[Java学习]在for循环外与循环内创建对象的区别

最近无意间将创建对象的操作放在外面，发现结果与预期不符。上网搜索了解如下：

创建的对象为全局变量，当进行for循环遍历时，他会不断往sb里添加内容。（我创建的是StringBuilder对象sb）

创建的对象为局部变量，此时每一次for循环均会创建新的变量sb，每一次内容也就不同。

就是在循环外创建对象会不断向那个对象添加内容

在for循环外创建对象

package com.itheima_集合到文件改进;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;

public class ArraylistToFileDemo {
    public static void main(String[] args) throws IOException {
        //创建ArrayList集合
        ArrayList array = new ArrayList();

        //创建学生对象
        Student s1 = new Student("itheima001", "林青霞", 30, "西安");
        Student s2 = new Student("itheima002", "赵曼玉", 35, "武汉");
        Student s3 = new Student("itheima003", "胡洁", 33, "安徽");

        //把学生添加到集合
        array.add(s1);
        array.add(s2);
        array.add(s3);


        //创建bufferedwriter对象
        BufferedWriter bw = new BufferedWriter(new FileWriter("itheima_study\src\com\itheima_集合到文件改进\ArrayToFile.txt"));
//        得到字符串写入

        for (Student s : array) {
            StringBuilder sb = new StringBuilder();
//            System.out.println(s.getAddress());
            sb.append(s.getSid()).append(",").append(s.getName()).append(",").append(s.getAge()).append(",").append(s.getAddress());
            //通过BufferWriter输出操作
            bw.write(sb.toString());
            bw.newline();
            bw.flush();
        }
        bw.close();

    }
}

 在for循环内创建对象：

package com.itheima_集合到文件改进;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;

public class ArraylistToFileDemo {
    public static void main(String[] args) throws IOException {
        //创建ArrayList集合
        ArrayList array = new ArrayList();

        //创建学生对象
        Student s1 = new Student("itheima001", "林青霞", 30, "西安");
        Student s2 = new Student("itheima002", "赵曼玉", 35, "武汉");
        Student s3 = new Student("itheima003", "胡洁", 33, "安徽");

        //把学生添加到集合
        array.add(s1);
        array.add(s2);
        array.add(s3);


        //创建bufferedwriter对象
        BufferedWriter bw = new BufferedWriter(new FileWriter("itheima_study\src\com\itheima_集合到文件改进\ArrayToFile.txt"));
//        得到字符串写入

        for (Student s : array) {
            StringBuilder sb = new StringBuilder();
//            System.out.println(s.getAddress());
            sb.append(s.getSid()).append(",").append(s.getName()).append(",").append(s.getAge()).append(",").append(s.getAddress());
            //通过BufferWriter输出操作
            bw.write(sb.toString());
            bw.newline();
            bw.flush();
        }
        bw.close();

    }
}