生成随机数的效率问题

这里有段代码

 1 import java.io.File;
 2 import java.io.FileOutputStream;
 3 import java.util.Random;
 4 
 5 public class GenerateFile {
 6     private String filePath;
 7     private long fileSize;
 8     public GenerateFile(String filePath, long fileSize) {
 9         this.filePath = filePath;
10         this.fileSize = fileSize;
11     }
12     
13     public long generate() {
14         int[] number = { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 };// 0~9的unicode码
15         File file = new File(filePath);
16         FileOutputStream out = null;
17         byte[] enter = { 10 };// 回车
18         Random r = new Random();
19         long beginTime = System.currentTimeMillis();
20         try {
21             out = new FileOutputStream(file);
22             while (file.length() < fileSize) {// 生成n行数字
23                 int length = (int) (Math.random() * 8) + 1;/* 长度为0~9 */
24                 boolean sign = (int) (Math.random() * 2) >= 1;// 随机写入正负数
25 
26                 if (!sign) {
27                     out.write(45);// 写入负号
28                 }
29                 out.write(number[(int) (Math.random() * 9) + 1]);// 第一个不能是0
30                 for (int j = 1; j < length; ++j) {
31                     out.write(number[(int) (Math.random() * 10)]);/* 生成数字 */
32                 }
33                 out.write(enter);
34             }
35             out.close();
36         } catch (Exception e) {
37             e.printStackTrace();
38         }
39         long endTime = System.currentTimeMillis();
40         System.out.println("##########" + (endTime-beginTime) / 1000 + "s");
41         return endTime - beginTime;
42     }
43 }

 1 import java.io.File;
 2 import java.io.FileOutputStream;
 3 import java.util.Random;
 4 
 5 public class GenerateFile {
 6     private String filePath;
 7     private long fileSize;
 8     public GenerateFile(String filePath, long fileSize) {
 9         this.filePath = filePath;
10         this.fileSize = fileSize;
11     }
12     
13     public long generate() {
14         int[] number = { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 };// 0~9的unicode码
15         File file = new File(filePath);
16         FileOutputStream out = null;
17         byte[] enter = { 10 };// 回车
18         Random r = new Random();
19         long beginTime = System.currentTimeMillis();
20         try {
21             out = new FileOutputStream(file);
22             while (file.length() < fileSize) {// 生成n行数字
23                 int length = (int) (Math.random() * 8) + 1;/* 长度为0~9 */
24                 out.write(length);
25                 out.write(enter);
26             }
27             out.close();
28         } catch (Exception e) {
29             e.printStackTrace();
30         }
31         long endTime = System.currentTimeMillis();
32         System.out.println("##########" + (endTime-beginTime) / 1000 + "s");
33         return endTime - beginTime;
34     }
35 }

调用：

1 public static void main(String[] args) {
2         GenerateFile gen = new GenerateFile("c:\test\numbers.txt", 1024*1000);
3         
4         gen.generate();
5     }

不知道为什么前面代码的效率比后面代码的效率要高，记录一下