Why Does the Java Constant Pool Treat Integers Between -128 and 127 Differently?

The Enigma of the Integer Constant Pool: The Boundary at 127

The Java Constant Pool holds an array of literals used within a class file, including integer constants. However, the peculiar behavior of these integer constants at the value 127 has left many bewildered. Let's delve into the mystery and unravel the reasoning behind this phenomenon.

The Constant Pool's mechanics are akin to those of the String Constant Pool, where only compile-time string literals are interned. However, for integer wrapper types, any boxing operation, regardless of whether it's a compile-time constant or not, utilizes the pool if the value qualifies.

This seemingly innocuous detail leads to a significant shift in behavior after the integer threshold of 127. For instance:

int x = 10;
int y = x + 1;
Integer z = y; // Not a compile-time constant!
Integer constant = 11;
System.out.println(z == constant); // true; reference comparison

According to the Java Language Specification (JLS), the Constant Pool guarantees a small range of pooled values. However, implementations have the flexibility to expand this range. This explains why the behavior is not uniform across different Java Virtual Machines (JVMs).

Interestingly, JLS mandates that boxing certain primitive values, including integers between -128 and 127, must always yield an identical reference. This ensures predictable behavior in common scenarios without incurring a substantial performance overhead.

For example, the following code snippet, which uses the static valueOf method to manually perform boxing, exhibits the same behavior:

Integer x = Integer.valueOf(100);
Integer y = Integer.valueOf(100);
System.out.println(x == y); // true

In conclusion, the integer Constant Pool's behavior around 127 is dictated by the rules established in the JLS. While common implementations cache values in this range, they may exhibit slight variations depending on the specific JVM. Understanding this behavior is crucial for accurately interpreting code and optimizing performance in Java applications.

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