Why 1 == 1 is True but 128 == 128 is False in Java: The Mystery Unveiled! 🔍

Hey there! 👋 Let’s dive into a quirky but crucial aspect of Java that could surprise you if you're not familiar with how Java handles object comparisons. Today, we’ll explore why comparing integers in Java with == sometimes gives unexpected results—like why 1 == 1 returns true, but 128 == 128 might return false. Confusing, right? 🤔 Let’s break it down.

The Curious Case of 1 and 128

When we compare two numbers in Java using ==, our natural assumption is that if the numbers are the same on both sides, the result will always be true. That’s what we expect, right?

Let’s take a look at some code:

Integer a = 128; Integer b = 128; System.out.println(a == b); // false Integer x = 1; Integer y = 1; System.out.println(x == y); // true

Surprisingly, the first comparison (a == b) returns false, while the second one (x == y) returns true. But why? 🤔 The answer lies in a Java optimization known as Integer Caching.

Integer Caching: The Hidden Optimization

Java uses a caching mechanism to optimize memory usage for Integer objects. By default, Java caches Integer values in the range -128 to 127. This means if you create an Integer within this range, Java doesn’t create a new object each time. Instead, it reuses the same object from the cache.

So, when you do this:

Integer x = 1; Integer y = 1;

Java doesn’t create two separate Integer objects. Both x and y reference the same object in memory. This is why x == y returns true.

However, when you do this:

Integer a = 128; Integer b = 128;

Since 128 is outside the cached range, Java creates two separate Integer objects. Even though both a and b hold the value 128, they are not the same object in memory, which is why a == b returns false.

Understanding == vs .equals()

Now, it’s important to note the difference between == and .equals() in Java. When you use ==, Java checks if both variables point to the same object in memory (i.e., they reference the same memory location). On the other hand, .equals() compares the values inside the objects.

Here’s an example using .equals():

Integer a = 128; Integer b = 128; 
System.out.println(a.equals(b)); // true

This time, a.equals(b) returns true because .equals() compares the values inside the Integer objects, and both a and b hold the value 128.

The Cached Range: -128 to 127

By default, Java caches Integer objects for values between -128 and 127. This range is chosen because these small numbers are frequently used in applications, so caching them saves memory and improves performance.

If you want, you can even adjust this range by setting the JVM argument -XX:AutoBoxCacheMax=<size>, but the default is up to 127.

Code Example Recap

To make this clearer, here’s a breakdown of how memory addresses and caching work:

Integer c = 128; Integer d = 128; System.out.println(System.identityHashCode(c)); // Unique HashCode for c System.out.println(System.identityHashCode(d)); // Unique HashCode for d Integer e = 1; Integer f = 1; System.out.println(System.identityHashCode(e)); // Same HashCode for e (cached) 
System.out.println(System.identityHashCode(f)); // Same HashCode for f (cached)

For numbers like 128 that are outside the cache, Java allocates different memory locations for c and d, so their hash codes will be different. But for numbers in the cached range (like 1), the hash codes of e and f will be the same, showing that they reference the same cached object.

Why Does This Matter?

This behavior can lead to unexpected results if you rely on == for comparison. If you’re comparing two Integer objects with ==, it will return true only if both objects reference the same memory location, which isn’t guaranteed for numbers outside the cached range.

If you’re comparing values, it’s safer to use .equals(). Always remember:

• Use == to compare object references (i.e., memory locations).

• Use .equals() to compare the values inside the objects.

  
  

Conclusion

Java’s Integer Caching is an optimization that works behind the scenes to save memory and improve performance for commonly used numbers. It’s important to understand how this works, especially when working with Integer comparisons, to avoid bugs and unexpected behavior in your code.

Next time you see 128 == 128 returning false, you’ll know exactly why! And remember: when comparing objects in Java, prefer .equals() over == unless you’re specifically checking if two objects refer to the same memory location.

Happy coding! 😊