java - Why does 128==128 return false but 127==127 return true when converting to Integer wrappers? -

class d {     public static void main(string args[]) {         integer b2=128;         integer b3=128;         system.out.println(b2==b3);     } } 

output:

false 

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class d {     public static void main(string args[]) {         integer b2=127;         integer b3=127;         system.out.println(b2==b3);     } } 

output:

true 

note: numbers between -128 , 127 true.

when compile number literal in java , assign integer (capital i) compiler emits:

integer b2 =integer.valueof(127) 

this line of code generated when use autoboxing.

valueof implemented such numbers "pooled", , returns same instance values smaller 128.

from java 1.6 source code, line 621:

public static integer valueof(int i) {     if(i >= -128 && <= integercache.high)         return integercache.cache[i + 128];     else         return new integer(i); } 

the value of high can configured value, system property.

-djava.lang.integer.integercache.high=999

if run program system property, output true!

the obvious conclusion: never rely on 2 references being identical, compare them .equals() method.

so b2.equals(b3) print true logically equal values of b2,b3.

note integer cache not there performance reasons, rather comform jls, section 5.1.7; object identity must given values -128 127 inclusive.

integer#valueof(int) documents behavior:

this method yield better space , time performance caching requested values. method cache values in range -128 127, inclusive, , may cache other values outside of range.