How does + operator work in Java?

Question

I am providing some code below with its compiled class.

public class PlusOperator {

public static void main(String[] args) {

    int a=10;
    int b=20;
    int c =a+b;

    String s1 = 1+3+"f";
    String s2 = "f"+1+2;
    String s3 = (1+3)+"f";
    String s4 = "f"+(1+3);

}
}

Compiled class with Java 1.6

  // Compiled from PlusOperator.java (version 1.6 : 50.0, super bit)
public class question.PlusOperator {

  // Method descriptor #6 ()V
  // Stack: 1, Locals: 1
  public PlusOperator();
    0  aload_0 [this]
    1  invokespecial java.lang.Object() [8]
    4  return
      Line numbers:
        [pc: 0, line: 3]
      Local variable table:
        [pc: 0, pc: 5] local: this index: 0 type: question.PlusOperator

  // Method descriptor #15 ([Ljava/lang/String;)V
  // Stack: 2, Locals: 8
  public static void main(java.lang.String[] args);
     0  bipush 10
     2  istore_1 [a]
     3  bipush 20
     5  istore_2 [b]
     6  iload_1 [a]
     7  iload_2 [b]
     8  iadd
     9  istore_3 [c]
    10  ldc <String "4f"> [16]
    12  astore 4 [s1]
    14  ldc <String "f12"> [18]
    16  astore 5 [s2]
    18  ldc <String "4f"> [16]
    20  astore 6 [s3]
    22  ldc <String "f4"> [20]
    24  astore 7 [s4]
    26  return
      Line numbers:
        [pc: 0, line: 7]
        [pc: 3, line: 8]
        [pc: 6, line: 9]
        [pc: 10, line: 11]
        [pc: 14, line: 12]
        [pc: 18, line: 13]
        [pc: 22, line: 14]
        [pc: 26, line: 16]
      Local variable table:
        [pc: 0, pc: 27] local: args index: 0 type: java.lang.String[]
        [pc: 3, pc: 27] local: a index: 1 type: int
        [pc: 6, pc: 27] local: b index: 2 type: int
        [pc: 10, pc: 27] local: c index: 3 type: int
        [pc: 14, pc: 27] local: s1 index: 4 type: java.lang.String
        [pc: 18, pc: 27] local: s2 index: 5 type: java.lang.String
        [pc: 22, pc: 27] local: s3 index: 6 type: java.lang.String
        [pc: 26, pc: 27] local: s4 index: 7 type: java.lang.String
}

My Questions:

1. How + operator works with String and int?
2. If you see the compiled code then String "+" is evaluated at the time of compilation with int also but in case of "+" only with int is performed at runtime. Why?

Answer 1

a and b are variables -- their values are only known at runtime¹. However, 1, f and 3 are constants so the compiler is smart enough to compute the result and include it in the bytecode. So expressions like 3 + f are automatically compiled to the constant 3f.

This has nothing to do with the operands being of type int or String. Try the same with Strings:

String str = a + b;

and you'll see a call to StringBuilder#append(String) in the compiled code.

---

¹ An exception is when the variables are declared final, so in the following code:

final int a=10;
final int b=20;
int c = a + b;

c is directly assigned the value 30 in the bytecode:

bipush 10
istore_1
bipush 20
istore_2
bipush 30
istore_3