Why Do Java\'s Integer Wrapper Objects Share Instances Only Between -128 and 127?

Integer Wrapper Objects and Shared Instances: Beyond the Value of 127

The Java programming language provides wrapper classes to represent primitive data types as objects. Integer is one such wrapper class that wraps the int primitive type. An intriguing aspect of Integer wrapper objects is their behavior regarding shared instances.

Within the Realm of 127

As the question correctly observes, Integer wrapper objects indeed share the same instance within the value range of [-128, 127]. This behavior is explicitly defined by the Java Language Specification (JLS) in section 5.1.7:

If the value p being boxed is ... an int or short number between -128 and 127 (inclusive), then ... any two boxing conversions of p ... will always yield an identical reference.

This means that within this value range, all Integer wrapper objects representing the same primitive int value will point to the same instance in memory.

Beyond 128

However, this sharing behavior does not continue beyond the value 128. When an int value outside the specified range [-128, 127] is boxed into an Integer object, the implementation is free to create a new instance for each unique value. This is confirmed by the JLS in the same section:

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.

In summary, Integer wrapper objects share the same instance only within the value range [-128, 127] because it is explicitly specified by the Java Language Specification. This optimization provides performance benefits for common scenarios involving primitive values within this range, without compromising the flexibility of using different instances for unique values beyond this range.

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