生成随机数的效率问题

这里有段代码

import java.io.File;
import java.io.FileOutputStream;
import java.util.Random;

public class GenerateFile {
    private String filePath;
    private long fileSize;
    public GenerateFile(String filePath, long fileSize) {
        this.filePath = filePath;
        this.fileSize = fileSize;
    }
    
    public long generate() {
        int[] number = { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 };// 0~9的unicode码
        File file = new File(filePath);
        FileOutputStream out = null;
        byte[] enter = { 10 };// 回车
        Random r = new Random();
        long beginTime = System.currentTimeMillis();
        try {
            out = new FileOutputStream(file);
            while (file.length() < fileSize) {// 生成n行数字
                int length = (int) (Math.random() * 8) + 1;/* 长度为0~9 */
                boolean sign = (int) (Math.random() * 2) >= 1;// 随机写入正负数

                if (!sign) {
                    out.write(45);// 写入负号
                }
                out.write(number[(int) (Math.random() * 9) + 1]);// 第一个不能是0
                for (int j = 1; j < length; ++j) {
                    out.write(number[(int) (Math.random() * 10)]);/* 生成数字 */
                }
                out.write(enter);
            }
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("##########" + (endTime-beginTime) / 1000 + "s");
        return endTime - beginTime;
    }
}

import java.io.File;
import java.io.FileOutputStream;
import java.util.Random;

public class GenerateFile {
    private String filePath;
    private long fileSize;
    public GenerateFile(String filePath, long fileSize) {
        this.filePath = filePath;
        this.fileSize = fileSize;
    }
    
    public long generate() {
        int[] number = { 48, 49, 50, 51, 52, 53, 54, 55, 56, 57 };// 0~9的unicode码
        File file = new File(filePath);
        FileOutputStream out = null;
        byte[] enter = { 10 };// 回车
        Random r = new Random();
        long beginTime = System.currentTimeMillis();
        try {
            out = new FileOutputStream(file);
            while (file.length() < fileSize) {// 生成n行数字
                int length = (int) (Math.random() * 8) + 1;/* 长度为0~9 */
                out.write(length);
                out.write(enter);
            }
            out.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("##########" + (endTime-beginTime) / 1000 + "s");
        return endTime - beginTime;
    }
}

调用：

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

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