CODEWITHSUNDEEP
javagenericscomparison
mynameisJEFF
mynameisJEFF

Reputation: 4239

Java: Generic class compare method

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);    
     }
  }

Upvotes: 2

Views: 1204

Answers (1)

Elliott Frisch
Elliott Frisch

Reputation: 201429

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);
    }
}

Upvotes: 3

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