Java Practices -> Determine if Strings are equal

Determine if Strings are equal

To determine if two String objects match exactly, you should almost always use the equals method, and not the == operator.

The equals method compares the actual content of the Strings, using the underlying Unicode representation, while == compares only the identity of the objects, using their address in memory (which is usually not desired).

Example


/**
* Illustrates the incorrectness of using == for typical 
* String comparisons.
*/
public final class EqualStrings {

  /**
  * Pass in the String "blah" on the command line in
  * order to exercise the code.
  */
  public static void main (String... arguments) {
    String text = (arguments.length == 1 ? arguments[0] : null);
    testUsingEquals(text);
    testUsingOperator(text);
  }

  // PRIVATE //
  private static final String BLAH = "blah";

  private static void testUsingEquals( String aString ) {
    if (BLAH.equals(aString)) {
      log("Equals: YES, the user entered \"blah\".");
    }
    else {
      log("Equals: NO, the user did not enter \"blah\".");
    }
  }

  private static void testUsingOperator( String aString ) {
    if (aString == BLAH) {
      log("== operator: YES, the user entered \"blah\".");
    }
    else {
      log("== operator: NO, the user did not enter \"blah\".");
    }
  }
  
  private static void log(String aMessage){
    System.out.println(aMessage);
  }
}

An example run of this class :

>java -cp . EqualStrings blah
Equals: YES, the user entered "blah".
== operator: NO, the user did not enter "blah".

Note that x.equals(y) will generate a NullPointerException if x is null, but will not generate such an exception if only y is null. In the above example, for instance, BLAH.equals(aString) is used instead of aString.equals(BLAH), since aString may be null.

It is true that the intern method may be used to allow meaningful comparisons using ==. (See The Java Programming Language by Arnold, Gosling, and Holmes for further information.) The intern technique is meant as a performance optimization, and should usually be avoided since it's more complex.

Would you use this technique?