instead of java -jar
|Typically you are reading a UTF-8 stream and encounter an
illegal binary pattern that has no meaning.|
|class has wrong version
||class file has wrong version 49.0, should be 48.0|
|You trying to run class files compiled with JDK (Java Development Kit)
version 1.5 on an older JRE (Java Runtime Environment)
|class file contains wrong class
||class file contains wrong class. Please remove or make sure it appears in the correct
subdirectory of the classpath.|
|If the name of your class is HelloWorld then the name of the source file must be
HelloWorld.java, and case matters. You also get this error if you have the wrong
or missing package statement. You could also have the source file in the wrong directory if you have a
complex package structure. If your package is called com.mindprod.com.mindprod.mypackage and your class MyClass, then
you had better be in the root directory and the class file for MyClass hand
better be called MyClass.class and better live in com/mindprod/mypackage, with matching case!|
- You have cast a Class object to a Class that
is not the same class or a super class of the Class object. This exception
happens when you cast a generic reference to an Object to an invalid class.
You must cast it to the class of the Object or one of its superclasses. You
can find out what class the Object actually is with getClass().toString().
- You can cast null into anything without raising a ClassCastException. Generally, this is a Good Thing™. Otherwise every cast
would need an if to specially handle the null case. You might be tempted to
count on ClassCastExceptions to filter out nulls for you. The following
code will not raise a java.lang.
Cat c = null;
Object o = c;
Dalmatian d = (Dalmatian)o;
In other words, there is one universal representation for null, not a
special one for each class. For more detail see gotchas.
- If the error message complains you have tried to cast class X to class
X, it is complaining that the two different class Xes belong to different class loaders. Even though they have the same name, they are
treated like logically separate classes.
- If the ClassCastException occurs during a sort, chances are you forgot
the implements Comparable. See the
gotchas for details.
- If you write a factory method like this:
protected CameraButton cameraButtonFactory ( int buttonNumber )
return new LabelledCameraButton ( buttonNumber, this );
and it seems to be producing the wrong kind of objects — ones from the base class’s
version of the method, check the precise spelling and signature of the method. It had better exactly
match the one in the base class. If it does not exactly match, it will not override. To guard against
this error Bali proposes an explicit overrides keyword.
- You mangled the class file FTP
upload by doing it as ASCII
instead of binary.
- Further, your web server must be configured to send *.class files as binary rather than
ASCII. It needs a
MIME (Multipurpose Internet Mail Extensions)
configuration entry to define *.class files. See MIME for
- Sometimes it is actually a CLASSPATH problem. It
can’t find the class.
- ClassFormatError: class already loaded. You have a class file twice on the classpath. It has
nothing to do with the DLL
already being loaded. Thankfully, having a DLL
still loaded from the last time you ran is not considered an error.
- ClassFormatError: bad major version. Your ancient 1.1 Java runtime does not know what to do with a
program compiled under JDK
|Thrown when an application tries to load a class through its string name using
Class.forName, ClassLoader. findSystemClass or ClassLoader. loadClass. It is similar to NoClassDefFoundError.
- this occurs only at run time. Usually it means a class that was present during compilation has
disappeared, or the classpath has changed so it is no longer accessible. It could also happen if you
dynamically load a class and the class is not on the classpath.
- Are you spelling the fully qualified class name correctly on your Applet tag. Case matters. Double
check the case of every letter.
- The class itself references another class that can’t be found. Are you using
APIs (Application Programming Interfaces)
from versions of Java that aren’t supported by your browser?
- Try specifying CODEBASE=. as an Applet parameter,
to ensure that the browser is looking in the right place.
- Are there packages involved? If so, the Applet class needs to be in a package-appropriate
subdirectory, or better still in a jar, not in the same directory as the HTML (Hypertext Markup Language)
file. If not, you ought to look into putting every class in a some package before you deploy code; the
default (nameless) package is intended only for test code that won’t be distributed.
- Look at your jar with WinZip. Are all the classes and package names correct with the correct case?
Make sure all the case information is perfect on your jar-building line.
- Class.forName(String) is short hand for Class.forName
( String, initaliseTrue, currentClassLoader). If the class you are looking for was loaded by a different
ClassLoader you won’t find the class.
- If you are deploying with FStart, make sure you have included all the jars used in your
JNLP (Java Network Launching Protocol). You may have forgotten some that were on
the classpath or in the ext directory that the app needs. Make sure you spelled all the jar names
correctly and they actually exist on the download site in the directory you expected.
- In the Java control panel ⇒ advanced ⇒ Java plug-in, trying
unticking the option enable the next generation Java plug-in.
- If you have been playing games with class loaders, the problem can be that you are using the wrong
classloader to load some particular class. You might have meant: ClassLoader. getSystemClassLoader, or
Thread.currentThread(). getContextClassLoader() or some other.
Keep in mind that the same class loaded in different classloaders are effectively different
- See bad magic number
|You were using JAX to access AWS (Amazon Web Services) or something similar. A request failed. You can catch the more general
|You tried to delete or add to a Collection while you were in the middle of
running an Iterator over it. To get around this, you must remove elements via
the Iterator object using Iterator.remove rather than the usual Collection. remove. You can add elements if you use ListIterator.add. See sample code.|
|You were probing a server on the net. A request timed out. You can catch the more general
java.net.SocketException or java.io.IOException instead.|
|Could not find main-class
||Could not find main-class com.mindprod.setclock.SetClock in
|Check that you have the correct package name and class name in the Java source, in the
*.mft file and the *.jnlp file line <application-desc main-class=com.mindprod.setclock.SetClock
|Could not reserve enough space for object heap.
||Error occurred during initialization of VM. Could not reserve enough space for object
heap. Could not create the Java virtual machine.|
|You asked for too much RAM
on java.exe -Xmx parameter.|
|does not contain expected
||myClass.class does not contain myClass as expected, but MyClass. Please remove the file.
Class myClass not found in new.|
- missing caps on new MyClass() reference.
- missing caps on MyClass() obj declaration.
|EOFException in ZIP
||java.io.EOFException: Unexpected end of ZLIB input
|Try using ZipFile instead of ZipInputStream to read
a zip created by ZipOutputStream. See Zip for why.|
|Thrown when code that is dependent on a keyboard, display, or mouse is called in an
environment that does not support a keyboard, display, or mouse. Check the java.awt.headless=true Java System property. If you are
using X11, check that the DISPLAY environment variable points to the X11
- In Netscape, inner classes can’t seem to access private variables of the enclosing class. You
don’t discover this until run time.
- Failing to use both the <APPLET ARCHIVE= parameter to specify a comma-delimited list of jar file
names and the CODEBASE= parameter in the form of an absolute http://-style
reference to where the jar file is.
- Generally problems with CLASSPATH, CODEBASE, and ARCHIVE.
- Classes in jar have the wrong amount of qualification stored with them.
- Classes in packages are not stored in the correspondingly named directories. See CLASSPATH and java.exe in the Java glossary for a fuller discussion.
- Using an old library with new code. In the old library a method might be private where in the new
it might be public.
: Input length must be multiple of 8 when decrypting with padded cipher.|
|Literally, the problem is the message to be decrypted is not a multiple of 8
bytes, when the encryption padding algorithm should have made it a multiple of 8 bytes. Possible causes:
- The message was truncated or garbled in the process of storing, armouring or transmitting it. Dump it out in hex immediately after encrypting and immediately before decrypting to make sure it is
- You specified different encryption and decryption algorithm options.
- For good measure, make sure both the encryption and decryption were done with the same version of
the JCE (Java Cryptography Extension).
||An IllegalMonitorStateException thrown when an object’s
wait(), notify(), or notifyAll() method is called from a thread that does not own the object’s monitor. In
other words, the Thread using these methods must be executing inside a
synchronised block locking on the object whose wait etc. method you are
||Illegal use of nonvirtual function call|
|An inner class is not permitted to call private methods of the enclosing outer class. It is permitted to
call protected methods. This error message appears in the old Symantec 2.5a Java only when you have debugging turned off. Normally the compiler
should catch this error, but if it doesn’t it will show up at run time with this strange error
|Image won’t paint
||Your app uses drawImage, paintComponent or simply Container.add. Yet nothing
|See the list of possible causes.|
||When generating Javadoc, you get Identifier Expected|
|You are using old version of javadoc.exe that does not understand
the newer Java syntax, or perhaps you are trying to generate Javadoc before you have a clean compile.|
||Incompatible types, found : java.lang.Object, required: java.io.File: for ( File file : wantedFiles )|
|Make sure you declare your File collection class with
implements Iterable< File> and its iterator method with public Iterator< File>
- You forgot the static on your main method.
- Any illegal use of a legal class.
- You have made a change to a class and are still referencing it from an another class by its old
signatures. Try deleting all class files and recompiling everything.
||OutOfMemoryError: string intern table overflow|
|You have too many interned strings. Some older
JVM (Java Virtual Machine)
’s may limit you to 64K Strings, which leaves perhaps 50,000 for your application.|
||java.io.InvalidClassException: local class incompatible:
stream classdesc serialVersionUID = -3714695503693357733.|
- You serialised some objects, then modified the class source code a tad, say by adding another
convenience constructor. Java panicked and decided the old objects were no longer compatible when you
tried to read them back in. You can soothe it by manually assigning your objects class version numbers
- It is up to you to change that number any time the class changes in a way that would affect the old
object’s ability to be read back with the new class definition. Java has some flexibility for the
old and new objects not to have a perfect match. serialVersionUID is a magic
variable name. Spell it exactly like that. If you have used a serialVersionUID, perhaps you have actually changed the layout of the class fields without
a new serialVersionUID. Finally, perhaps you failed to provide a no-arg
- You serialised some objects, then changed the format of the objects, rightly changed the
serialVersionUID, then tried to read the old object back with the new class
Oracle’s JDK Platform Guide to InvalidClassException
|This is a wrapper exception. You will get one if a method or constructor you invoke with
reflection throws an exception. Because the compiler cannot analyse methods called through reflection, the
reflection runtime wraps any errors thrown in the called routines in a predictable exception called the
InvocationTargetException which you can catch. Use InvocationTargetException. getCause and InvocationTargetException. getTargetException to find more
One way to trigger one is to initialise variables in an Applet rather than setting their initial values in the init
||IOException: invalid header field|
|Jar Not Signed With Same Certificate
||Your JAR-resources in JNLP-File are not signed from the same certificate.|
|You will have to resign all jars mentioned in the JNLP
file with the same certificate.|
||You have an obsolete JavaMail or Java Activation Framework jar.|
|Make sure you don’t have more that one mail.jar or
jmf.jar on the classpath/ext directory.|
||The application has requested a version of the JRE
(version 1.5+) that currently is not locally installed.|
|If the JRE is not installed, install it. Normally, you only need
the latest version,1.8.0_05, none of the older ones. This is a common Java Web Start problem. The
is indeed installed and works fine for non Java Web Start apps. Java Web Start apps sometimes work fine
launched from a browser, but fail when launched from a desktop shortcut. My theory is somehow the
registry or the Java Web Start development cache gets corrupted in a way that causes Java to use a
version of javaws.exe with the wrong matching directory. There will be at least
three copies on your machine C:\Program Files\java\jre8\bin\javaws.exe,
\bin\javaws.exe and C:\Windows\System32\javaws.exe.
Kludges around the bug include:
Here is how to fix the problem permanently. Unfortunately, this is a difficult, time-consuming procedure
and does not always work:
- Always launch Java Web Start apps inside a browser.
- Create your own desktop shortcuts with an explicit link to a local copy of the *.jnlp file and possibly C:\Program Files\java\jre8
\bin\javaws.exe if you are unsure associations are set to use that version of javaws.exe.
- Explicitly launch the JRE
javaws.exe with C:\Program Files\java\jre8
\bin\javaws.exe -viewer and use it to launch your apps or create your
- Uninstall any JREs and JDKs including the SQL (Standard Query Language)
- Manually strip out any remaining JRE/JDK files, including those in C:\Windows\System32, and C:\Users\
- Manually delete any references to the JRE/JDK from the registry.
- Remove any Java Web Start desktop shortcuts or menu items.
- run a registry cleaner.
- run NTRegOpt.
- Reinstall the JDK
with associated JRE, making
sure you select a drive explicitly for each of the three pieces, the JDK,
the database, and the JRE.
- Fix up the set environment and the ext directories.
|load: class not found
||load: class X not found|
|Likely you are using Microsoft’s ancient old JVM
in Internet Explorer that supports only up to Java 1.1.4. Check which version of Java you are using with
Wassup. Make sure you are using the most recent
JRE in C:\Program Files\java\jre8
|method not found
||method X not found|
|This is caused by using obsolete jar files. The version of the class in the jar is missing
the method, but at compile time, the java file had the method. The way to avoid this is to do a clean
compile periodically deleting all class and jar files and rebuilding them.|
||MissingResourceException: Can’t find bundle base name …|
|The resources you need are missing from the main jar or the jar they live in is not on the
classpath. Keep in mind that jars have an internal manifest classpath and Java Web Start using the
file to locate the jars.|
Detail from The Scream, by Edvard Munch
java.lang.NoClassDefFoundError: mypackage/ MyClass
This one is a bitch to track down. It has so many causes. Java is so picky! A misplaced comma, quote,
semicolon, extraneous extension or even the wrong upper/lower case can derail the whole program. I hope
some will implement the NoClassDefFoundError
Amanuensis to make the problem easier to track down.
Sun explains that it is thrown when Java Virtual Machine or a ClassLoader instance tries to load in
the definition of a class (as part of a normal method call or as part of creating a new instance using
the new expression) and no definition of the class could be found. The searched-for class definition
existed when the currently executing class was compiled, but the definition can no longer be found. It is
similar to a ClassNotFoundException. This gives you the hint that a NoClassDefFoundError problem was not with caused by a
- Essentially the problem is the place and name where you put the class file, free-standing on disk
or inside a jar, does match the place and name it should have, based on the classpath, the package name
and the class name. Everything is case sensitive, and sensitive to the tiniest typo. Unfortunately,
there is currently no tool to tell
you where the class should be placed (usually there are several possibilities) or where it is now.
- You got the class name wrong on the java.exe command line. It must include
the full package name and class separated by dots, without the *.class e.g.
not just the class:
Newbies often get in trouble by adding a *.java or *.class suffix. In contrast, javac.exe demands the *.java extension.
- You get an error message like this: Exception in thread main java.lang.NoClassDefFoundError: XXX/java. When attempting to compile, you
keyed java.exe XXX.java instead of javac.exe
- Java can’t find the class mentioned. It is missing from the jar or classpath. Java may need
it because it occurs as a class or superclass in a serialised ObjectStream
or because you called a method of that class. You don’t find out about the problem at compile
time or even load time. You only find out when you actually go to use that missing class. The missing
class was present when the class referencing it was compiled (perhaps on somebody else’s machine)
but the missing class is no longer accessible. To fix a vanilla NoClassDefFoundError, search the all the jar files on your machine for the missing
class. Funduc Search and Replace will do that. If you
can’t find it, scour the web for a download. When you find it, make sure its jar is on the
general classpath, the project classpath, the command line classpath or in the ext directories of the various JREs (Java Runtime Environments)
you have installed. There will be at least two ext directories.
- Three diagnostic tools you may find helpful:
- To dump the classpath that you are actually using at some point in your code insert: You must
first determine if the classpath is correct with:
- If you have a jar or more than one class, assign each class to a package! Package-less classes
are for one-shot, single-class throw-away experiments only.
- Then when you are sure the jar or directory you want is truly on the classpath (it is not
sufficient for the directory that the jar is in be on the classpath), then check the contents of
the jar with jarlook to make sure your package/class is in there, spelled
precisely, including case. You can download
jarlook with source.
- download jarcheck as well. It will help
you detect problems with unexpected target versions of class files in your jars.
- Recall that reconstitution of serialized files uses Class.
forName to instantiate all the classes buried in the files. Genjar or equivalent does not know to include these classes in your
jar. It is up to you to include all the classes you need manually. You will keep getting the dreaded
NoClassDefFoundError until you have nailed them all.
- Look for a typo in spelling the class file name in the file or in the command line invoking its
main method. Copy/paste to make sure they are absolutely identical including case.
- Check your classpath. Make sure you used semicolons or
colons as appropriate. Don’t specify null fields in your classpath, e.g. two semicolons in a row.
- The problem is essentially either the class is not on the classpath or the classpath is wrong. You
must first determine if the classpath is correct with:
- If your program used to work and it suddenly stopped working, did you
upgrade your JDK/JRE? If so, you likely forgot to copy over the jars to the
C:\Program Files\java\jre8\lib\ext and C:\Program Files\java\jre8
- You wrote some code like this:
private static final int a;
a = someMethod();
}If the class you are loading throws an Exception during
the static initialisation, it will show up as a ExceptionInInitializerError rather than the original
Exception, which your code may not be prepared to handle. When your code
later goes to use the class, you will get a NoClassDefFoundException.
- If you see the phrase wrong name in the error message, it means you are
not consistently naming the packagename. ClassName everywhere. Perhaps you left off the packagename, or got the case wrong, or
made a typo in either the package or class name.
- If you have two packages that use classes from each other, each jar must have all the classes of
the other package that it uses, or that it uses indirectly uses.
- You must put the -classpath option before the class name on the java
command line. To remember the order, you can think of it like this. Java.exe
needs to know the classpath before it can search for the class.
- You got package name wrong at the top of the source file, or left it out entirely. It does not
match the one when you built the jar. It is case sensitive.
- If this is an Applet, you got the <applet
code=com.mindprod.mypackage.MyClass.class wrong. Perhaps you forgot the
package part of the name or you forgot the .class. The package names in the
jar may not be right. The <applet archive=myjar.jar may not match a jar file containing the needed class.
- If you use java.exe -jar, it ignores your classpath environment variable
and any -classpath option without comment! All your classes must be in the
java.exe -cp C:\javamail-1.3.3_01\mail.jar -jar bulk.jarwon’t be able to find any of the classes in
mail.jar. You must copy mail.jar to the ext
directory or mention the mail.jar in the manifest Class-Path: entry.
- Did you remember to include. on your classpath?
- If you got the exception trying to execute a class in a jar, check with WinZip that the pathnames
inside the jar on that class exactly match the package structure, including case. If you don’t
know what I am talking about, see jar for details. Make sure the
class you are trying to call has the expected package statement. Check the manifest to make sure the
fully qualified name of the main-class is properly specified.
- You are trying to use a program compiled under Java version 1.4 or later
with an old Java 1.1.5 runtime, often the one in Internet Explorer. The old class library is missing
many of the new methods. Any other version mismatch where you run new code on old runtimes can cause
the similar problems. Check the runtime version
is one of the most likely candidates for a method from 1.5+ not available under earlier
JVMs (Java Virtual Machines).
- Have you got a simple HelloWorld working yet? If not,
your problem may be in the way you installed the JDK or
JRE. If you have trouble, read up on Getting Started, javac.exe,
java.exe and classpath.
- Java is case sensitive. Have you got the name of the class spelled exactly the same way everywhere
it is used, including the name of the *.java file, with class names always
starting with a capital letter and package names pure lower case?
- One of your static initialisers invokes a function that uses a
static field not yet initialised.
- Did you remember to make your kickoff class, the one you mentioned in the java command line,
- Does your main method have signature exactly like this?
public static void main( String args )
- Somehow your runtime is having trouble finding classes it could find at compile time. Check your
CLASSPATH. Make sure all the classes are available.
- Check your java.exe command line. Your current directory
is crucial. You must specify fully qualified class names, without the .class extension.
- Consider combining class files into a single jar file. Jars
cause far less trouble than individual class files, especially when you export them to a server or
someone else’s machine.
- Check that your browser supports jar files. Fortunately, all the modern ones mentioned under
- You can get this error if you try to run an Applet on an old browser
since it will not have the needed classes.
- Watch \ and /. In string literals
\ appears as \\.
- Make sure when developing classes for an extension i.e. class or jar files that live in the
lib/ext/ folder, make sure you do not have duplicate classes. Classes from
extension folders will always be loaded before any other classpath, therefore making it impossible to
test other development versions not in the extensions folder. Make sure extensions *.jar’s are complete (i.e., not reliant any additional classes). Make sure the
*.jar in the extensions folder is up to date. Basically only
thoroughly-tested, stable code should live in the extensions folder.
- Have you accidentally used a system package name for your own classes?
- If every program gives NoClassDefFoundErrors, try uninstalling all Java
tools and JDK
s, clean out the registry, and then reinstall. Make sure you have only the latest Java on your
- If your NoClassDefFoundError occurs only sporadically, here is a
possible cause: You are running some classes in a jar file which is stored on a network mapped drive in
MS Windows. Sun keeps the jar files open in order to re-load classes which haven’t been used in a
while, and occasional network problems will force the jar to close. The JVM
doesn’t know to re-open the jar when the network mapped drive reconnects, and simply reports that
various classes (different each time) no longer exist. The solution is either make sure the network is
reliable, use a network file system which doesn’t close if drives are temporarily unavailable, or
locate all jar files on local drives.
- If you change the jars used by your JSP (Java Server Pages)
code, make sure you manually delete the generated class files (classes in the WEB-INF tree for Caucho Resin). The womb won’t be smart enough to do that on its
own and you will get all manner of NoClassDefFoundErrors.
- If you have classes included in your project that don’t live in the same directory as the
project, in Café you must also do a Project ⇒ Options
⇒ Directory ⇒ Classes to include their directories in the invisible CLASSPATH for the project. Simply including the java or class files in the project is
not sufficient. Other IDEs (Integrated Development Environments)
may have similar gotchas.
- If your code works then suddenly stops working, chances are you have uninstalled a JRE/JDK or
installed a new one, and the current JDK/JRE does not have the full complement of jars in the
Sooner or later you will have to reinstall the JDK/JRE and you will lose your ext
You can quickly rebuild them if you
maintain a bat file like this and run it after every JRE/JDK install. Adjust the
file to account for where your ext dirs are and where
the jars are you need.
- Sometimes the problem is with your build. Try using ant, and
a script based on one of my examples. Ant has a better chance of getting it right that some ad hoc
- You overrode JPanel.getParent
or perhaps some other crucial piece of Sun’s code without reproducing its functionality typically
by calling super.xxx().
- Check the integrity of all jars. Some may have been truncated during upload. Verify with
jarsigner.exe or winzip.exe.
- An the app works fine locally but fails when you deploy it on a server because you failed to upload
all the class files needed to the server (usually in jar form).
|You did a new javax.naming.InitialContext
() when your program was not running inside a servlet womb which provides
the default Initial context factory. You can provide a factory class by naming it is a SET environment parameter, or a Java system property java.naming.factory.initial. Here is some typical code:
|You called Iterator.next when
there were no more elements. You must either call isNext first, or use the
for ( Thing: thing : thingCollection ) syntax which handles it all for you
|Likely you are mixing an old library with new code or vice versa. The definitions and
references to a class differ in whether they include some field. If you have source for the libraries,
recompile everything to discover the precise error. If not, check to make sure you are using the most
up-to-date libraries (or ones of the appropriate vintage) and clean compile all your code making sure the
libraries you run with are also on the classpath at compile time. Compilation may clear up the problem or
at least make it more specific.|
|Use Transport.sendMessage not Transport.send.|
|You forgot to put implements Serializable on the
class you are doing a writeObject on. Since writeObject also writes out all the objects pointed to by that object, by the objects
those objects point to, ad infinitum, all those classes too must be marked implements Serializable. Any references to non-serializable
classes must be marked transient. While you are at it, give each of those
Oracle’s JDK Platform Guide to NotSerializableException
||NTLDR is missing|
- You are trying to boot from a non-system diskette (or disk) in Windows
- The crucial loader program used in booting has been deleted from your hard disk.
|If you try to execute a piece of code like this when s is null you will get a
s.someMethod();To learn about exceptions in general and try/catch to recover from them, see
exceptions. Here are some of the possible causes:
- A reference variable is null, when you used it to access a field or method.
- Autounboxing can make NullPointerExceptions hard to track down. Innocent
looking code like int result = getResult(); can throw a NullPointerException if
autounboxing is hiding what is really happening, namely int result = getResult().intValue();
It will fail if getResult returns null instead
of an Integer object.
- Examine the stack trace to figure out where in the code it happening and how you got there. The
stack trace shows the exact line the problem was detected (not necessarily the line at fault though).
That gives you a clue to get started. You can put some code of the form:
if ( p == null )
System.out.println( "p was null" );
assert p != null : "p was null";
- Narrow it down to the precise variable giving the trouble. Then you have to sit and think about
why that variable is null. The real problem could be much earlier in the code, and the
problem propagated through to the point where it eventually surfaced as an exception. You must figure
out where it was supposed to be set, and have a look at that code and why it is failing. Most of the
time you simply forgot to initialise. The easiest way to track the causes down is using a source-level
debugger such as Intellij Idea or Eclipse You can trace
the flow of execution, or examine the state of the universe at the time of failure.
- If you don’t want to use a debugger, pepper your code with assertions to track down just what
ensureNotNull( thing, "thing" );
assert thing != null : "thing is null";
- Your ensureNotNull( Object toTest ) method might print an error message if toTest is null,
then throw a new NullPointerException. By making ensureNotNull final, and using an if (DEBUG) inside ensureNotNull, the inline expansions will have no overhead in production, and you can
leave the debugging assertions in place to be reactivated later if ever needed.
- Most commonly you forgot statements of the form: myArray = new int;
or myThing = new Thing(); See the array initialisation gotchas. These are the most common of runtime
- You may not call getParameter in the Applet
constructor. Move that code to init.
- Sometimes the problem is redeclaring a local variable with the same name as an instance variable.
You might have initialised one but not the other. Beware having two variables with the same name, one
local and one instance, It is very easy to do inadvertently, and Javac will not warn you, though Jikes will
ArrayList thelist = new ArrayList( 149 );instead
thelist = new ArrayList( 149 );You work with the local variable thinking you are working with the instance
variable, and poof, your values are lost as soon as the local goes out of scope, leaving the instance version null.
This is the way I most commonly create NullPointerExceptions for myself, when I convert a
local variable to an instance one when I have to break a method up that uses the variable and forget to take out the
- Be very careful calling any methods inside your constructors. If subclasses override them, you will
be invoking the subclass’s version, which may be attempting to use fields that have not been
initialised yet. You won’t get a warning message! The problem usually shows up as puzzling
NullPointerExceptions. The way out is to move code out of the constructor,
often to the addNotify method. However, addNotify can get in analogous problem to the constructor since it too is overridden,
and it may use overridden methods.
- If the problem is in Sun’s code that you call or indirectly call, (i.e. no source code line
number) look at the place in your code where you trigger the chain of calls. Insert some debug code
there to examine the parameters and the fields in any objects you pass. If you can’t track it
from that, use a source code debugger like Eclipse that lets you follow program flow inside Sun’s
code. For example, your code creates an event, asking it to operate on some null component, and the
problem does not show up until later when the event is dispatched and handled by Sun’s code.
- The Nice language manages to detect NullPointerExceptions at compile time instead of run time the way Java does.
|In converting a String to internal int/ long etc. with parseInt or
brethren, the number was not a valid string of digits. The usual problem is leading or trailing spaces on
your number. The sign must come ahead of the number, no commas. Localisation may have an effect on what
constitutes a valid number. Sometimes it can be the number is too big to fit in the int, long etc.|
|Attempting to use readObject on a stream that does not contain
objects, or attempting to read past the end of file.|
|You have run out of memory. You have filled up virtual RAM
with objects and the garbage collector can’t free any more RAM.
You can expand the effective size of virtual ram by
putting options on the java.exe command line. See the-Xss64k
-Xoss300k -Xms4m and -Xmx10m options described under java.exe. Perhaps you have read some ridiculously large number from a
DataInputStream to use as the size of an array. Perhaps you have used a method
recursively without terminating at reasonable depth and filled memory that way.
|IBM (International Business Machines)
Visual Age sometimes makes a noise like an elevator arriving every time you click anything. No error
message is visible anywhere. It means you are debugging a multi-thread program and a higher priority thread
is blocking the one you want to debug.|
||No error while developing. Security Violation in production.|
|Applets can only access the server they were loaded from.|
|signal 10 error
||Unexpected Signal : 10|
|Signal 10 is a bus error. It’s generally caused by attempting to perform an unaligned
bus operation (for example, reading a long from an odd address), so it isn’t something that will
normally occur in a pure java application unless there is a bug in the JVM.
If you have native code in your application though,
that could be the cause. On some platforms, an application will get a bus error if its executable file or
one of its shared libraries are replaced while the application is running.|
||StackOverflowError Stack size too small Use java -Xss to
increase default stacksize.|
|These usually happen when you have recursion, a method that calls itself, perhaps indirectly via a second
method. You have simply nested to deeply. Another source of the problem is calling method x() or this.x() when you meant
to call super. x(), usually when inside method
x. If you legitimately overflowed the stack, you may rescue yourself by
getting the runtime to allocate more memory for the stack for each thread with java.exe
|Start Service Failed
||StartService FAILED 1056: Instance of the service is already running.|
|Just ignore this spurious error message. There is nothing the matter. Window is being
prissy and chastising you for asking it to start a service, when it already did so earlier.|
|The object stream is scrambled. Perhaps you are trying to read human-readable data, e.g. something that
was not prepared with writeObject. Perhaps you have classes missing or the
classes used in the stream are obsolete. Perhaps you tried to use append mode to tack more onto the end
of your stream file after it was closed and later reopened in append mode. That does not work.
Oracle’s JDK Platform Guide to StreamCorruptedException
||java.lang.StringIndexOutOfBoundsException: String index out
|The parameters you gave to the substring, charAt,
index or related String method were outside the
range of the base String, or the start point was after the end point. See
substring gotchas. To track it down, dump the base
String and the indexing parameters with System.
|Applets behind a firewall may not have access to DNS (Domain Name Service).
You must encode your CODEBASE as an
IP (Internet Protocol)
instead of a DNS
|unable to load for debugging
||Unable to load MyClass.class for debugging.|
- Symantec support suggests copying all the DLLs
in VCP\JAVA\BIN to VCP\BIN to correct stability
problems. If you do this, all debugging will cease to function. Delete any DLL
in VCP\BIN that also exists is VCP\JAVA\BIN.
- Check that you fully qualified the name of your class with its package name, e.g. com.mindprod.business.TestDate in the Project section of Visual Cafe, no trailing
.class and that the upper and lower case is precisely correct. Read the
section in the glossary on CLASSPATH and under java.exe on how packages,
classnames, the directory structure and the CLASSPATH all interact. If that does not solve it, I have
- Windows 95 has the most incredibly inept scheme for dealing with DLLs
from different vendors that just happen to have the same 8+3 name, (even when they are stored in
totally different directories). Whichever application starts first gets its DLLs
installed, and every other app that comes afterward has to use them. If you start some app before VC, it
may load incompatible DLLs.
To avoid this, load VC first. Then the other apps may stop working. Phttt! Mr. Gates has guaranteed
himself a seat in hell for this (and for the confounded registry where all configuration information
for all applications is lumped in one unrestorable basket). There should be separate system
and application DLLs
with no possibility of DLLs
provided by one vendor being foisted on some other. Try starting Visual Cafe before any other app and
keep it running. It stands a better chance then of getting its own DLLs
|Unable to locate tools.jar
||Unable to locate tools.jar. Expected to find it in
|It seems to be looking for tools.jar in the JRE
instead of the JDK. Make sure all
directories contain the expected files, including tools.jar. I found that
uninstalling the JRE
and reinstalling them cleared this up. Especially for beta software you are safer to uninstall and
reinstall rather than install over top.|
|Unable to open file xxx.exe
||Fatal error (0): Unable to open file xxx.exe|
|In JET (Just Enough Time)
when compiling, you forgot to terminate execution of your app before redoing the compile.|
|unable to run
||Unable to run MyClass.class: Check that you have properly specified name of the class
containing main in your project settings.|
|Your SlickEdit vep project file is not in the same directory with the *.java and *.class files that compose the class containing
||javax.jnlp.UnavailableServiceException: Running a
app without JWS.|
|If you run a Java Web Start apps and try to use the JWS
service library, you will get this exception. You can only use that library in the context of Java Web
||java.rmi.UnmarshalException: invalid method hash.|
- the receiver does not have the needed class file.
- the receiver and sender have different versions of the class file.
- your stubs are out of date.
- you screwed up something doing RMI. Try a very simple case and get that going, then gradually add
complexity. When it stops working you have a much better idea where to look for the problem.
: Cannot recover key|
|You gave the wrong password when you accessed the keystore with KeyStore. getKey.|
|This means you had some trouble loading a library containing native class implementations.
major.minor SomeClass version (nn.n).|
|You used a method that your JVM
does not support. This most commonly happens when you target for Java version 1.6
but inadvertently use a Java version 1.7
method. The compiler will not warn you at compile time. I run into it most commonly when I run jet whose
run time is Java version 1.6. The problem goes away when I use a Java version 1.7
JVM. Look at the since comments
in the Javadoc to make sure you are not using any too new methods. If you use the -bootpath option to provide Java version 1.6
jars, then the compiler can warn you about using a Java version 1.7
method. Try upgrading or reverting your JVM. This is likely a bug — some native method not prepared to function
under the current version. Try uninstalling all your java, purging the files, then reinstall your
JDKs (Java Development Kits)
s, working from oldest to newest. The problem is caused by some sort of clash with code from one version
interacting with code from another. Watch your classpath. You could have no classes on it for a more
advanced JVM than you are using. Check which version you are using
or use Wassup for more detail.
||javax.activation.UnsupportedDataTypeException: no object DCH
|You have an obsolete JavaMail or Java Activation Framework jar.|
||Exception in thread xxx java.lang.VerifyError: Accessing value from universalised
|This is a compiler
bug. It comes from redefining a local variable. Just use two different names. Could come from other
compiler bugs. Try using a the current JDK
compiler. It can also come from using Outer.this from a non-inner class. In
1.4.2+, the compiler should detect the problem rather than runtime.|
||This is a NoClassDefFoundError where you got package
name wrong at the top of the source file. It does not match the one when you built the jar. It is case
||class file has wrong version 48.0, should be 47.0|
|You have partly a Java version 1.4
and partly an earlier version of Java installed. Check out each copy of java.exe and with the -version option. If necessary uninstall all
s, prune the registry of references to Java, prune the hard
disk of java directories and reinstall from scratch. Watch out for any JDKs
that came embedded with an IDE (Integrated Development Environment). Watch out specially for the C:\WINNT\system32\java.exe. It too should be 1.4. You may have the rt.jar file from a different version.|
||java.util.zip.ZipException: The system cannot find the file
|Usually the zip file in question is the distribution jar containing your class files and resources. For
some reason it can’t be found. You may have misspelled it or used the wrong syntax to specify it.