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1. 备忘录模式的原理与实现

• snapshot模式
  • Memento Design Pattern
  • Captures and externalizes an object’s internal state so that it can be restored later, all without violating encapsulation
  • 在不违背封装原则的前提下，捕获一个对象的内部状态，并在该对象之外保存这个状态，以便之后恢复对象先前的状态。
    • 存储副本以便后期恢复
    • 在不违背封装原则的前提下，进行对象的备份和恢复

    public class InputText {
      private StringBuilder text = new StringBuilder();

      public String getText() {
        return text.toString();
      }

      public void append(String input) {
        text.append(input);
      }

      public Snapshot createSnapshot() {
        return new Snapshot(text.toString());
      }

      public void restoreSnapshot(Snapshot snapshot) {
        this.text.replace(0, this.text.length(), snapshot.getText());
      }
    }

    public class Snapshot {
      private String text;

      public Snapshot(String text) {
        this.text = text;
      }

      public String getText() {
        return this.text;
      }
    }

    public class SnapshotHolder {
      private Stack<Snapshot> snapshots = new Stack<>();

      public Snapshot popSnapshot() {
        return snapshots.pop();
      }

      public void pushSnapshot(Snapshot snapshot) {
        snapshots.push(snapshot);
      }
    }

    public class ApplicationMain {
      public static void main(String[] args) {
        InputText inputText = new InputText();
        SnapshotHolder snapshotsHolder = new SnapshotHolder();
        Scanner scanner = new Scanner(System.in);
        while (scanner.hasNext()) {
          String input = scanner.next();
          if (input.equals(":list")) {
            System.out.println(inputText.toString());
          } else if (input.equals(":undo")) {
            Snapshot snapshot = snapshotsHolder.popSnapshot();
            inputText.restoreSnapshot(snapshot);
          } else {
            snapshotsHolder.pushSnapshot(inputText.createSnapshot());
            inputText.append(input);
          }
        }
      }
    }

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