Strange behavior with string interning in Java

Question

There is code as following:

String s = new String(\"1\");
s.intern();
String s2 = \"1\";
System.out.println(s == s2);

String s3 = new String(\"1\")+new          


        

Answer 1

For the scenario 1:

String s = new String("1");
s.intern();
String s2 = "1";
System.out.println(s == s2);

with bytecode:

   0: new           #2                  // class java/lang/String
   3: dup
   4: ldc           #3                  // String 1
   6: invokespecial #4                  // Method java/lang/String."":(Ljava/lang/String;)V
   9: astore_1
  10: aload_1
  11: invokevirtual #5                  // Method java/lang/String.intern:()Ljava/lang/String;
  14: pop
  15: ldc           #3                  // String 1

for String s = new String("1"); it will create a new String object, it will have a new address with "1" that it is already in String Pool:

ldc #3 // String 1

and for s2, as the bytecode:

15: ldc #3 // String 1

s2 is pointing to String Pool variable: "1", so s and s2 have the different address and result is false.

For the scenario 2:

String s3 = new String("1")+new String("1");
s3.intern();
String s4 = "11";
System.out.println(s3 == s4);

with bytecode:

   0: new           #2                  // class java/lang/StringBuilder
   3: dup
   4: invokespecial #3                  // Method java/lang/StringBuilder."":()V
   7: astore_1
   8: aload_1
   9: ldc           #4                  // String 1
  11: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  14: ldc           #4                  // String 1
  16: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  19: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  22: astore_2
  23: aload_2
  24: invokevirtual #7                  // Method java/lang/String.intern:()Ljava/lang/String;
  27: astore_3
  28: ldc           #8                  // String 11

As the bytecode, you can see new String("1")+new String("1"); is created by using StringBuilder

new #2 // class java/lang/StringBuilder

it's totally a new Object without String Pool variable.

and after s3.intern(), this method will add current s3 to the Memory String Pool and 8: aload_1.

and s4 is trying to load from

ldc #8 // String 11

so s3 and s4 address should equal and result is true.