Java String Optimization

Question

How many lines does this create?

String test(String text) { return "string 1 " + text + " string 2 " + "string 3"; } 

Answer 1

There are no new String calls here, the only one of which exists in StringBuilder.toString, therefore

1

 java.lang.String test(java.lang.String); Code: Stack=3, Locals=2, Args_size=2 0: new #16; //class java/lang/StringBuilder 3: dup 4: ldc #18; //String string 1 6: invokespecial #20; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 9: aload_1 10: invokevirtual #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 13: ldc #27; //String string 2 15: invokevirtual #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 18: ldc #29; //String string 3 20: invokevirtual #23; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 23: invokevirtual #31; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 26: areturn 

Answer 2

It is quite easy to answer by compiling the code and then checking the bytecode with javap -c . In this particular case, the compiler should generate code like

 String result = new StringBuilder("string1"). append(text).append("string2").append("string3").toString(); 

therefore, depending on how you look at it, you can say only one, or if you are one of those people who like to consider literals “created” when they are used, you can say four.

Answer 3

javap is your friend. I put your code in S.java and pulled it with javap -c S :

 Compiled from "S.java" public class S extends java.lang.Object{ public S(); Code: 0: aload_0 1: invokespecial #1; //Method java/lang/Object."<init>":()V 4: return public java.lang.String test(java.lang.String); Code: 0: new #2; //class java/lang/StringBuilder 3: dup 4: invokespecial #3; //Method java/lang/StringBuilder."<init>":()V 7: ldc #4; //String string 1 9: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 12: aload_1 13: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 16: ldc #6; //String string 2 18: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 21: ldc #7; //String string 3 23: invokevirtual #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 26: invokevirtual #8; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 29: areturn } 

With OpenJDK 1.6.0_22, only one StringBuilder and one string are created.

Answer 4

this is the equivalent

  (new StringBuilder((new StringBuilder((new StringBuilder("String 1")).append(text).toString())).append("string 2 ").toString()).append("string 3").toString(); 

that 3 string constructions and 6 lines. Optimization Method:

  StringBuilder sb = new StringBuilder("string 1"); sb.append(test); sb.append(" string 2 "); sb.append("string 3"); return sb.toString();