Java gotchas

2007-06-22T20:34:08Z

Arunkv: Added link to JLS 5.1.7

== Equality ==
Object equality is tested using the == operator, while value equality is tested using the .equals(Object) method. For example:
String one = new String("abc");
String two = new String("abc");
String three = one;
if (one != two) System.out.println("The two objects are not the same.");
if (one.equals(two)) System.out.println("But they do contain the same value");
if (one == three) System.out.println("These two are the same, because they use the same reference.");

The output is:
The two objects are not the same.
But they do contain the same value
These two are the same, because they use the same reference.
=== Autoboxing ===
Java 5's autoboxing and unboxing features add some new gotchas to testing for equality. Consider the following example:
Integer i = 100;
Integer p = 100;
if (i == p) System.out.println("i and p are the same.");
if (i != p) System.out.println("i and p are different.");
if(i.equals(p)) System.out.println("i and p contain the same value.");
The output is:
i and p are the same.
i and p contain the same value.

The code above shows that i == p because the values are first unboxed into the primitive int type, and then the comparison is performed. But this is only true for Integer values >= -128 and <= 127! In other words, if the above example is changed so that i = 200 and p = 200, then i == p evaluates to false and the output would be:
i and p are different.
i and p contain the same value.

This behavior is documented in the [http://java.sun.com/docs/books/jls Java Language Specification] [http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.7 section 5.1.7]. Quoting from there:
:If the value ''p'' being boxed is true, false, a byte, a char in the range \u0000 to \u007f, or an int or short number between -128 and 127, then let ''r1'' and ''r2'' be the results of any two boxing conversions of ''p''. It is always the case that ''r1'' == ''r2''.

Notice in the above example that the declaration was:
Integer i = 100;
If this had been:
Integer i = new Integer(100);
Then the output would have been different:
i and p are different.
i and p contain the same value.

== Incrementing values ==
Be careful of the post-increment operator:

int x = 5;
x = x++;
System.out.println( x );

Prints 5.

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Java gotchas