this : Java Glossary

this means the current object, the instance we are currently running a method on. There are 5 types of variable:

1. Local in the current method in the current thread
2. instance in the current object.
3. instance in some other object.
4. static in the current class.
5. static in some other class.

If you don’t specify otherwise, Java presumes (1) local, but if it can’t find the name, it presumes (2). If you want to explicitly use (2), use the keyword this.

If you mention any instance variables in your methods without any associated pointer, e.g. height, rather than joe.height, this is automatically assumed, i.e. as if you had said this.height, meaning the height variable in the current object. Ditto for instance methods.

Uses for this

this has many uses:

• Sometimes you might choose to use an explicit this just to make it clear what you are up to, that you are not using a static or local variable.
• this is most useful in constructors and get/set accessor functions where you have a parameter with the same name as an instance variable. this.dangerous refers to the instance variable, whereas plain dangerous refers to the parameter. Be careful using this idiom. You can easily misspell the parameter, and the compiler will not warn you.

  /* constructor */
  public Dog ( boolean dangerous )
     {
     this.dangerous = dangerous;
     }
  private boolean dangerous;

• this is also often used in convenience constructors to provide default values. In that context this means call another constructor of the same class.

  /** Convenience constructor to provide defaults
   */
  public Con ( int thing )
     {
     this( thing, false, 0 );
     }

• Often you need to refer to the current object when you want to pass a reference to it as a parameter, e.g. to say the current object is the child or parent of some other object.

  node.setParent( this );

• For convenience you sometimes want a method that does something to an object then returns a reference to it.

  public Dog groomDog()
     {
     washDog();
     dryDog();
     return this;
     }

• Beware, inside an (anonymous) inner class, this refers to the inner class, not the usual enclosing one. If the outer class were called MyOuter, you could get at its this with MyOuter.this.

Delphi and Smalltalk programmers will recognize this as self.

If you don’t speak assembler please ignore the following. Looking at this from the point of view of an assembler programmer, for any instance method, there is a hidden first parameter on every instance method call, a pointer to the current this object that is automatically generated by the Java compiler. You can refer to this parameter by the word this inside the called method.

This Class

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