How to delete a specific element from a priority queue?

Question

I have created a priority queue using the Java API and I want to remove a specific element from the priority queue at the end of the program. I know it has to do something with the comparator but I can't figure it out. Can someone help? Here's my code:

public static void main(String[] args)
{
    PriorityQueue<Element> X = new PriorityQueue<Element>(100, new ElementComparator());
    X.add(new Element(30, 3));
    X.add(new Element(700, 4.5));
    X.add(new Element(100, 6.2));
    X.add(new Element(2, 8.1));
    System.out.println(X.remove(new Element(100, 6.2)));
}

and here's my Element class:

private int index;
private double value;

public Element(int i, double v) 
{
    index = i;
    value = v;
}

public int getIndex() { return index;};
public double getValue() { return value;};
public void setValue(double v) { value = v;};

And here's the comparator that I created:

public int compare(Element o1, Element o2)
{
    int idx1 = o1.getIndex();
    int idx2 = o2.getIndex();
    if (idx1 < idx2) {
        return -1;
    } else if (idx1 > idx2) {
        return 1;
    } else {
        return 0;
    }
}

public boolean equals(Element o1, Element o2) 
{
    return o1.getIndex() == o2.getIndex();
}

I appreciate your help...

System.out.println(X.remove(new Element(100, 6.2))); will print if the element was actually removed from X, is that what you are trying to achieve? or are you trying to print X? — amit
– amit, Commented Feb 6, 2012 at 0:42
The compareTo function is for sorting. The equality method is needed for remove. See @johncarl's answer below for the correct signature of this method. — Perception
– Perception, Commented Feb 6, 2012 at 0:42
Also: It is a good practice and a java convention to name non-constant variables with a name starting with lower case letter. — amit
– amit, Commented Feb 6, 2012 at 0:43

John Ericksen · Accepted Answer · 2012-02-06 00:38:42Z

1

You need to define equals() and hashcode() on your Element object as such:

public class Element{
    private int index;
    private double value;

    public Element(int i, double v)
    {
        index = i;
        value = v;
    }

    public int getIndex() { return index;}
    public double getValue() { return value;}
    public void setValue(double v) { value = v;}

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Element)) return false;

        Element element = (Element) o;

        if (index != element.index) return false;

        return true;
    }

    @Override
    public int hashCode() {
        return index;
    }
}

Defining equals() on the ElementComparator does not perform the same task.

answered Feb 6, 2012 at 0:38

John Ericksen

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