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Where the author realizes the significance of the String.intern() method

I might have hinted about in in my previous post on the subject of strings in Java, yet I did not realize the significance of String.intern() method. The following code sample demonstrates the behavior of the String.intern() method, similar to what I demonstrated in the post.

public class TestClass2 {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = new String("hello");

        // This is going to be false.
        if (s1 == s2) System.out.println("s1 == s2");

        // This is going to be true.
        if (s1 == s2.intern()) System.out.println("s1 == s2.intern()");
    }
}

It’s a didactic example at best. It’s when you consider that strings also come from input/output that it String.intern() becomes a thing of interest.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class TestClass2 {
    public static void main(String[] args) {
        String s1 = "hello";
        String s2 = null;

        try {
            // Enter hello at this point.
            s2 = new BufferedReader(new InputStreamReader(System.in)).readLine();

            // This is going to be false.
            if (s1 == s2) System.out.println("s1 == s3");

            // This is going to be true.
            if (s1 == s2.intern()) System.out.println("s1 == s2.intern()");

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

As you can see in that example, the String.intern() method returns a reference to the string “hello” already in the constant pool. The virtual machine maintains an table of string instances that can be shared between all the string references in the program.

An immediate and obvious benefit of this technique called String interning is reduced memory footprint because of object reuse. Wikipedia also describes that the technique is also used by programs that need to do fast string comparisons such as compiler. This allow to compare strings by simple comparing references instead of possibly scanning the full length of both strings.

The JDK documentation gives a better description of the behavior of the String.intern() method. It’s a surprise to me that I never took the time to understand this behavior of such a core class of the Java library.

Microsofties might also find interesting that the .NET Framework also has a String.intern() method which behaves approximatively in the same way.