Is it better to reuse a StringBuilder in a loop?

Question

I've a performance related question regarding use of StringBuilder. In a very long loop I'm manipulating a StringBuilder and passing it to another method like this:

for (loop condition) {     StringBuilder sb = new StringBuilder();     sb.append("some string");     . . .     sb.append(anotherString);     . . .     passToMethod(sb.toString()); } 

Is instantiating StringBuilder at every loop cycle is a good solution? And is calling a delete instead better, like the following?

StringBuilder sb = new StringBuilder(); for (loop condition) {     sb.delete(0, sb.length);     sb.append("some string");     . . .     sb.append(anotherString);     . . .     passToMethod(sb.toString()); } 

Answer 1

The second one is about 25% faster in my mini-benchmark.

public class ScratchPad {      static String a;      public static void main( String[] args ) throws Exception {         long time = System.currentTimeMillis();         for( int i = 0; i < 10000000; i++ ) {             StringBuilder sb = new StringBuilder();             sb.append( "someString" );             sb.append( "someString2"+i );             sb.append( "someStrin4g"+i );             sb.append( "someStr5ing"+i );             sb.append( "someSt7ring"+i );             a = sb.toString();         }         System.out.println( System.currentTimeMillis()-time );         time = System.currentTimeMillis();         StringBuilder sb = new StringBuilder();         for( int i = 0; i < 10000000; i++ ) {             sb.delete( 0, sb.length() );             sb.append( "someString" );             sb.append( "someString2"+i );             sb.append( "someStrin4g"+i );             sb.append( "someStr5ing"+i );             sb.append( "someSt7ring"+i );             a = sb.toString();         }         System.out.println( System.currentTimeMillis()-time );     } } 

Results:

25265 17969 

Note that this is with JRE 1.6.0_07.

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Based on Jon Skeet's ideas in the edit, here's version 2. Same results though.

public class ScratchPad {      static String a;      public static void main( String[] args ) throws Exception {         long time = System.currentTimeMillis();         StringBuilder sb = new StringBuilder();         for( int i = 0; i < 10000000; i++ ) {             sb.delete( 0, sb.length() );             sb.append( "someString" );             sb.append( "someString2" );             sb.append( "someStrin4g" );             sb.append( "someStr5ing" );             sb.append( "someSt7ring" );             a = sb.toString();         }         System.out.println( System.currentTimeMillis()-time );         time = System.currentTimeMillis();         for( int i = 0; i < 10000000; i++ ) {             StringBuilder sb2 = new StringBuilder();             sb2.append( "someString" );             sb2.append( "someString2" );             sb2.append( "someStrin4g" );             sb2.append( "someStr5ing" );             sb2.append( "someSt7ring" );             a = sb2.toString();         }         System.out.println( System.currentTimeMillis()-time );     } } 

Results:

5016 7516