Comparing strings with == which are declared final in Java

I have a simple question about strings in Java. The following segment of simple code just concatenates two strings and then compares them with ==.

String str1="str";
String str2="ing";
String concat=str1+str2;

System.out.println(concat=="string");

The comparison expression concat=="string" returns false as obvious (I understand the difference between equals() and ==).

When these two strings are declared final like so,

final String str1="str";
final String str2="ing";
String concat=str1+str2;

System.out.println(concat=="string");

The comparison expression concat=="string", in this case returns true. Why does final make a difference? Does it have to do something with the intern pool or I'm just being misled?

When you declare a String (which is immutable) variable as final, and initialize it with a compile-time constant expression, it also becomes a compile-time constant expression, and its value is inlined by the compiler where it is used. So, in your second code example, after inlining the values, the string concatenation is translated by the compiler to:

String concat = "str" + "ing";  // which then becomes `String concat = "string";`

which when compared to "string" will give you true, because string literals are interned.

From JLS §4.12.4 - final Variables:

A variable of primitive type or type String, that is final and initialized with a compile-time constant expression (§15.28), is called a constant variable.

Also from JLS §15.28 - Constant Expression:

Compile-time constant expressions of type String are always "interned" so as to share unique instances, using the method String#intern().

This is not the case in your first code example, where the String variables are not final. So, they are not a compile-time constant expressions. The concatenation operation there will be delayed till runtime, thus leading to the creation of a new String object. You can verify this by comparing byte code of both the codes.

The first code example (non-final version) is compiled to the following byte code:

  Code:
   0:   ldc     #2; //String str
   2:   astore_1
   3:   ldc     #3; //String ing
   5:   astore_2
   6:   new     #4; //class java/lang/StringBuilder
   9:   dup
   10:  invokespecial   #5; //Method java/lang/StringBuilder."<init>":()V
   13:  aload_1
   14:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   17:  aload_2
   18:  invokevirtual   #6; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   21:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   24:  astore_3
   25:  getstatic       #8; //Field java/lang/System.out:Ljava/io/PrintStream;
   28:  aload_3
   29:  ldc     #9; //String string
   31:  if_acmpne       38
   34:  iconst_1
   35:  goto    39
   38:  iconst_0
   39:  invokevirtual   #10; //Method java/io/PrintStream.println:(Z)V
   42:  return

Clearly it is storing str and ing in two separate variables, and using StringBuilder to perform the concatenation operation.

Whereas, your second code example (final version) looks like this:

  Code:
   0:   ldc     #2; //String string
   2:   astore_3
   3:   getstatic       #3; //Field java/lang/System.out:Ljava/io/PrintStream;
   6:   aload_3
   7:   ldc     #2; //String string
   9:   if_acmpne       16
   12:  iconst_1
   13:  goto    17
   16:  iconst_0
   17:  invokevirtual   #4; //Method java/io/PrintStream.println:(Z)V
   20:  return

So it directly inlines the final variable to create String string at compile time, which is loaded by ldc operation in step 0. Then the second string literal is loaded by ldc operation in step 7. It doesn't involve creation of any new String object at runtime. The String is already known at compile time, and they are interned.

As per my research, all the final String are interned in Java. From one of the blog post:

So, if you really need to compare two String using == or != make sure you call String.intern() method before making comparison. Otherwise, always prefer String.equals(String) for String comparison.

So it means if you call String.intern() you can compare two strings using == operator. But here String.intern() is not necessary because in Java final String are internally interned.

You can find more information String comparision using == operator and Javadoc for String.intern() method.

Also refer this Stackoverflow post for more information.