The memento pattern is used to save the state of an object without knowing it's internal data structures.
I try to explain it with an Iterator
example
public class MementoListIterator<E> implements Iterator<E> {
public static class Memento {
private int savedIndex;
private Memento(MementoListIterator<?> mementoListIterator) {
this.savedIndex = mementoListIterator.index;
}
}
private List<E> elements;
private int index = 0;
public MementoListIterator(List<E> elements) {
this.elements = elements;
}
public Memento save() {
return new Memento(this);
}
public void restore(Memento memento) {
this.index = memento.savedIndex;
}
@Override
public boolean hasNext() {
return this.index < elements.size();
}
@Override
public E next() {
return elements.get(index++);
}
@Override
public void remove() {
throw new UnsupportedOperationException("Not implemented yet");
}
}
A client can now save any state of the iterator without knowing how the iterator internally manages it's state.
public class Main {
public static void main(String[] args) {
List<String> list = Arrays.asList("A", "B", "C", "D", "E");
MementoListIterator<String> mementoListIterator = new MementoListIterator<String>(
list);
Memento initialState = mementoListIterator.save();
while (mementoListIterator.hasNext()) {
String string = mementoListIterator.next();
System.out.println(string);
}
// Normally we can not re-use the iterator, but
// fortuanatly we saved the initial state.
// restore the initial state and we can use the Iterator again
mementoListIterator.restore(initialState);
while (mementoListIterator.hasNext()) {
String string = mementoListIterator.next();
System.out.println(string);
}
}
}