根據對象的內部狀態變化生成相應快照，以便以後恢復。

下面舉一個使用筆記軟件的歷史版本控制的例子來實現備忘錄模式。

原始類

public class Note {
    private String title;
    private String content;
    private Long createTime;
    private Long updateTime;
    private Integer version;

    public Note(String title) {
        this.title = title;
        this.createTime = System.currentTimeMillis();
        this.version = 0;
    }

    /**
     * 更新筆記
     * @param title
     * @param content
     */
    public void update(String title,String content){
        NoteMemento memento = new NoteMemento(this.title,this.content,this.version);
        NoteStack.push(memento);
        if (title != null)this.title = title;
        if (content != null)this.content = content;
        this.updateTime = System.currentTimeMillis();
        this.version++;
    }

    /**
     * 返回最近更改記錄
     * @return
     */
    public List<NoteMemento> history(){
        return NoteStack.history();
    }

    /**
     * 恢復上一個版本的信息
     */
    public void restore(){
        NoteMemento memento = NoteStack.lastVersion();
        this.title = memento.getTitle();
        this.content = memento.getContent();
        this.updateTime = System.currentTimeMillis();
        this.version++;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

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

    public Long getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Long createTime) {
        this.createTime = createTime;
    }

    public Long getUpdateTime() {
        return updateTime;
    }

    public void setUpdateTime(Long updateTime) {
        this.updateTime = updateTime;
    }

    public Integer getVersion() {
        return version;
    }

    public void setVersion(Integer version) {
        this.version = version;
    }

    @Override
    public String toString() {
        return "Note{" +
                "title='" + title + '\'' +
                ", content='" + content + '\'' +
                ", createTime=" + createTime +
                ", updateTime=" + updateTime +
                ", version=" + version +
                '}';
    }
}

備忘錄類（舊版本類）

public class NoteMemento {
    private String title;
    private String content;
    private Integer version;

    public NoteMemento(String title, String content, Integer version) {
        this.title = title;
        this.content = content;
        this.version = version;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

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

    public Integer getVersion() {
        return version;
    }

    public void setVersion(Integer version) {
        this.version = version;
    }

    @Override
    public String toString() {
        return "NoteMemento{" +
                "title='" + title + '\'' +
                ", content='" + content + '\'' +
                ", version=" + version +
                '}';
    }
}

版本存儲類

public class NoteStack {
    private static Stack<NoteMemento> noteMementos = new Stack<>();
    public static void push(NoteMemento memento){
        noteMementos.push(memento);
    }

    /**
     * 返回最近的5條歷史記錄
     * @return
     */
    public static List<NoteMemento> history(){
        return noteMementos.subList(noteMementos.size()>5?noteMementos.size()-5:0,noteMementos.size());
    }

    /**
     * 返回上一個版本
     * @return
     */
    public static NoteMemento lastVersion(){
        return noteMementos.lastElement();
    }
}

測試類

public class MemorandumTest {
    @Test
    public void test(){
        Note note = new Note("母豬護理從入門到精通");
        System.out.println(note);
        note.update(null,"好一大頭母豬，真難護理！！！！");
        note.update("母豬養護從入門到精通",null);
        System.out.println(note);
        System.out.println(note.history());
        note.update(null,"還要護理嗎它真的好大！");
        note.update(null,"不護理了，我去找頭公豬護理！");
        note.update(null,"啊！公豬也好大！");
        note.update(null,"我還是繼續護理母豬吧！");
        System.out.println(note);
        System.out.println(note.history());
        note.restore();
        System.out.println(note);
    }
}
=====測試結果=====
Note{title='母豬護理從入門到精通', content='null', createTime=1655912034992, updateTime=null, version=0}
Note{title='母豬養護從入門到精通', content='好一大頭母豬，真難護理！！！！', createTime=1655912034992, updateTime=1655912034994, version=2}
[NoteMemento{title='母豬護理從入門到精通', content='null', version=0}, NoteMemento{title='母豬護理從入門到精通', content='好一大頭母豬，真難護理！！！！', version=1}]
Note{title='母豬養護從入門到精通', content='我還是繼續護理母豬吧！', createTime=1655912034992, updateTime=1655912034996, version=6}
[NoteMemento{title='母豬護理從入門到精通', content='好一大頭母豬，真難護理！！！！', version=1}, NoteMemento{title='母豬養護從入門到精通', content='好一大頭母豬，真難護理！！！！', version=2}, NoteMemento{title='母豬養護從入門到精通', content='還要護理嗎它真的好大！', version=3}, NoteMemento{title='母豬養護從入門到精通', content='不護理了，我去找頭公豬護理！', version=4}, NoteMemento{title='母豬養護從入門到精通', content='啊！公豬也好大！', version=5}]
Note{title='母豬養護從入門到精通', content='啊！公豬也好大！', createTime=1655912034992, updateTime=1655912034996, version=7}