I want to implement a Fail-Fast Iterator to remove Entrys from my own List and test it on correct behaviour. The List holds Elements of the type MyEntry.
These Entrys hold an Generic Value and a reference to the next Entry.
class MyEntry<E> {
MyEntry<E> next;
E o;
MyEntry() {
this(null, null);
}
MyEntry(E o) {
this(o, null);
}
MyEntry(E o, MyEntry<E> e) {
this.o = o;
this.next = e;
}
}
The List itself tracks its position with the pos Entry and behaves like a Stack. Now I would like to implement a fail-fast iterator that allows me to iterate over the List and remove Entrys instead of throwning an UnsupportedOperation Exception.
import java.util.Iterator;
import java.util.NoSuchElementException;
public class MyList<E> implements Cloneable, java.lang.Iterable {
private MyEntry<E> begin;
private MyEntry<E> pos;
public MyList() {
pos = begin = new MyEntry<E>();
}
public boolean empty() {
return begin.next == null;
}
public boolean endpos() {
return pos.next == null;
}
public void reset() {
pos = begin;
}
/**
* Advances one step in this List.
*
* @throws NoSuchElementException if the last Entry of this List already has been reached.
*/
public void advance() {
if (endpos()) {
throw new NoSuchElementException("Already at the end of this List");
}
pos = pos.next;
}
/**
* Returns the actual element of this List.
*
* @return the actual element
* @throws RuntimeException if the last Entry of this List already has been reached.
*/
public E elem() {
if (endpos()) {
throw new NoSuchElementException("Already at the end of this List");
}
return pos.next.o;
}
/**
* Inserts <code>o</code> in this List. It will be placed before the actual
* element. After insertion the inserted element will become the actual
* element.
*
* @param x the element to be inserted
*/
public void add(E x) {
MyEntry<E> newone = new MyEntry<E>(x, pos.next);
pos.next = newone;
}
/**
* Deletes the actual element of this List. The element after the actual
* element will become the new actual element.
*
* @throws NoSuchElementException if the last Entry of this List already has been reached.
*/
public void delete() {
if (endpos()) {
throw new NoSuchElementException("Already at the end of this List");
}
pos.next = pos.next.next;
}
@Override
public Iterator<E> iterator() {
return new Iterator<E>() {
private MyEntry<E> it = null;
@Override
public boolean hasNext() {
return pos != null;
}
@Override
public E next() {
if (it==null)
it = begin;
else
it = it.next;
return it.o;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
};
}
}
First of I found three cases which need to be handled.
- The first one would be that i want to remove an Entry that has a previous and a next Entry. In that case the iterator should change the next element of the previous Entry to the next Entry, so the Entry that i want to delete gets skipped.
- The second case would be the first Entry of the List. In that case the next Entry should be set as first.
- The last case is that I delete the last Entry, in this case the previous Entrys next should be pointed to null.
I also already tried to implement an Iterator to my List and tested it with my test Class
public class MyListTest {
@Test
public void test() {
MyList list = new MyList();
list.add("a");
list.add("b");
list.add("c");
while(list.iterator().hasNext()==true) {
System.out.println(list);
}
list.iterator().remove();
while(list.iterator().hasNext()==true) {
System.out.println(list);
}
}
}
But the output loops an c and doesnt even get to remove an Entry.
I am now stuck at the correct implementation of an Fast-Fail Iterator that can iterate over MyList and remove Entrys. Because i couldnt break it down to a single problem i made a list of a few questions that came up while i tried to implement the Iterator
- Should the Iterator be implemented in the MyList class or should it be implemented in an own class?
- How do i get the Iterator to advance over MyList like the advance() method did?
- Is the while-loop in the test class convenient or should another method be used instead?
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