Java Stack peek() behavior

Question

The following code returns false for all values except between -128 and 127. Is there any particular reason for this? I know I have to use equals because peek() returns a reference to the object, but I'm curious to know why it works only for the above range of values.

public boolean test(int x) {
    Stack<Integer> s1 = new Stack<Integer>();
    Stack<Integer> s2 = new Stack<Integer>();
    s1.push(x);
    s2.push(x);
    return (s1.peek() == s2.peek());
}

`Integer` caches values between -128 and 127 to improve performance. — Dragondraikk, Jul 01 '15 at 08:21
refer this http://stackoverflow.com/questions/1700081/why-does-128-128-return-false-but-127-127-return-true-in-this-code — Sasikumar Murugesan, Jul 01 '15 at 08:24

score 4 · Accepted Answer · answered Jul 01 '15 at 08:22

There are two reasons for this:

autoboxing;
the fact that an Integer implementation, as per the documentation, has an internal cache of at least all values between, precisely, -128 and 127.

When you autobox, s1.push(x) for some int x is transformed into s1.push(Integer.valueOf(x)). And since this is a cached value, the same instance is reused for any of these values.

Depending on the implementation of the JRE, Integer may have a larger cache than that as well -- but don't rely on that ;)

score 0 · Answer 2 · answered Jul 01 '15 at 08:24

In the lines

s1.push(x);
s2.push(x);

the primitive int x is autoboxed into an Integer instance using Integer.valueOf(x) (java does that by itself). That method uses a Cache to store Integer instances in the range of -127 to 128, meaning that for all other int values, a new instance will be created in both of the lines of code above, resulting in the == comparison to return false.

Java Stack peek() behavior

2 Answers2