|| : Java Glossary

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||
Performs a short-circuit logical or, on two booleans. || is sometimes called the short circuit or operator or the McCarthy or operator. John McCarthy was one of the LISP (List Processing language) pioneers. The | character is created by hitting shift-\. Sometimes it is displayed with a small gap in the bar, sometimes not. It can be hard to tell apart from Il1! in some fonts.

false || false → false
false || true → true
true || false → true
true || true → true

The evaluation is guaranteed to proceed in strict left to right order and if any boolean is true, it does not evaluate any expressions to the right of 
// if s is null, Java won't bother to test the length of s
if ( s == null || s.length() == 0 )
   {
   out.println( "empty string" );
   }

Learning More

JLS (Java Language Specification): expression evaluation order

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