StringBuilder 沒有提供clear或empty方法。
清空有3種方法：
1）新生成一個，舊的由系統(tǒng)自己主動回收
2）使用delete
3）使用setLength

將三種方法循環(huán)1000萬次，代碼：

1.public class sbbm {

2.

3.??? static String a;

4.??? static long time ;

5.??? public static void main( String[] args ) throws Exception {

6.???????

7.???????? StringBuilder sb = new StringBuilder();

8.???????? StringBuilder sb3 = new StringBuilder();

9.????????

10.???????? time = System.currentTimeMillis();

11.????????? for( int i = 0; i < 10000000; i++ ) {

12.????????????? StringBuilder sb2 = new StringBuilder();

13.????????????? sb2.append( "someStr6ing" );

14.????????????? sb2.append( "someS5tring2" );

15.????????????? sb2.append( "some3Strin4g" );

16.????????????? sb2.append( "so3meStr5ing" );

17.????????????? sb2.append( "so2meSt7ring" );

18.????????????? a = sb2.toString();

19.????????? }

20.????????? System.out.println( "Way2="+(System.currentTimeMillis()-time) );

21.??????????

22.

23.??????? time = System.currentTimeMillis();

24.??????? for( int i = 0; i < 10000000; i++ ) {

25.??????????? sb.delete( 0, sb.length() );

26.??????????? sb.append( "someString" );

27.??????????? sb.append( "someString2" );

28.??????????? sb.append( "someStrin4g" );

29.??????????? sb.append( "someStr5ing" );

30.??????????? sb.append( "someSt7ring" );

31.??????????? a = sb.toString();

32.??????? }

33.??????? System.out.println( "Way1="+(System.currentTimeMillis()-time) );

34.

35.??????? time = System.currentTimeMillis();

36.??????? for( int i = 0; i < 10000000; i++ ) {

37.

38.??????????? sb3.setLength( 0 );

39.??????????? sb3.append( "someStr55ing" );

40.??????????? sb3.append( "some44String2" );

41.??????????? sb3.append( "som55eStrin4g" );

42.??????????? sb3.append( "some66Str5ing" );

43.??????????? sb3.append( "so33meSt7ring" );

44.??????????? a= sb3.toString() ;

45.??????? }

46.??????? System.out.println( "Way3="+(System.currentTimeMillis()-time) );

47.?

48.?????

49.??? }

50.}

注意append的字符串要都不同樣，否則會由于java 的String? pool對結果造成影響（即3好于2）

結果：

1.Way2=9438

2.Way1=6281

3.Way3=6469

能夠將各方法多反復幾次，點到順序，等
整體來看：方法2好于方法3好于方法1

轉載于:https://www.cnblogs.com/bhlsheji/p/4290917.html

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