Collections.sort和Object []

Question

so just writing a Abstract classes and interfaces program. my code so far:

package lesson8;

import java.util.Collections;

public class ShapeTest {

 public static void main(String[] args) {
    Object[] shape = {new Square(4),
            new Rectangle(3, 2), 
            new RightTriangle(5,2), 
            new Square(2),
            new RightTriangle(2, 4),
            new Plus(3)};  
    Collections.sort(shape);
    }

}

my question is, why doesn't the Collection.sort() work with Object[]? how can i make it work? thanks!

Answer 1

You need to use Arrays.sort() to sort arrays (in place). Moreover, there is no "natural" ordering for Object s, so you will have to pass a Comparator<Object> to the overloaded version of Arrays.sort() that takes a comparator, unless your Object s all implement Comparable and are all mutually comparable, as per this .

Answer 2

First of all, Collections.sort() expects a Collection as the argument, not an array. For arrays you have Arrays.sort() .

Second of all, in order for either Collection.sort() or Arrays.sort() to work for a Collection or array of Object s respectively, all these objects must be comparable to each other - they must implement the Comparable interface, and e1.compareTo(e2) must not throw a ClassCastException for each two elements of the Collection or array.

Answer 3

You could also use the BeanComparator from apache commons beanutils, like this:

Collections.sort(personList, new BeanComparator("name"));

Example

Answer 4

Collections.sort work by calling the compareTo method on the objects in the array, in order to determine the natural ordering of the objects. However, the class Object does not have a defined compareTo method, so the sort method has no way of knowing what the correct order is. As such it cannot sort the list.

If you want to sort the list using Collections.sort , then make sure all contained objects have a compareTo method defined.