Java == for String objects ceased to work?

Question

  public class Comparison {
        public static void main(String[] args) {
            String s = "prova";
            String s2 = "prova";
            System.out.println(s == s2);
            System.out.println(s.equals(s2));
        }
    }

outputs:

true
true

on my machine. Why? Shouldn't be == compare object references equality?

Answer 1

Because String instances are immutable, the Java language is able to make some optimizations whereby String literals (or more generally, String whose values are compile time constants) are interned and actually refer to the same (i.e. ==) object.

JLS 3.10.5 String Literals

Each string literal is a reference to an instance of class String. String objects have a constant value. String literals-or, more generally, strings that are the values of constant expressions -are "interned" so as to share unique instances, using the method String.intern.

This is why you get the following:

System.out.println("yes" == "yes"); // true
System.out.println(99 + "bottles" == "99bottles"); // true
System.out.println("7" + "11" == "" + '7' + '1' + (char) (50-1)); // true
System.out.println("trueLove" == (true + "Love")); // true
System.out.println("MGD64" == "MGD" + Long.SIZE);

That said it needs to be said that you should NOT rely on == for String comparison in general, and should use equals for non-null instanceof String. In particular, do not be tempted to intern() all your String just so you can use == without knowing how string interning works.

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On new String(...)

If for some peculiar reason you need to create two String objects (which are thus not == by definition), and yet be equals, then you can, among other things, use this constructor:

public String(String original) : Initializes a newly created String object so that it represents the same sequence of characters as the argument; in other words, the newly created string is a copy of the argument string. Unless an explicit copy of original is needed, use of this constructor is unnecessary since Strings are immutable.

Thus, you can have:

System.out.println("x" == new String("x")); // false

The new operator always create a new object, thus the above is guaranteed to print false. That said, this is not generally something that you actually need to do. Whenever possible, you should just use string literals instead of explicitly creating a new String for it.

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