StringBuilder vs String concatenation in toString() in Java

Question

Given the 2 toString() implementations below, which one is preferred:

public String toString(){
    return \"{a:\"+ a + \", b:\" + b + \", c: \"         


        

Answer 1

Using latest version of Java(1.8) the disassembly(javap -c) shows the optimization introduced by compiler. + as well sb.append() will generate very similar code. However, it will be worthwhile inspecting the behaviour if we are using + in a for loop.

Adding strings using + in a for loop

Java:

public String myCatPlus(String[] vals) {
    String result = "";
    for (String val : vals) {
        result = result + val;
    }
    return result;
}

ByteCode:(for loop excerpt)

12: iload         5
14: iload         4
16: if_icmpge     51
19: aload_3
20: iload         5
22: aaload
23: astore        6
25: new           #3                  // class java/lang/StringBuilder
28: dup
29: invokespecial #4                  // Method java/lang/StringBuilder."":()V
32: aload_2
33: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
36: aload         6
38: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
41: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
44: astore_2
45: iinc          5, 1
48: goto          12

Adding strings using stringbuilder.append

Java:

public String myCatSb(String[] vals) {
    StringBuilder sb = new StringBuilder();
    for(String val : vals) {
        sb.append(val);
    }
    return sb.toString();
}

ByteCdoe:(for loop excerpt)

17: iload         5
19: iload         4
21: if_icmpge     43
24: aload_3
25: iload         5
27: aaload
28: astore        6
30: aload_2
31: aload         6
33: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
36: pop
37: iinc          5, 1
40: goto          17
43: aload_2

There is a bit of glaring difference though. In first case, where + was used, new StringBuilder is created for each for loop iteration and generated result is stored by doing a toString() call(29 through 41). So you are generating intermediate Strings that your really do not need while using + operator in for loop.