How many Strings does this create?
String test(String text) {
return "string 1 " +
text + " string 2 " +
"string 3";
}
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motivation? is this an interview question or what?leonbloy– leonbloy2011-05-06 13:36:46 +00:00Commented May 6, 2011 at 13:36
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3Which version of Java? I believe the compiler has been optimised over the years which changes the performance of string concatentation, and therefore the answer will differ.Codemwnci– Codemwnci2011-05-06 13:37:50 +00:00Commented May 6, 2011 at 13:37
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No it's not homework or an interview question, I came across this at work. But why does it matter? @codemwnci, it's 1.6Michael– Michael2011-05-06 13:43:34 +00:00Commented May 6, 2011 at 13:43
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1it helps the answerers frame their work, and it helps YOU get a better answer when your question is clear and shares context.Jeff Atwood– Jeff Atwood2011-05-07 10:44:14 +00:00Commented May 7, 2011 at 10:44
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4 Answers
No new String calls are present here, the only one that exists is in StringBuilder.toString, so
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
Comments
This is pretty easy to answer by compiling the code, then inspecting 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();
so depending on how you look at it, you might say only one, or if you're one of those people who likes to count literals as being "created" when they're used, you could say four.
answered May 6, 2011 at 13:41
Ernest Friedman-Hill
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1 Comment
MeBigFatGuy
and if you look at the compiled code you will see NO new String calls. (Only one that happens is in the StringBuilder.toString())
javapis your friend. I put your code into S.java and deassembled it using 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 there is only one StringBuilder and one String created.
Comments
this is the equivalent of
(new StringBuilder((new StringBuilder((new StringBuilder("String 1")).append(text).toString())).append("string 2 ").toString()).append("string 3").toString();
that's 3 stringbuilders and 6 strings. The way to optimize this is:
StringBuilder sb = new StringBuilder("string 1");
sb.append(test);
sb.append(" string 2 ");
sb.append("string 3");
return sb.toString();
4 Comments
MeBigFatGuy
i only count 1 string this method 'creates', the other strings are created when the class is loaded, not due to this method.
Ernest Friedman-Hill
this is completely untrue. Only one
StringBuilder is constructed, and append is called for each subsequent string. The code you've shown as being "optimized" is actually what the compiler emits. Check it out using javap -c.Ernest Friedman-Hill
@gabuzo: All JDKs have always done this; they've always been smart enough to use "append" for concatenation on a single line. It's concatenating in multiple statements in a loop that's always been the bugbear.
gabuzo
@Ernest yes I removed my comment because I just used jad with various -source -target parameters and look at the result with jad. It happens that jad gave different results when in pre 1.5 version using StringBuffer than in 1.5+ version using StringBuilder.