Difference between revisions of "Java gotchas"

Revision as of 14:41, 23 August 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.

Also, be aware that:

String abc = "abc"

and

String abc = new String("abc");

are different. For example, consider the following:

String letters = "abc";
String moreLetters = "abc";

System.out.println(letters==moreLetters);

The output is:

true

This is due to the compiler and runtime efficiency. In the compiled class file only one set of data "abc" is stored, not two. In this situation only one object is created, therefore the equality is true between these object. However, consider this:

String data = new String("123");
String moreData = new String("123");

System.out.println(data==moreData);

The output is:

false

Even though one set of data "123" is stored in the class this is still treated differently at runtime. An explicit instantiation is used to create the String objects. Therefore, in this case, two objects have been created, so the equality is false. It is important to note that "==" is always used for object equality and does not ever refer to the values in an object. Always use .equals when checking looking for a "meaningful" comparison.

Immutable Objects / Wrapper Class Caching

Since Java 5 wrapper class caching was introduced. The following is an examination of the cache created by an inner class, IntegerCache, located in the Integer cache.

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.

The other wrapper classes (Byte, Short, Long, Character) also contain this caching mechanism. The Byte, Short and Long all contain the same caching principle to the Integer object. The Character class caches from 0 to 127. The negative cache is not created for the Character wrapper as these values do not represent a corresponding character. There is no caching for the Float object.

As per Java Language Specification(JLS) the values discussed above are stored as immutable wrapper objects. This caching has been created because it is assumed these values / objects are used more frequently.

Incrementing values

Be careful of the post-increment operator:

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

The output is:

5

Remember that the assignment completes before the increment, hence post-increment. Using the pre-increment will update the value before the assignment. For example:

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

The output is:

6

Garbage Collection

By overriding "finalize()" will allow you to define you own code for what is potentially the same concept as a destructor. There are a couple of important points to remember:

• "finalize()" will only ever by called once (at most) by the Garbage Collector.
• It is never a guarantee that "finalize()" will be called i.e that an object will be garbage collected.
• By overriding "finalize()" you can prevent an object from ever being deleted. For example, the object passes a reference of itself to another object.
• Garbage collection behaviour differs between JVMs.