Difference between revisions of "Java gotchas"

Revision as of 07:11, 25 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.

Integer Caching

Since Java 5 Integer caching was introduced. When creating an Integer using the following code:

Integer myNumber = 10

Or

Integer myNumber = Integer.valueOf(10);

256 Integer objects are created in the range of -128 to 127 which are all stored in an Integer array. This caching functionality can be seen by looking at the inner class, IntegerCache, which is found in Integer:

 private static class IntegerCache 
 {
   private IntegerCache(){}
   
   static final Integer cache[] = new Integer[-(-128) + 127 + 1];
 
   static 
   {
     for(int i = 0; i < cache.length; i++)
     cache[i] = new Integer(i - 128); 
   }
 }
    
 public static Integer valueOf(int i) 
 {
	final int offset = 128;
	if (i >= -128 && i <= 127) // must cache 
        { 
	    return IntegerCache.cache[i + offset];
	}
        return new Integer(i);
 }

So when creating an object using Integer.valueOf or directly assigning a value to an Integer within the range of -128 to 127 the same object will be returned. Therefore, 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.

It is important to note that object i and p only equate to true because they are the same object, the comparison is not based on the value, it is based on object equality. If Integer i and p are outside the range of -128 or 127 the cache is not used, therefore new objects are created. When doing a comparison for value always use the “.equals” method. It is also important to note that instantiating an Integer does not create this caching. So consider the following example:

Integer i = new Integer (100);
Integer p = new Integer(100);
if(i==p) System.out.println(“i and p are the same object”);
if(i.equals(p)) System.out.println(“ i and p contain the same value”);

In this circumstance, the output is only:

i and p contain the same value

Remember that “==” is always used for object equality, it has not been overloaded for comparing unboxed values.

This behavior is documented in the Java Language Specification 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.

Incrementing values

Be careful of the post-increment operator:

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

Prints 5.