使用System.setOut()

setOut()方法可以改变输出流

package java1200;

//可以向文件里写数据
import java.io.FileNotFoundException;
import java.io.PrintStream;

public class RedirectOutputStream {
    public static void main(String[] args) {
        try {
            PrintStream out = System.out;
            PrintStream ps = new PrintStream("./log.txt");

            System.setOut(ps);
            int age = 11;
            System.out.println("年龄变量成功定义，初始值为11");
            String sex = "女";
            System.out.println("年龄变量成功定义，初始值为女");
            // 整合2个变量
            String info = "这是个" + sex + "孩子，应该有" + age + "岁了";
            System.setOut(out);
            System.out.println("程序运行完毕，请查看日志");
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

    }

}

那么setIn()是不是可以改变输入流呢？

import java.io.InputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Read {
    public static void main(String[] args) {
        try {
            InputStream in = System.in;
            InputStream ps = new FileInputStream("./log.txt");
            System.setIn(ps);
            Scanner scanner = new Scanner(System.in);
            String line = "";
            while (scanner.hasNextLine()) {
                line = scanner.nextLine();
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

    }
}

哈哈 可以

那么以后就可以用setIn()把文件里面的内容读到程序里了