JavaIO RandomAccessFile 借助ArrayList方便的操作文件

小鱼儿 2022-05-25 00:47 143阅读 0赞

上一篇中，介绍了RandomAccessFile类，RandomAccessFile类的API,以及通过RandomAccessFile对文件进行增删改查(改没完成)

同时发现了借助数据结构我们能更好的操作文件，把对文件的操作转换成对集合(ArrayList)的操作。把对文件的增删改查转换成对集合(ArrayList)的增删改查。这样就比较简单了。

这里来介绍修改的方法。查询的话直接查询ArrayList即可

学生类：

import java.io.IOException;
    import java.io.RandomAccessFile;
    
    public class Student
    {
    	String id;// 学号
    	String name;// 名字
    	String sex;// 性别
    	int age;// 年龄
    	public Student()
    	{
    	}
    	public Student(String id, String name, String sex, int age)
    	{
    		this.id = id;
    		this.name = name;
    		this.sex = sex;
    		this.age = age;
    	}
    	public void writeStudent(RandomAccessFile raf) throws IOException
    	{
    		raf.writeUTF(this.id);
    		raf.writeUTF(this.name);
    		raf.writeUTF(this.sex);
    		raf.writeInt(this.age);
    	}
    	public void readStudent(RandomAccessFile raf) throws IOException
    	{
    		this.id = raf.readUTF();
    		this.name = raf.readUTF();
    		this.sex = raf.readUTF();
    		this.age = raf.readInt();
    	}
    	@Override
    	public String toString()
    	{
    		return "Student [id=" + id + ", name=" + name + ", sex=" + sex
    				+ ", age=" + age + "]";
    	}
    	/**
    	 * 获取到第n条学生记录
    	 */
    	public Student getTheNthStudent(RandomAccessFile raf, int n)
    			throws IOException
    	{
    		if (n < 1)
    		{
    			System.out.println("n要大于1");
    			return null;
    		}
    
    		Student student = new Student();
    		raf.seek(0);
    		n--;
    		while (n >= 0)
    		{
    			student.readStudent(raf);// 读取一条记录
    			n--;
    		}
    		return student;
    	}
    }

工具类：

import java.io.File;
    import java.io.IOException;
    import java.io.RandomAccessFile;
    import java.util.ArrayList;
    
    //文件列表
    public class StudentArrayList
    {
    	static ArrayList<Student> stuList=new ArrayList<Student>();
    	public static void initializeList() throws IOException
    	{
    		File file =new File(".\\text2.txt");
    		//以只读的方式打开文件
    		RandomAccessFile raf=new RandomAccessFile(file,"r");
    		while(raf.getFilePointer()<raf.length())
    		{
    			Student student=new Student();
    			student.readStudent(raf);//从文件中读取一条记录
    			stuList.add(student);//把这条记录生成的对象添加到集合中去。
    		}
    		raf.close();//关闭文件
    	}
    	public static void writeList(RandomAccessFile raf) throws IOException
    	{
    		//把集合中的所有Student对象写入到文件中
    		for (Student student : stuList)
    		{
    			student.writeStudent(raf);
    		}
    	}
    	//显示List中的所有学生
    	public static void showAllStudent()
    	{
    		for (Student student : stuList)
    		{
    			System.out.println(student);
    		}
    	}
    	public static void writeChange() throws IOException
    	{
    		RandomAccessFile raf=new RandomAccessFile(new File(".\\text2.txt"), "rw");
    		raf.setLength(0);
    		writeList(raf);
    		raf.close();
    	}
    }

测试：

import java.io.IOException;
    
    public class ModifyRAF
    {
    	
    	public static void main(String[] args) throws IOException
    	{
    		StudentArrayList.initializeList();//初始化ArrayList
    		StudentArrayList.showAllStudent();
    		Student student = new Student();
    		//获取第2条记录
    		student = StudentArrayList.stuList.get(2-1);
    		student.id="B2000";
    		student.name="小红";
    		student.sex="女";
    		student.age=21;
    		StudentArrayList.stuList.set(2-1, student);
    		System.out.println("--------------------------------");
    		StudentArrayList.showAllStudent();
    		StudentArrayList.writeChange();//更新记录文件
    	}
    }

运行结果：

Student [id=B10005, name=小蓝, sex=男, age=19]
    Student [id=B10005, name=小蓝, sex=男, age=19]
    Student [id=B1000, name=小明名名, sex=男, age=20]
    Student [id=B1001, name=小黄, sex=男, age=20]
    --------------------------------
    Student [id=B10005, name=小蓝, sex=男, age=19]
    Student [id=B2000, name=小红, sex=女, age=21]
    Student [id=B1000, name=小明名名, sex=男, age=20]
    Student [id=B1001, name=小黄, sex=男, age=20]