java generic casting in generic classes

Question

I'm writing a simple implementation of a generic max heap. If I write

public class FastMaxHeap<T>{

  T[] data;
  int size;

  static final int HEAP_SIZE = 10000;

  @SuppressWarnings("unchecked")
  public FastMaxHeap(){
    data = (T[]) new Object[HEAP_SIZE];
  size = 0;
  }
}

it compiles. Now to actually implement the heap, i.e. write maxHeapify(), I need to be able to compare two T's. One option that a priori seems possible would be to tell the compiler that T implements Comparable. But if I type replace < T > with < T implements Comparable > the compiler complains -- how can I do this?

Alternatively, I could define a class

public class HasValue{

  int value;

  public HasValue(int value){
        this.value = value;
  }

}

and in theory I should then be able to compare two HasValue objects like x.value > y.value. But if I type

public class FastMaxHeap<T extends HasValue>{

  T[] data;
  int size;

  static final int HEAP_SIZE = 10000;

  @SuppressWarnings("unchecked")
  public FastMaxHeap(){
    data = (T[]) new Object[HEAP_SIZE];
  size = 0;
  }
}

I now get a ClassCastException. What is going on here? Java generics hurt my brain.

Why not just store Comparables?

Perception
– Perception

2012-12-19 20:36:33 +00:00
Commented Dec 19, 2012 at 20:36 — Perception
– Perception, Commented Dec 19, 2012 at 20:36
+1 for "Java generics hurt my brain."

Peter Lawrey
– Peter Lawrey

2012-12-19 20:39:39 +00:00
Commented Dec 19, 2012 at 20:39 — Peter Lawrey
– Peter Lawrey, Commented Dec 19, 2012 at 20:39

Peter Lawrey · Accepted Answer · 2012-12-19 20:30:57Z

5

In the first case T extends Object which is erased to Object at runtime.

In the second case T extends HasValue is erased to HasValue so you need to have.

data = (T[]) new HasValue[HEAP_SIZE];

IMHO It is needlessly pedantic that Java doesn't allow new T[HEAP_SIZE] to do what you have to do anyway.

answered Dec 19, 2012 at 20:30

Peter Lawrey

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6 Comments

mikeslattery Over a year ago

I deleted my answer in favor of yours.

Peter Lawrey Over a year ago

@stefanbachert The casting will work provided the component type of the new array is a sub-type of what T extends.

newacct Over a year ago

It's not "needlessly pedantic"; it's very important. Try to return data from a method in your class with return type T[] and see what happens.

Peter Lawrey Over a year ago

@newacct Can you be more specific?

newacct Over a year ago

I am assuming you mean you want Java to allow new T[HEAP_SIZE] to mean (T[])new HasValue[HEAP_SIZE];. But that is unsafe so it is important to force the user to have to explicitly override it.

|

stefan bachert · Accepted Answer · 2012-12-19 20:34:08Z

0

You may try this one (not compiled, yet)

public class FastMaxHeap<T extends HasValue>{

  HasValue[] data;
  int size;

  static final int HEAP_SIZE = 10000;

   public FastMaxHeap(){
     data = new HasValue[HEAP_SIZE];
     size = 0;
   }
}

answered Dec 19, 2012 at 20:34

stefan bachert

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1 Comment

Peter Lawrey Over a year ago

If you drop the casting you don't need the suppress warning

maks · Accepted Answer · 2012-12-19 20:50:35Z

0

It is better to have type token to create arrays like this

public class FastMaxHeap<T>{

  T[] data;
  int size;

  static final int HEAP_SIZE = 10000;

  @SuppressWarnings("unchecked")
  public FastMaxHeap(Class<T> clazz){
    data = (T[])Array.newInstance(clazz, HEAP_SIZE);
    size = 0;
  }
}

In such way you will have no ClassCastExceptions in runtime

And also: < T implements Comparable > is not correct, correct one is < T extends Comparable >

answered Dec 19, 2012 at 20:50

maks

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Comments

Judge Mental · Accepted Answer · 2012-12-19 20:51:31Z

0

Your heap should accept a Comparator< T > as a constructor argument. Problem solved. Client can use any type he wants. You can also supply a simple overload which infers the comparator implementation for types T that already implement Comparable.

answered Dec 19, 2012 at 20:51

Judge Mental

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