Comparing strings with == which are declared final in Java

Question

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

Answer 1

If you take a look at this methods

public void noFinal() {
    String str1 = "str";
    String str2 = "ing";
    String concat = str1 + str2;

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

public void withFinal() {
    final String str1 = "str";
    final String str2 = "ing";
    String concat = str1 + str2;

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

and its decompiled with javap -c ClassWithTheseMethods versions you will see

  public void noFinal();
    Code:
       0: ldc           #15                 // String str
       2: astore_1      
       3: ldc           #17                 // String ing
       5: astore_2      
       6: new           #19                 // class java/lang/StringBuilder
       9: dup           
      10: aload_1       
      11: invokestatic  #21                 // Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
      14: invokespecial #27                 // Method java/lang/StringBuilder."":(Ljava/lang/String;)V
      17: aload_2       
      18: invokevirtual #30                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      21: invokevirtual #34                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      ...

and

  public void withFinal();
    Code:
       0: ldc           #15                 // String str
       2: astore_1      
       3: ldc           #17                 // String ing
       5: astore_2      
       6: ldc           #44                 // String string
       8: astore_3      
       ...

So if Strings are not final compiler will have to use StringBuilder to concatenate str1 and str2 so

String concat=str1+str2;

will be compiled to

String concat = new StringBuilder(str1).append(str2).toString();

which means that concat will be created at runtime so will not come from String pool.

---

Also if Strings are final then compiler can assume that they will never change so instead of using StringBuilder it can safely concatenate its values so

String concat = str1 + str2;

can be changed to

String concat = "str" + "ing";

and concatenated into

String concat = "string";

which means that concate will become sting literal which will be interned in string pool and then compared with same string literal from that pool in if statement.