java.lang.Object; 无法转换为[java.lang.comparable]

Question

I'm stuck at this java.lang.ClassException . In general, I'm trying with a simple method maxElement() that simply returns the highest Integer, the add() method works perfectly as well as overriding toString() , this last 2 methods are inherited from ArrayLinearList<T> . element is an array . ArrayLinearList does work perfectly as well .This is my code:

public class ArrayLinearList<T> implements LinearList<T>, Iterable<T>{
 protected T[] element; 
 protected int size; 
  *constructors and methods*
  // default constructor makes an T[]element of length 10

}

public class ArrayLinearListTaller1<T extends  Comparable<T>> extends ArrayLinearList<T>{
  public static void main(String[] args) {
    ArrayLinearListTaller1<Integer>x=new ArrayLinearListTaller1<Integer>();
    x.add(0, new Integer(10));
    x.add(1, new Integer(5));
    x.add(2, new Integer(7));
    System.out.println(x);
    System.out.println(x.maxElement());
    }// end of main

 public T maxElement() {
    Arrays.sort(element,0,size);
    T max= element[size-1];//*error here*
    return max;
}

This is what I get:

[10,5,7]  
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Comparable;

Answer 1

The elements passed to Arrays.sort() must implement Comparable interface.

Sorts the specified range of the specified array of objects into ascending order, according to the natural ordering of its elements. The range to be sorted extends from index fromIndex, inclusive, to index toIndex, exclusive. (If fromIndex==toIndex, the range to be sorted is empty.)

All elements in this range must implement the Comparable interface.

Furthermore, all elements in this range must be mutually comparable (that is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the array).

So the Objects present in element array must implement Comparable interface .

Answer 2

It seems to me like whatever your type of object for element is does not implement Comparable interface and you're getting exception from Arrays.sort(). Maybe sharing your full code will help.