Question

I am looking for advice, because I get an error when I run the following code:

public class Test9 {


    public static void main(String[] args) {        

        Object [] myObjects = {
                 new Integer(12),
                 new String("foo"),
                 new Integer(5),
                 new Boolean(true)
                 };
                 Arrays.sort(myObjects);
                 for(int i=0; i<myObjects.length; i++) {
                 System.out.print(myObjects[i].toString());
                 //System.out.print(" ");
                 }

    }

}

The error I am getting is:

   Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
    at java.lang.Integer.compareTo(Unknown Source)
    at java.util.Arrays.mergeSort(Unknown Source)
    at java.util.Arrays.sort(Unknown Source)
    at Practicequestions.Test9.main(Test9.java:18)

Answer 1

 Object [] myObjects = {
                 new Integer(12),
                 new String("foo"),
                 new Integer(5),
                 new Boolean(true)
                 };

This a generic type array. So when you trying to sort it with public static void sort(Object[] a) it populate runtime ClassCastException cause array contains elements that are not mutually comparable.

Array.sort() specified array of objects into ascending order, according to the natural ordering of its elements.
There is a another method you can use it sort(Object[] a,Comparator c) . Implement Comparator with your own logic and pass it.

Object[] myObjects = { new Integer(12), new String("foo"),
                new Integer(5), new Boolean(true) };
        Comparator<Object> comparator=new Comparator<Object>() {
            @Override
            public int compare(Object obj1, Object obj2) {
                if(obj1 instanceof String && obj2 instanceof String){
                    return String.valueOf(obj1).compareTo(String.valueOf(obj2));
                }else if(obj1 instanceof Integer && obj2 instanceof Integer){
                    return ((Integer) obj1).compareTo((Integer) obj2);
                }else if(obj1 instanceof Boolean && obj2 instanceof Boolean){
                    return ((Boolean) obj1).compareTo((Boolean) obj2);
                }else if(obj1 instanceof String && obj2 instanceof Integer){
                    return 1;
                }else if(obj1 instanceof Integer && obj2 instanceof String){
                    return -1;
                }else if(obj1 instanceof Boolean){
                    return -1;
                }else if( obj2 instanceof Boolean){
                    return 1;
                }
                return 0;
            }
        }; 
        Arrays.sort(myObjects,comparator);
        for (int i = 0; i < myObjects.length; i++) {
            System.out.print(myObjects[i].toString());
        }