依据对象的外部状态变动生成相应快照，以便当前复原。
上面举一个应用笔记软件的历史版本控制的例子来实现备忘录模式。
原始类
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; } /** * 更新笔记* @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++; } /*** 返回最近更改记录* @return* / public List<NoteMemento> history(){ return NoteStack.history(); } /*** 复原上一个版本的信息* / 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 + '}'; }}备忘录类（旧版本类）
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 + '}'; }}版本存储类
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条历史记录* @return* / public static List<NoteMemento> history(){ return noteMementos.subList(noteMementos.size()>5?noteMementos.size()-5:0,noteMementos.size()); } /*** 返回上一个版本* @return* / public static NoteMemento lastVersion(){ return noteMementos.lastElement(); }}测试类
public class MemorandumTest { @Test public void test(){ Note note = new Note("母猪护理从入门到精通"); System.out.println(note); note.update(null,"好一大头母猪，真难护理！！！！"); note.update("母猪养护从入门到精通",null); System.out.println(note); System.out.println(note.history()); note.update(null,"还要护理吗它真的好大！"); note.update(null,"不护理了，我去找头公猪护理！"); note.update(null,"啊！公猪也好大！"); note.update(null,"我还是持续护理母猪吧！"); System.out.println(note); System.out.println(note.history()); note.restore(); System.out.println(note); }}=====测试后果=====Note{title='母猪护理从入门到精通', content='null', createTime=1655912034992, updateTime=null, version=0}Note{title='母猪养护从入门到精通', content='好一大头母猪，真难护理！！！！', createTime=1655912034992, updateTime=1655912034994, version=2}[NoteMemento{title='母猪护理从入门到精通', content='null', version=0}, NoteMemento{title='母猪护理从入门到精通', content='好一大头母猪，真难护理！！！！', version=1}]Note{title='母猪养护从入门到精通', content='我还是持续护理母猪吧！', createTime=1655912034992, updateTime=1655912034996, version=6}[NoteMemento{title='母猪护理从入门到精通', content='好一大头母猪，真难护理！！！！', version=1}, NoteMemento{title='母猪养护从入门到精通', content='好一大头母猪，真难护理！！！！', version=2}, NoteMemento{title='母猪养护从入门到精通', content='还要护理吗它真的好大！', version=3}, NoteMemento{title='母猪养护从入门到精通', content='不护理了，我去找头公猪护理！', version=4}, NoteMemento{title='母猪养护从入门到精通', content='啊！公猪也好大！', version=5}]Note{title='母猪养护从入门到精通', content='啊！公猪也好大！', createTime=1655912034992, updateTime=1655912034996, version=7}