Difference between revisions of "Java gotchas"

Revision as of 07:31, 22 June 2007

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 > -127 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.

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.