读取文件代码如下：

package com.java.day23FileReader;

import java.io.FileReader;
import java.io.IOException;

public class RederFile {
    public static void main(String[] args) {
        readFile();
    }

    public static void readFile() {
        FileReader fr = null;
        try {
            fr = new FileReader("D:\\IdeaProjects\\StudyJava\\day01\\src\\com\\java\\day23FileReader\\test01");
            int len = 0;
            while ((len = fr.read()) != -1) {
                System.out.print((char) len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fr != null) {
                try {
                    fr.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

效果如图：

 

 写入文件代码如下：

package com.java.day25FileWrite;

import java.io.FileWriter;
import java.io.IOException;

public class WriteFile {
    public static void main(String[] args) {
        fileWrite();
    }

    public static void fileWrite() {
        FileWriter fw = null;
        try {
            fw = new FileWriter("D:\\IdeaProjects\\StudyJava\\day01\\src\\com\\java\\day25FileWrite\\test01.txt",true);
            fw.write("Hello世界"+"\r\n");
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fw != null) {
                try {
                    fw.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

运行效果如图：