switch : Java Glossary

Java’s N-way branch. In the JVM (Java Virtual Machine) there are two different ways of implementing a switch, tableswitch and lookupswitch. The efficient tableswitch is a jump table used when the switch values are reasonably dense e.g. numbered 0..N. The less efficient lookupswitch is a binary search used when the switch values all over the map. It thus pays to keep your switch values dense i.e. sequential.

The most common error with switch is to forget the break. Then one case falls through into the next.

// S W I T C H / C A S E
switch ( n )
   {
   case 1: out.println( "one" );
      break;
   case 2: out.println( "two" );
      break;
   default: out.println( "something else" );
   }// end switch (n)

Case Labels

• enum constants.
• character or numeric literals.
• the keyword default.
• A constant expression, i.e. An expression that can be evaluated at compile time.
• A constant variable (oxymoronic) for a static or instance final variable whose value is known at compile time.

• null.
• String literals. However, they are supported in Java version 1.7 or later
• Local final variables.
• floating point literals.
• references other than enum constants.
• In JDK (Java Development Kit) 1.7+, you can use Strings for case labels. The switch is implemented by switching on hashCode and resolvning duplicates by String comparison. This amounts to a linear search, do don’t toss out your HashMap lookups.

Example of Use

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