Comparison of int to Integer not working after adding to Linkedlist

Question

public static void main(String[] args) {
     int var = 128;
     Integer i = var;
     int j = var;
     LinkedList<Integer> ll = new LinkedList<>();
     ll.add(var);
     System.out.println(i==j);
     System.out.println(i==ll.peek()); 
 }

Output:
true
false

The values of variable var below number 128 though gives correct output as:

Output:
true
true

Please explain why comparison fails for peek() on values above 127?

You're not comparing an int to an Integer. You're comparing an Integer to another Integer. So you should use `equals` rather than `==` — khelwood, Apr 11 '20 at 10:28
I think you should read [this](https://stackoverflow.com/questions/13098143/why-does-the-behavior-of-the-integer-constant-pool-change-at-127) question and you will get partial answer. — Michał Krzywański, Apr 11 '20 at 10:42

Arvind Kumar Avinash · Accepted Answer · 2020-04-11T10:55:24.283

2

Do it as follows:

System.out.println(i.equals(ll.peek()));

Remember, == compares the references, not the content.

Check Why is 128==128 false but 127==127 is true when comparing Integer wrappers in Java? to understand why it returned true for a number less than 128.

edited Apr 11 '20 at 10:55

answered Apr 11 '20 at 10:28

Arvind Kumar Avinash

71,965
6
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how come == works at all if it is comparing the references? – JunisvaultCo Apr 11 '20 at 10:32
@JunisvaultCo - do you mean how `i==j` returns `true`? If yes, that is because of [Autoboxing and Unboxing](https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html). – Arvind Kumar Avinash Apr 11 '20 at 10:34
he stated that i==ll.peek() returns true for var<128. And indeed, I've tested it with 127 andi t does. Why? It is comparing references. – JunisvaultCo Apr 11 '20 at 10:37
1

@JunisvaultCo have a look at [this](https://stackoverflow.com/questions/13098143/why-does-the-behavior-of-the-integer-constant-pool-change-at-127) – Michał Krzywański Apr 11 '20 at 10:40
@michalk thanks! that's crazy – JunisvaultCo Apr 11 '20 at 10:46

score 1 · Answer 2 · answered Apr 11 '20 at 10:44

This is because of the Integer constant pool. Java maintains Integer pool from -128 to 127

private static class IntegerCache {
        static final int low = -128;
        static final int high;  //set to 127
}

So for values between -128 to 127, same reference would be returned from cache but for other values new Integer object would be created.

score 0 · Answer 3 · answered Apr 11 '20 at 10:52

0

the operator == checks for reference equality. Because Integer i is a Class type, as well as the returned value of ll.peak(), you should use the equals() method to compare them.

answered Apr 11 '20 at 10:52

Alon Parag

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Comparison of int to Integer not working after adding to Linkedlist

3 Answers3