Why Does the Java Constant Pool Treat Integers Differently Above 127?

Why 127 Is the Magic Number for Integer Constant Pool Behavior

In Java, the behavior of the constant pool for integer values differs when the value exceeds 127. This change in behavior can be puzzling, especially considering the consistent behavior of the string constant pool.

Behavior for Values <= 127

For integers less than or equal to 127, the constant pool operates similarly to the string constant pool. Any two integer constants with the same value are guaranteed to reference the same object in the constant pool. This means the following two statements return true:

Integer i1 = 127;
Integer i2 = 127;
System.out.println(i1 == i2);

Behavior for Values > 127

However, when the integer value exceeds 127, the behavior changes. In this case, boxing operations (which convert primitive types to their wrapper objects) no longer use the constant pool. Instead, new Integer objects are created for each value. As a result, the following statement returns false:

Integer i1 = 128;
Integer i2 = 128;
System.out.println(i1 == i2);

Reason for the Change

This change in behavior is due to the implementation details of the Java Virtual Machine (JVM). While the JLS mandates that certain primitive values always be boxed into indistinguishable objects (e.g., integers between -128 and 127), it is up to the JVM to determine if this rule is applied beyond that range.

Most JVMs choose not to apply this rule for values greater than 127. This is because it would require additional memory overhead and performance implications. By lazily creating Integer objects only when necessary, the JVM can optimize performance and reduce memory consumption.

Conclusion

Understanding the different behaviors of the constant pool for integer values is essential for writing efficient and correct Java code. By knowing that boxing operations use the constant pool for values less than or equal to 127, but not for larger values, developers can avoid unexpected results and optimize their code accordingly.

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