Question

    import java.util.*;  
    class A implements Comparable<A>  
    {  
        int a;  
        public int compareTo(A other)  
        {  
            A a1 = (A) other; //No need of cast  
            if(this.a == other.a ) return 0;  
            if(this.a < other.a )  return -1;  
            return 1;  
        }  
    }  
    class ctest  
    {  
        public static void main(String args[])  
        {    
            String[] names = {"OOP","BITS","PILANI"};  
            Arrays.sort(names);  
            int[] data = { 10,-45,87,0,20,21 };  
            Arrays.sort(data);    

            A[] arr = new A[5];  
            arr[0] = new A();  
            arr[1] = new A();  
            arr[2] = new A();  
            arr[3] = new A();   
            arr[4] = new A();  
            Arrays.sort(arr);  
        }  
    } 

This gives the error:

Exception in thread "main" java.lang.ClassCastException: A cannot be cast to java.lang.Comparable  
    at java.util.ComparableTimSort.countRunAndMakeAscending(Unknown Source)  
    at java.util.ComparableTimSort.sort(Unknown Source)  
    at java.util.ComparableTimSort.sort(Unknown Source)  
    at java.util.Arrays.sort(Unknown Source)

Answer 1

Exception in thread “main” java.lang.ClassCastException: A cannot be cast to java.lang.Comparable

This means the version of A you are running is not Comparable. Most likely you have two versions of A or you didn’t build the class correctly. I suggest using an IDE to build you classes and doing a clean build should fix this.

BTW: All your A are the same, so sorting them won’t do anything. You can fix these once you have fixed the build.