Java面向对象

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封装：是指一种将类实现细节部分包装，隐藏起来的方法。只暴露使用的方法。
低耦合是暴露的信息较少，解耦相对简单。

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继承：可以抽离公共部分，进行代码的简化。子类对父类进行拓展，拓展可以是新创建也可以是对原有的修改。

package com.company;

import java.io.*;

public class File {
    private String file;

    public File(String file) {
        this.file = file;
    }

    public String readToString() throws IOException {
        CharArrayWriter writer = new CharArrayWriter();
        InputStreamReader reader =
                new InputStreamReader(
                        new BufferedInputStream(
                                new FileInputStream(this.file)), "UTF-8");
        char[] charArr = new char[512];
        int readChars;
        String ret;

        try {
            while ((readChars = reader.read(charArr)) != -1) {
                writer.write(charArr, 0, readChars);
            }
            ret = writer.toString();
        }finally {
            reader.close();
            writer.close();
        }

        return ret;
    }
    public void writeString(String content)throws IOException{
        PrintWriter writer = new PrintWriter(this.file);
        try {
            writer.print(content);
        }finally {
            writer.close();
        }
    }
}

如上代码，主要暴露给用户使用的接口，不暴露内部具体的实现。

package com.company;

public class News {
    protected String title;  //拓展子类功能，使用protected
    protected String content;

    public News(String title,String content){
        this.title = title;
        this.content = content;
    }

    protected News () {

    }
    /* 阻止改动title
    public void setTitle(String title) {
        this.title = title;
    }
    */
    public String getTitle() {
        return title;
    }

    public String getContent() {
        return content;
    }

    //控制显示
    public String display(){
        return title + "\n" + content;
    }
}

package com.company;

import java.io.*;
import java.io.File;

public class FileNews extends News {
    public FileNews(String title, String content) {
        super(title, content);
    }

    public FileNews() {
    }

    public void read(String url) {
        try {
            BufferedReader reader = new BufferedReader(new FileReader(new File(url)));
            title = reader.readLine();  //读取title
            reader.readLine(); // 跳过空行
            content = reader.readLine(); //读取content
        } catch (java.io.IOException e) {
            System.out.println("出错");
        }
    }
}

以上代码，子类继承父类，并可对功能进行了拓展。