Inconsistent behavior on java's ==

integer, java, primitive

Solution

Autoboxing of primitives into objects (as used in your calls to `method` uses a cache of small values. From the Java Language Specification section 5.1.7:

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 discussion part of the spec immediately following that is interesting too. Notably a JVM can cache more values if it wants to - you can't be sure of the results of doing:

Integer i1 = 129;
Integer i2 = 129;
boolean b = (i1 == i2);

Problem

Consider this code: ``` class test { public static void main(String[] args) { test inst_test = new test(); int i1 = 2000; int i2 = 2000; int i3 = 2; int i4 = 2; Integer Ithree = new Integer(2); // 1 Integer Ifour = new Integer(2); // 2 System.out.println( Ithree == Ifour ); inst_test.method( i3 , i4 ); inst_test.method( i1 , i2 ); } public void method( Integer i , Integer eye ) { System.out.println(i == eye ); } } ``` It prints: ``` false true false ``` I understand the first `false`, the == operator only checks if two references are working on the same object, which in this case aren't. The following `true` and `false` have me scratching my head. Why would Java consider `i3` and `i4` equal but `i1` and `i2` different? Both have been wrapped to Integer, shouldn't both evaluate to false? Is there a practical reason for this inconsistency?

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