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