nullPointerException I cannot figure out in Java generic class

Question

I am trying to write program and keep getting a nullPointerException when I call a particular method, what does this mean ?

This question and the code looks extremely familiar. I've seen this definitely before a few hours ago, but can't find it now... It was answered there. — Boris
– Boris, Commented Mar 5, 2011 at 16:09
@stackoflow, Hm.. Have you recreate topic and delete previous? (-1 until answer) — Stan Kurilin
– Stan Kurilin, Commented Mar 5, 2011 at 16:12
This question is extremely similar to stackoverflow.com/questions/5185577/java-generic-arguments/… — Zach L
– Zach L, Commented Mar 5, 2011 at 16:19
This is not homework, and yes this is a previously submitted topic - I phrased my last question badly, it got spammed, I spoke to a mod who deleted it so that it wouldn't confuse any people trying to learn from it. Acceptable answer ? — Frank Burns
– Frank Burns, Commented Mar 5, 2011 at 16:58

Stan Kurilin · Accepted Answer · 2011-03-05 17:53:11Z

1

I think it should be

private int size; //non static

private static <S extends Comparable<S>> MyList<S> leftHalf(MyList<S> list) {
    MyList<S> leftSide = new MyList<S>();
    int middle = list.size() /2;
    for (int countToMiddle = 0; countToMiddle < middle; countToMiddle++) {
        leftSide.addEnd(list.head());
    }

    return leftSide;
}

if no, please provide more information about what this method should do.

upd: construction issue

public MyList() {   //takes no arguments
    nodes = null;
}
public MyList(T... args) {  //takes any number of arguments
    this();
    for(T t : args){
        add(t);
    }
}

upd: addEnd issue

public void addEnd(T item) {
    if (nodes == null) {
        nodes = new NodesList<T>(item, null);
        return;
    }
    if (nodes.tail == null) {
        nodes.tail = new NodesList<T>(item, null);
    } else {
        nodes.tail == new NodesList<T>(nodes.tail, item);
    }
}

edited Mar 5, 2011 at 17:53

answered Mar 5, 2011 at 16:07

Stan Kurilin

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17 Comments

Frank Burns Over a year ago

This method should split the list into two lists, in the first(leftSize) it will contain the elements from 0 to n/2, in the second, rightSide, it will contain the elements from n/2 to n. Unfortunately my implementation does not seem to work, as, when I call 'MyList list = new MyList<Long>( 1L, 2L, 3L ); //3 entries list.print();', I get the error cannot find symbol symbol : constructor MyList(long,long,long)

Stan Kurilin Over a year ago

@stackoflow, I think, this method should do exactly what you want) , correct? Optionally you can remove static modifier and list argument.

Stan Kurilin Over a year ago

@stackoflow, about constructor : you should create constructor with vararg to do things like this. It's other question)

Frank Burns Over a year ago

I believe it should, by my logic, which is often incorrect! I cannot test it, however. Unfortunately I am still having the error that when I call the MyList constructor, I get the message "cannot find symbol symbol : constructor MyList(long,long,long)".

Frank Burns Over a year ago

@Stas , could you please elaborate on using vararg ? Thanks.

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