双链表无进口

Question

I'm currently working on Junit testing for implementing a Doubly Linked list in java. I understand how it needs to work drawn pictures over and over but I cannot get my removeFirst method to work. addLast(); works up until removeFirst is called.

private Link<I> first;
private Link<I> last;

public boolean isEmpty() {
    return size() == 0;
}

public int size() {
    int count = 0;
    Link<I> aLink = first;
    while (aLink != null) {
        count++;
        aLink = aLink.getAfter();
    }
    return count;
}

public I get(int index) {
    Link<I> aLink = first;
    int count = 0;
    while (count != index) {
        aLink = aLink.getAfter();
        count++;
    }
    return aLink.getItem();

}

public void addLast(I anItem) {
    Link<I> aLink = new Link<I>(anItem);
    if (isEmpty()) {
        first = aLink;
        first = last;
    } else {
        last.setAfter(aLink);
        aLink.setBefore(last);
        last = aLink;

    }
}

public void addFirst(I anItem) {
    Link<I> aLink = new Link<I>(anItem);
    if (isEmpty()) {
        first = aLink;
        last = first;
    } else {
        aLink.setAfter(first);
        first.setBefore(last);
        first = aLink;
    }

}

public I removeFirst() {
    I removed = first.get(0);
    if(size()==3) {
        first = first.getAfter();
        first.setBefore(null);
        first.setAfter(last);
        return removed;
    } else if(size()==2) {
        first = first.getAfter();
        first.setBefore(null);
        first.setAfter(null);
        return removed;
    }else {
        first = null;
        first.setBefore(null);
        first.setAfter(null);
        return removed;
    }

}

public I removeLast() {
    I removed = last.getItem();
    if (isEmpty()) {
        removed = null;
        return removed;
    } else {
        last = last.getBefore();
        removed = last.getItem();
        return removed;
    }

}

}

public class Link<I> {
private Link<I> after;
private Link<I> before;
private I item;

public Link(I anItem) {
    item = anItem;
}

public Link<I> getAfter(){
    return after;
}

public void setAfter(Link<I> aLink) {
    after = aLink;
}

public Link<I> getBefore(){
    return before;
}

public void setBefore(Link<I> aLink) {
    before = aLink;
}

public I getItem() {
    return item;
}

public void setItem(I anItem) {
    item = anItem;
}

}

here is the test unit

void testAddLast() {
    notes.addLast("do");
    notes.addLast("re");
    notes.addLast("mi");
    String note = notes.removeFirst();
    assertTrue(notes.size()==2);
    assertTrue("do".equals(note));
    note = notes.removeFirst();
    assertTrue(notes.size()==1);
    assertTrue("re".equals(note));
    note= notes.removeFirst();
    assertTrue(notes.isEmpty());
    assertTrue("mi".equals(note));
    note = notes.removeFirst();
    assertTrue(note == null);
    assertTrue(notes.isEmpty());
    assertTrue(notes.size() == 0);
}

I feel like what I have now is getting me close but I removed = first.get(0); keeps giving me a null pointer exception. I was given extra time on this assignment over 2 weeks ago and I am still having a really hard time with this. I have tried a bunch of variations of the removeFirst() method following the debugger to help.

any insight would help me alot. I have more test cases I can add to post aswell.

Answer 1

This seems to be the method that's giving you trouble

public I removeFirst() {
    I removed = first.get(0);
    if(size()==3) {
        first = first.getAfter();
        first.setBefore(null);
        first.setAfter(last);
        return removed;
    } else if(size()==2) {
        first = first.getAfter();
        first.setBefore(null);
        first.setAfter(null);
        return removed;
    }else {
        first = null;
        first.setBefore(null);
        first.setAfter(null);
        return removed;
    }

}

You say you're getting a NullPointerException when you call first.get(0) . This means that first is null.

This is how you add a node:

public void addLast(I anItem) {
    Link<I> aLink = new Link<I>(anItem);
    if (isEmpty()) {
        first = aLink;
        first = last;
    } else {
        last.setAfter(aLink);
        aLink.setBefore(last);
        last = aLink;
    }
}

You assign to first aLink , then overwrite that assignment with last , which is null.

You probably means to say last = first there.

Try this test case:

void testAddOne() {
    assertTrue(notes.count() == 0);
    notes.addLast("do");
    assertTrue(notes.count() == 1);
}

Generally, your test case is very complicated. You want simpler tests to check the postconditions of an operations. For example, inserting one element should increase the count by one. Removing an element should decrease the count by one. Adding two elements, check get returns the first for index 0 and the second for index 1, and so on.