string - Differences between "abc" + "de" and "abc" + de (de = "de") in Java?

I run the following code and get the results shown in the comments. I know the differences between == and .equals(). What I don't understand is why my code in the second line has different results from the code in the third line.

    String de = "de";
//  String abcde = "abc" + "de"; // abcde == "abcde" reture true
    String abcde = "abc" + de;   // abcde == "abcde" reture false;
    System.out.println();

    System.out.println(abcde=="abcde");
    System.out.println(de=="de");

In trying to debug this I used the javap -c command and got the following output 'code' for the first string concatenation:

         Code:
 0:   ldc     #9; //String de
 2:   astore_1
 3:   new     #10; //class java/lang/StringBuilder

 6:   dup
 7:   invokespecial   #11; //Method java/lang/StringBuilder."":()V
 10:  ldc     #4; //String abc
 12:  invokevirtual   #12; //Method java/lang/StringBuilder.append:(Ljava/lang
 String;)Ljava/lang/StringBuilder;
 15:  aload_1
 16:  invokevirtual   #12; //Method java/lang/StringBuilder.append:(Ljava/lang
 String;)Ljava/lang/StringBuilder;
   19:  invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/l
 ng/String;

   22:  astore_2
   23:  getstatic       #14; //Field java/lang/System.out:Ljava/io/PrintStream;
   26:  invokevirtual   #15; //Method java/io/PrintStream.println:()V
   29:  getstatic       #14; //Field java/lang/System.out:Ljava/io/PrintStream;
   32:  aload_2
   33:  ldc     #16; //String abcde
   35:  if_acmpne       42
   38:  iconst_1
   39:  goto    43
   42:  iconst_0

   43:  invokevirtual   #17; //Method java/io/PrintStream.println:(Z)V
   46:  getstatic       #14; //Field java/lang/System.out:Ljava/io/PrintStream;
   49:  aload_1
   50:  ldc     #9; //String de
   52:  if_acmpne       59
   55:  iconst_1
   56:  goto    60
   59:  iconst_0
   60:  invokevirtual   #17; //Method java/io/PrintStream.println:(Z)V
   63:  return

And the output for the second string concatenation:

I am not so familiar with this 'code' and can't see any reason why these differences exists. So could anyone explain why those differences happen?

Related post

Answer

The "problem" is simply that the compiler is too smart for you. When it sees "abc" + "de" it immediately concatenates that into the literal "abcde". But when it sees "abc" + de it's not allowed (per Java rules) to "optimize" that to a literal but must instead implement the + function, creating a new String object.

String literals are always handled as interned Strings, so == will work on them.