Understanding Integer Boxing Behavior in Java: Why Does `==` Return True for Certain Values?

Question

What explains the behavior of Java's Integer boxing, particularly why `==` returns true for certain small values and false for larger ones?

public class Scratch {
    public static void main(String[] args) {
        Integer a = 1000, b = 1000;
        System.out.println(a == b);  // Output: false

        Integer c = 100, d = 100;
        System.out.println(c == d);  // Output: true
    }
}

Answer

In Java, the behavior observed when using the `==` operator with Integer objects stems from Java's caching mechanism for small integers. This article will explain why the comparisons yield different results based on the values being compared.

Integer x = 100; // cached instance
Integer y = 100; // references the same cached instance
System.out.println(x == y); // true

Integer m = 1000; // new Integer object
Integer n = 1000; // another new Integer object
System.out.println(m == n); // false

Causes

  • Java caches Integer objects for values between -128 and 127.
  • When comparing small integers (like 100), Java uses the same cached instance, leading the `==` comparison to return true.
  • For larger integers (like 1000), new Integer objects are created, making the `==` comparison return false.

Solutions

  • Use `.equals()` method when comparing Integer objects to avoid confusion with reference comparison.
  • Understand the implications of autoboxing and caching in Java to predict behavior accurately.

Common Mistakes

Mistake: Assuming `==` checks value equivalency for Integer objects.

Solution: Always use `.equals()` for value comparison between Integer objects.

Mistake: Not knowing about Integer caching range.

Solution: Remember that Integer instances from -128 to 127 are cached in Java.

Helpers

  • Java Integer boxing
  • Integer comparison in Java
  • Java autoboxing
  • Integer caching in Java
  • Java Integer equality

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