Java: Why can String equality be proven with ==?

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It doesn’t. It’s still a bad thing to do – you’ll still be testing reference equality instead of value equality.

public class Test
{
    public static void main(String[] args)
    {
        String x = "hello";
        String y = new String(x);
        System.out.println(x == y); // Prints false
    }
}

If you’re seeing == testing “work” now then it’s because you genuinely have equal references. The most common reason for seeing this would probably be due to interning of String literals, but that’s been in Java forever:

public class Test
{
    public static void main(String[] args)
    {
        String x = "hello";
        String y = "hel" + "lo"; // Concatenated at compile-time
        System.out.println(x == y); // Prints true
    }
}

This is guaranteed by section 3.10.5 of the Java Language Specification:

Each string literal is a reference
(§4.3) to an instance (§4.3.1, §12.5)
of class String (§4.3.3). String
objects have a constant value. String
literals-or, more generally, strings
that are the values of constant
expressions (§15.28)-are “interned” so
as to share unique instances, using
the method String.intern.

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