intern() behaving differently in Java 6 and Java 7

Question

class Test {
    public static void main(String...args) {
        String s1 = \"Good\";
        s1 = s1 + \"morning\";
        System.out.println(s1.intern());
              


        

Answer 1

The result code dependents runtime:

class Test {
     public static void main(String... args) {
        String s1 = "Good";
        s1 = s1 + "morning";
        System.out.println(s1 == s1.intern()); // Prints true for jdk7, false - for jdk6.
    }
}

If you write like this:

class Test {
     public static void main(String... args) {
        String s = "GoodMorning";
        String s1 = "Good";
        s1 = s1 + "morning";
        System.out.println(s1 == s1.intern()); // Prints false for both jdk7 and jdk6.
    }
}

the reason is ' ldc #N ' (Load string from constant pool) and String.intern() both will use StringTable in hotspot JVM. For detail I wrote a pool english article: http://aprilsoft.cn/blog/post/307.html

Answer 2

In jdk6: String s1="Good"; creates a String object "Good" in constant pool.

s1=s1+"morning"; creates another String object "morning" in constant pool but this time actually JVM do: s1=new StringBuffer().append(s1).append("morning").toString();.

Now as the new operator creates an object in heap therefore the reference in s1 is of heap not constant pool and the String s2="Goodmorning"; creates a String object "Goodmorning" in constant pool whose reference is stored in s2.

Therefore, if(s1==s2) condition is false.

But what happens in jdk7?

Answer 3

You need to use s1.equals(s2). Using == with String objects compares the object references themselves.

Edit: When I run your second code snippet, I do not get "both are equal" printed out.

Edit2: Clarified that references are compared when you use '=='.