How to prevent to change element of list in Java-CodePudding

I have Snapshot class. Inside class I have restore method which needs to restore previous values of list. How I can prevent list element to change values..

Blockquotepackage package_1;

import java.util.ArrayList;

public class Snapshot {

// Modify the implementation of the Snapshot class so that an ArrayList stored
// in the snapshot is not affected by modifications to either the original or restored ArrayList.

private ArrayList<Integer> data;


public Snapshot(ArrayList<Integer> data) {
    this.data = data;
    

}
public ArrayList<Integer> restore() {
    return this.data;
}

public static void main(String[] args) {

    ArrayList<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);

    Snapshot snap = new Snapshot(list);

    list.set(0, 3);

    list = snap.restore();
    System.out.println(list); //It should log "[1,2]"
    list.add(4);
    list = snap.restore();
    System.out.println(list); //It should log "[1,2]"

}

}

Blockquote

CodePudding user response：

You should read about Immutable Objects pattern https://docs.oracle.com/javase/tutorial/essential/concurrency/imstrat.html

To make your Snapshot class work, it should:

Make sure your list item is an Immutable class. Integer is the Immutable class.
Copy all original list items to new list (snapshot data)
Do not expose directly snapshot data reference to outside by return copying items instead

Source code:

public final class Snapshot {

   private final List<Integer> data;

   public Snapshot(List<Integer> data) {
       this.data = new ArrayList<>(data);
   }

   public List<Integer> restore() {
       return new ArrayList<>(this.data);
   }

}

Main method:

public static void main(String[] args) {
    List<Integer> list = new ArrayList<>();
    list.add(1);
    list.add(2);
    Snapshot snap = new Snapshot(list);
    list.set(0, 3);
    System.out.println("It should log [3, 2], actual "   list);
    list = snap.restore();
    System.out.println("It should log [1,2], actual "   list);
    list.add(4);
    System.out.println("It should log [1,2, 4], actual "   list);
    list = snap.restore();
    System.out.println("It should log [1,2], actual "   list);
}

CodePudding user response：

package package_1;

import java.util.ArrayList;

public class Snapshot {

// Modify the implementation of the Snapshot class so that an ArrayList stored
// in the snapshot is not affected by modifications to either the original or restored ArrayList.

private ArrayList<Integer> data;

public Snapshot(ArrayList<Integer> list) {
    data = new ArrayList<>(list);
}
public ArrayList<Integer> restore() {
   return new ArrayList<>(data);

}
public static void main(String[] args) {

    ArrayList<Integer> list = new ArrayList<Integer>();
    list.add(1);
    list.add(2);

    Snapshot snap = new Snapshot(list);

    list.set(0, 3);


    list = snap.restore();

    System.out.println(list); //It should log "[1,2]"
    list.add(4);
    list = snap.restore();

    System.out.println(list); //It should log "[1,2]"
}

}