StringBuilder vs. .concat vs. “+” Operator relative performance different in eclipse than command line?

Question

I was reading about how when possible the java compiler will compile strings concatenated with the \"+\" operator into instances of StringBuilder, and how this makes it bett

Answer 1

On Oracle JDK 1.7 (javac 1.7.0_17), the "+" operator is still implemented using StringBuilder, as shown by running javap -c on the class to get the bytecode (only showing the loops here):

public static void plusOperator();
Code:

  16: iload         4
  18: ldc           #10                 // int 100000000
  20: if_icmpge     53
  23: new           #11                 // class java/lang/StringBuilder
  26: dup           
  27: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V
  30: aload_0       
  31: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  34: aload_1       
  35: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  38: aload_2       
  39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  42: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  45: astore        5
  47: iinc          4, 1
  50: goto          16


public static void stringBuilder();
Code:

  16: iload         4
  18: ldc           #10                 // int 100000000
  20: if_icmpge     50
  23: new           #11                 // class java/lang/StringBuilder
  26: dup           
  27: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V
  30: aload_0       
  31: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  34: aload_1       
  35: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  38: aload_2       
  39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  42: astore        5
  44: iinc          4, 1
  47: goto          16

The only difference between these two is that the version with "+" converts the StringBuilder to a String within the loop.

So the question becomes: why does your test show such different results for the same code. Or more completely, why is this not a valid micro-benchmark. Here are some possible reasons:

• You're counting wall-clock time. This means that you're actually measuring everything that the JVM is doing while running your test. Which includes garbage collection (which is important because you're creating a lot of garbage). You can mitigate this by getting the thread CPU time.
• You don't verify when or if HotSpot is compiling the methods. This is why you should do a warmup phase before any micro-benchmarks: basically, run your main() multiple times, before you run your actual test.

Answer 2

Try to place StringBuilder toBuild = new StringBuilder() above the loop. The same with String toPrint and do += for string and you will see the difference.
Don't create new String and StringBuilder in the loop.