I wrote some sample programs and decompiled them, finding nothing of interest. Then i tried to investigate auto-boxing. I wrote a sample program that calls a method that has an Integer class as parameter. Something like that:
public void bar(Integer i) { ... } int i = 1; System.out.println(i);When i used auto-boxing, the compiler used the Integer.valueOf(int) method. So i started searching if it is more optimized to use auto-boxing, or not.
When i changed the i variable to Integer type, and called normally the constructor of the Integer class, i realized that the constructor was invoked normally, with int as parameter.
The mystery solved when i read the original Integer class sources.
The contructor simply instantiates a new Integer object. On the other hand, the Integer.valueOf(int) does the following:
public static Integer valueOf(int i) {pre final int offset = 128; if (i >= -128 && i <= 127) { // must cache return IntegerCache.cache[i + offset]; } return new Integer(i); }The valueOf method uses an cache object and keeps 256 integers cached all the time. The pre for the integer cache follows:
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); } }So, if you use auto-boxing (or simply use the Integer.valueOf() method) instead of the normal constructor, you get a cached integer reference instead of a new object. This is an documented feature (see the Integer.valueOf(int) javadoc entry), and is demostrated by the following code.
public class Foo { public static void main(String[] args) { Integer i = new Integer(10); Integer ii = new Integer(10); Integer iii = Integer.valueOf(10); Integer iiii = Integer.valueOf(10); System.out.println("i == ii - " + (i == ii)); System.out.println("ii == iii - " + (ii == iii)); System.out.println("iii = i - " + (i == iii)); System.out.println("iiii = iii - " + (iiii == iii)); } }Sample executiion output follows:
nefarian:~/devel/ew bkarak$ java Foo i == ii - false ii == iii - false iii = i - false iiii = iii - trueBut the caching works only with the Integer.valueOf(int) method. See the implementation of the Integer.valueOf(String,int) for example:
public static Integer valueOf(String s, int radix) throws NumberFormatException { return new Integer(parseInt(s,radix)); }This method always returns a new Integer object. Doh!