Java, compare values of ArrayList

Question

My code is here:

ArrayList<Integer> list1 = new ArrayList<>();

list1.add(4444);
list1.add(4444);

System.out.println(list1.get(0) == list1.get(1));

It print out as false. What is the reason? normally 4444 == 4444 should be true. Thank you.

Answer 1

List (and other Collection subclasses) store objects, not primitives.

In Java 4 and before, list1.add(4444); would have been a compilation error.

Since Java 5, there is autoboxing: if you pass a primitive in a place that expects an instance of an object, the JVM automatically converts it to its wrapper value. So you end with your code working like list1.add(Integer.valueOf(4444)).

When you do the get, you are comparing not two primitives but two objects, so you must use .equals() to compare them.

Funny nitpick, because of how Integer.valueOf, if you had used 111 (or anything in the -128 to 127 range) in your example, your comparation would have returned true, since for those values the instances of integer are in a cache and Integer.valueOf returns always the same instance, not a new one.