Interface and class with generics and Comparable

Question

I have a little problem connected with interfaces, classes and generics in Java.

First of all, I have an interface which is meant to represent the idea of Priority Queue:

public interface PriorityQueue<T> {

    // insert object o of class T into priority queue with appropriate element
    public void insert(T o);

    // remove an element with the highest priority
    public T remove();  

}

As we know we can implement a priority queue by heap or list. Here's my class of heap:

public class Heap <T implements Comparable> implements PriorityQueue<T>

I want to have an ArrayList which will have elements of type T. I want my heap to be prepared for all types which are comparable (classes which implement interface Comparable). T could be a String, Double, Integer or just my own type (then I know that I have to write a compareTo method...).

How can I do that? I have a lot of errors in my NetBeans...

you also want types which are comparable *to themselves*: `>` — newacct, Feb 10 '14 at 08:13
OK, but could you explain me your idea? I used `public class Heap implements IPriorityQueue` and everything seems fine. I tested my heap on Strings, Integers and Doubles and I see that the whole heap is working properly. — TheOpti, Feb 14 '14 at 15:33

score 0 · Accepted Answer · edited May 23 '17 at 12:28

0

Instead of

public class Heap <T implements Comparable> implements PriorityQueue<T>

write:

public class Heap<T extends Comparable> implements PriorityQueue<T>

It works (Of course implement inherited methods). See here for more information.

edited May 23 '17 at 12:28

Community

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answered Feb 09 '14 at 21:03

Ashot Karakhanyan

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score 0 · Answer 2 · answered Feb 09 '14 at 21:27

You're pretty close. Try: public class Heap<T extends Comparable>... This is one of the many weird and, IMO, unfortunate things about Java Generics. You never use the implements keyword inside of the < >, only extends. Here's a JUnit test showing it in action:

import org.junit.Test;
import java.util.ArrayList;
import java.util.List;
import static org.junit.Assert.*;

public class CalcIEProjectTreeTest {
public static interface Priority<T> {
    public void insert(T item);
    public T remove();
}

public static class Heap<T extends Comparable> implements Priority<T> {
    private List<T> storage = new ArrayList<T>();

    public void insert(T item){
        storage.add(item);
    }

    public T remove() {
        return storage.remove(0);
    }
}

 @Test
 public void testStuff() throws Exception {
    Heap h = new Heap<String>();
    h.insert("testString");
    assertEquals("testString", h.remove());
 }
}

never mind the bogus formatting.

Interface and class with generics and Comparable

2 Answers2