代码
package com.eblly;
import org.junit.Test;
import java.math.BigDecimal;
/**
* Created by eblly on 2017/5/11.
*/
public class StringTs {
@Test
public void test2() {
long startLong = System.currentTimeMillis();
String str = new String();
for (int i = 0; i <= 10000; i++) {
str = str + "3333";
}
System.out.println(System.currentTimeMillis() - startLong); // 403
//==================================================================
startLong = System.currentTimeMillis();
String str2 = new String();
for (int i = 0; i <= 10000; i++) {
StringBuilder stringBuilder = new StringBuilder();
String tmpStr = "2222";
stringBuilder.append(str2).append(tmpStr);
str2 = stringBuilder.toString();
}
System.out.println(System.currentTimeMillis() - startLong); // 337
//==================================================================
startLong = System.currentTimeMillis();
StringBuffer strBuff = new StringBuffer();
for (int i = 0; i <= 10000; i++) {
strBuff.append("4444");
}
System.out.println(System.currentTimeMillis() - startLong); // 1
//==================================================================
startLong = System.currentTimeMillis();
StringBuilder strBuilder = new StringBuilder();
for (int i = 0; i <= 10000; i++) {
strBuilder.append("5555");
}
System.out.println(System.currentTimeMillis() - startLong); // 1
// System.out.println(str.length());
// System.out.println(str2.length());
System.out.println();
}
}
运行结果:
388
310
0
1
3333和2222和运行速度差不多。
使用javap -c反编译查看代码。
Compiled from "StringTs.java"
public class com.eblly.StringTs {
public com.eblly.StringTs();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
public void test2();
Code:
0: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
3: lstore_1
4: new #3 // class java/lang/String
7: dup
8: invokespecial #4 // Method java/lang/String."<init>":()V
11: astore_3
12: iconst_0
13: istore 4
15: iload 4
17: sipush 10000
20: if_icmpgt 49
23: new #5 // class java/lang/StringBuilder
26: dup
27: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
30: aload_3
31: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
34: ldc #8 // String 3333
36: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
39: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
42: astore_3
43: iinc 4, 1
46: goto 15
49: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
52: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
55: lload_1
56: lsub
57: invokevirtual #11 // Method java/io/PrintStream.println:(J)V
60: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
63: lstore_1
64: new #3 // class java/lang/String
67: dup
68: invokespecial #4 // Method java/lang/String."<init>":()V
71: astore 4
73: iconst_0
74: istore 5
76: iload 5
78: sipush 10000
81: if_icmpgt 123
84: new #5 // class java/lang/StringBuilder
87: dup
88: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
91: astore 6
93: ldc #12 // String 2222
95: astore 7
97: aload 6
99: aload 4
101: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
104: aload 7
106: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
109: pop
110: aload 6
112: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
115: astore 4
117: iinc 5, 1
120: goto 76
123: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
126: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
129: lload_1
130: lsub
131: invokevirtual #11 // Method java/io/PrintStream.println:(J)V
134: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
137: lstore_1
138: new #13 // class java/lang/StringBuffer
141: dup
142: invokespecial #14 // Method java/lang/StringBuffer."<init>":()V
145: astore 5
147: iconst_0
148: istore 6
150: iload 6
152: sipush 10000
155: if_icmpgt 172
158: aload 5
160: ldc #15 // String 4444
162: invokevirtual #16 // Method java/lang/StringBuffer.append:(Ljava/lang/String;)Ljava/lang/StringBuffer;
165: pop
166: iinc 6, 1
169: goto 150
172: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
175: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
178: lload_1
179: lsub
180: invokevirtual #11 // Method java/io/PrintStream.println:(J)V
183: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
186: lstore_1
187: new #5 // class java/lang/StringBuilder
190: dup
191: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
194: astore 6
196: iconst_0
197: istore 7
199: iload 7
201: sipush 10000
204: if_icmpgt 221
207: aload 6
209: ldc #17 // String 5555
211: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
214: pop
215: iinc 7, 1
218: goto 199
221: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
224: invokestatic #2 // Method java/lang/System.currentTimeMillis:()J
227: lload_1
228: lsub
229: invokevirtual #11 // Method java/io/PrintStream.println:(J)V
232: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream;
235: invokevirtual #18 // Method java/io/PrintStream.println:()V
238: return
}
其中
17: sipush 10000
20: if_icmpgt 49
23: new #5 // class java/lang/StringBuilder
26: dup
27: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
30: aload_3
31: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
34: ldc #8 // String 3333
36: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
39: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
42: astore_3
和
78: sipush 10000
81: if_icmpgt 123
84: new #5 // class java/lang/StringBuilder
87: dup
88: invokespecial #6 // Method java/lang/StringBuilder."<init>":()V
91: astore 6
93: ldc #12 // String 2222
95: astore 7
97: aload 6
99: aload 4
101: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
104: aload 7
106: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
109: pop
110: aload 6
112: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
115: astore 4
类似。
只不过后者重排序的结果不一样，运行效率相近。String str += "3333";会先创建StringBuilder再使用append()拼接字符串，之后再toString()返回给String;如果String需要拼接多次，则会创建多个StringBuilder,调用多次toString()。因此效率比较低。
StringBuffer和StringBuilder效率相近。
