How can I delete a specific element in priority queue?

Question

import java.util.*;

public class test4 {
  public static void main(String[] args){
    PriorityQueue[] P = new PriorityQueue[10];
    P[1] = new PriorityQueue<ClassEntry>();
    P[1].add(new ClassEntry(1.2,1));
    P[1].add(new ClassEntry(1.5,2));
    P[1].add(new ClassEntry(1.2,3));
    P[1].add(new ClassEntry(10,4));

    P[1].remove(new ClassEntry(10,4));//I can't delete this object???

    System.out.println(P[1].size());
    ClassEntry ce = (ClassEntry) P[1].peek();
    System.out.println(P[1].size());
    System.out.println(ce.sim+"\t"+ce.index);
  }
}

Why i can't delete (10,4)? Can somebody teach how to implement...thanks!

rodion · Accepted Answer · 2010-12-28 03:53:16Z

6

ClassEntry has to override and implement Object.equals(Object o) for remove to work. For example:

class ClassEntry{
  float a;
  int b;

  public ClassEntry(float a, int b){
    //...
  }

  @Override
  public boolean equals(Object o){
    if(o instanceof ClassEntry){
      ClassEntry c = (ClassEntry)o;
      return a == c.a && b == c.b;
    }
    return false;
  }
}

EDIT: As was kindly pointed out by @Jim Garrison, if you do not implement equals the default behaviour, which is to compare objects by reference, is used. In this case for your code to work you will need to remove like this:

PriorityQueue[] P = new PriorityQueue[10];
P[1] = new PriorityQueue<ClassEntry>();
ClassEntry entry = new ClassEntry(10,4);
P[1].add(entry);
//remove object with the same reference as the added one
P[1].remove(entry);

edited Dec 28, 2010 at 3:53

answered Dec 28, 2010 at 3:17

rodion

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6 Comments

Jim Garrison Over a year ago

+1 with a clarification: the queue has to know how to determine if two objects are "equal" in order to find the one you want to delete. If you don't provide code to implement the "equal" criteria you are interested in, you get the default equals() from java.lang.Object, which compares the object addresses in memory.

LoveTW Over a year ago

thank you! but i don't know how can i let equals function works? Should i call equals() in test4 class?

rodion Over a year ago

equals just needs to be implemented by ClassEntry class. You don't need to call it. PriorityQueue will use it when you call remove function.

LoveTW Over a year ago

but i found that return a == o.a && b == o.b; will be error, since o is not an classEntry object? thank you:)

Alexandr Over a year ago

Actually instanceof return false if o == null, so it's not needed to check it for null. And of course you need to cast Object o to ClassEntry when you try to compare a and b variables.

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