Java: Generic class compare method

Question

I am new to Java. Basically, I created my own generic Heap data structure that will accept any type of objects. And the Heap data structure contains a method that compares 2 objects like this: heap[ parent(i)] <= heap[i]. Obviously, this will cause an error because I cannot use <= to compare two objects.

Question 1: I need to write a method to replace <= such that if I pass in any two objects it will automatically do the equivalent of the <= comparison . But I do not know how to do so.

class Heap<E>{
      E[] heap;
      // constructor
      public Heap(E[] heap){
          this.heap = heap;
      }

      public int parent(int i){
          return  (int) Math.floor(i/2.0);
      }   
      ...

      // This is where the problem occurs    
      public boolean minHeapProperty(){
          for(int i = 0; i < heap.length; i++){
             // <= operator need to be replaced by a method can be used for comparison by all objects
             if( !( heap[ parent(i) ] <= heap[i] )){
                 return false;
             }
          }
          return true;
       }

    ........
}

Question 2: I have another object class Vertex and I am trying to write a method to compare 2 Vertex objects by an instance variable node. But I kept failing. Note that I will be creating a heap of Vertex objects later on. So, I really only need a unique comparison method that can be used in both the Heap generic class and the Vertex class. This is why two questions are related and I do not want to post two separate questions on this website.

  Vertex{
     public int node;

     public Vertex(int node){
        this.node = node;
     }

     // compare two vertex objects (I do not know how to write this in the right way)
     public boolean compareTwoVertexObjects(Vertex v2){
         // need an answer here. 
         return (node <= v2.node);    
     }
  }

Have all possible classes used implement Comparable<T> and then use the compareTo(T other) method. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Dec 22, 2014 at 2:51

Elliott Frisch · Accepted Answer · 2014-12-22 02:54:47Z

3

I suggest you modify the type of E to specify that it must be Comparable like

class Heap<E extends Comparable<E>>

then you can compare two instances a and b like

E a;
E b;
// ...
int c = a.compareTo(b);
if (c < 0) {
  // a < b
} else if (c > 0) {
  // a > b
} else {
  // a == b
}

Then you might modify Vertex to also be Comparable with something like

class Vertex implements Comparable<Vertex> {
    public int node;

    public Vertex(int node) {
        this.node = node;
    }

    @Override
    public int compareTo(Vertex o) {
        return Integer.valueOf(node).compareTo(o.node);
    }
}

answered Dec 22, 2014 at 2:54

Elliott Frisch

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7 Comments

mynameisJEFF Over a year ago

How come a generic object E can extend an interface ? I thought interface can only be implemented but not extend

Elliott Frisch Over a year ago

@mynameisJEFF An interface extends another interface. This is how generics were implemented as an add-on in Java 5. The language did not have generics initially.

Dawood ibn Kareem Over a year ago

The meaning of extends in this context is slightly different from the meaning of extends in a class or interface definition. When you use it with a type parameter (that is, the E or T that's commonly used in a generic), it means "is a subtype of" - in other words, it either extends the type OR implements the type OR is the type itself. This is so that you can use E to substitute for either a class or an interface.

mynameisJEFF Over a year ago

Is there a difference using between E and T ? Or they are just the same thing ? Because I seen others symbols like ? and I am not entirely sure what they all mean ?

Dawood ibn Kareem Over a year ago

That sounds like a new question.

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