String memory allocation using + operator

Question

String a = "abc";
String b = "xyz";
String result = a + b;

I was wondering if "result" string is a String constant allocated memory in string pool or a new object created on heap.

I know that new String() creates object on heap and String constants like a,b in the above example in permgen string pool space.

Answer 1

An important note:

String a = "abc";
String b = "xyz";
String result = a + b;

is the same as

// creates a number of objects.
String result = new StringBuilder().append(a).append(b).toString();

but

final String a = "abc";
final String b = "xyz";
String result = a + b;

is the same as

String result = "abcxyz"; // creates no new objects.

Answer 2

如果你编译和反编译你的代码，它会给出以下结果：

String result = new StringBuilder().append(a).append(b).toString();

Answer 3

The concatenation results in the allocation of a StringBuilder to create the concatenated string.

Source:

public class Hello {

    public static final String CONST1 = "cafe";
    public static final String CONST2 = "babe";

    public static void main(String[] args){
        String a = "abc";
        String b = "xyz";
        String result = a + b;

        String result2 = CONST1 + CONST2;
    }
}

Disassembled via javap:

public class Hello extends java.lang.Object{
public static final java.lang.String CONST1;

public static final java.lang.String CONST2;

public Hello();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   ldc     #2; //String abc
   2:   astore_1
   3:   ldc     #3; //String xyz
   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:  ldc     #8; //String cafebabe
   27:  astore  4
   29:  return

}

You can see the StringBuilder allocation at line 10 for concatenating String a and b . Notice the concatenation of CONST1 and CONST2 is processed by the compiler at line 25. So if your Strings are final it will not result in a StringBuilder allocation