备忘录模式——实现撤销操作

思路

IMG_20200307_152700.png

image.png

Editor

/*
带撤销功能的文字编辑器——编辑器类
 */
public class Editor {
    //保存当前内容
    private String content;
    //当使用这个方法时，编辑器会以一个编辑器状态对象保存现在的状态，并返回它
    public EditorState createState(){
        return new EditorState(content);
    }

    public void restore(EditorState state){
        content = state.getContent();
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

}
/*
    //1.（撤销一次）添加一个新的字段，叫prevContent，每次我们要修改content的值的时候，我们先把当
前的值保存到prevContent，然后再修改content private String prevContent;
    //2.（撤销多次）List类型——不易扩展
    private List prevContents;
 */

EditorState

public class EditorState {
    private final String content;

    public EditorState(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }
}

History

import java.util.ArrayList;
import java.util.List;

public class History {
    //这个尖括号中表示我们想要保存的数据类型
    private List<EditorState> states = new ArrayList<>();

    //存入
    public void push(EditorState state){
        states.add(state);
    }

    //取出
    public EditorState pop(){
        var lastIndex = states.size() - 1;
        var lastState = states.get(lastIndex);
        states.remove(lastState);

        return lastState;
    }
}

Main

public class Main {
    public static void main(String[] args) {
        var editor = new Editor();
        var history = new History();

        editor.setContent("a");
        history.push(editor.createState());

        editor.setContent("b");
        history.push(editor.createState());
        editor.setContent("c");
        editor.restore(history.pop());
        editor.restore(history.pop());

        String content = editor.getContent();
        System.out.println(content);
    }
}