Avoiding overuse of autoboxing and auto-unboxing

It is probably not a good idea to overuse this new freedom.

Consider the following program, and work out how much boxing/unboxing is happening:

0182: public class Integers                                       // Integers.java
0183: {
0184:    public static void main(String[] pArgs)
0185:    {
0186:       Integer tWrapper = 127;
0187:       Integer tNewWrapper = tWrapper + tWrapper;
0188:       System.out.println(tNewWrapper);
0189:       if (tWrapper<tNewWrapper)
0190:       {
0191:          System.out.println(tWrapper + " is less than " + tNewWrapper);
0192:       }
0193:       tNewWrapper = 127;
0194:       if (tWrapper==tNewWrapper)
0195:          System.out.println(tWrapper + " is equal to " + tNewWrapper);
0196:       else
0197:          System.out.println(tWrapper + " differs from " + tNewWrapper);
0198:       tWrapper = 128;
0199:       tNewWrapper = 128;
0200:       if (tWrapper==tNewWrapper)
0201:          System.out.println(tWrapper + " is equal to " + tNewWrapper);
0202:       else
0203:          System.out.println(tWrapper + " differs from " + tNewWrapper);
0204:    }
0205: }

The above program produces the following output:

0206: 254
0207: 127 is less than 254
0208: 127 is equal to 127
0209: 128 differs from 128

The last two lines of output need some explanation.
Brett McLaughlin and David Flanagan have produced an excellent book on Java 5 ([21]).
In it, they say that ‘the Java specification indicates that certain primitives are always to be boxed into the same immutable wrapper objects. These objects are then cached and reused, with the expectation that these are commonly used objects. These special values are the boolean values true and false, all byte values, short and int values between -128 and 127, and any char in the range \u0000 to \u007F.’
So you may get surprising results from using the equality operator.
McLaughlin and Flanagan conclude: ‘you have to watch out for this, as it can result in some very tricky, hard-to-find bugs’.