Here they are the same instance:
Integer integer1 = 127; Integer integer2 = 127; System.out.println(integer1 == integer2); // outputs "true"
But here they are different instances:
Integer integer1 = 128; Integer integer2 = 128; System.out.println(integer1 == integer2); // outputs "false"
Why do the wrapper objects share the same instance only within the value 127?
Integer class is a wrapper class for the primitive type int which contains several methods to effectively deal with an int value like converting it to a string representation, and vice-versa. An object of the Integer class can hold a single int value.
Comparing Integers Integer is a wrapper class of int, and it provides several methods and variables you can use in your code to work with integer variables. One of the methods is the compareTo() method. It is used to compare two integer values. It will return a -1, 0, or 1, depending on the result of the comparison.
Because it's specified by Java Language Specification.
JLS 5.1.7 Boxing Conversion:
If the value
p
being boxed istrue
,false
, abyte
, or achar
in the range\u0000
to\u007f
, or anint
orshort
number between -128 and 127 (inclusive), then letr1
andr2
be the results of any two boxing conversions ofp
. It is always the case thatr1 == r2
.Ideally, boxing a given primitive value
p
, would always yield an identical reference. In practice, this may not be feasible using existing implementation techniques. The rules above are a pragmatic compromise. The final clause above requires that certain common values always be boxed into indistinguishable objects. The implementation may cache these, lazily or eagerly. For other values, this formulation disallows any assumptions about the identity of the boxed values on the programmer's part. This would allow (but not require) sharing of some or all of these references.This ensures that in most common cases, the behavior will be the desired one, without imposing an undue performance penalty, especially on small devices. Less memory-limited implementations might, for example, cache all
char
andshort
values, as well asint
andlong
values in the range of -32K to +32K.
The source of java.lang.Integer:
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); }
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