Example

public class ClassA implements Comparable<Integer> {
public int compareTo (Object o) {
// ...
}
}

Solution
Always use the standard signature for compareTo methods - public int compareTo(ObjectType o).
Ensure that the ObjectType matches the type identified in the Comparable interface argument.

public class ClassA implements Comparable<Integer> {
public int compareTo (Integer i) {
//...
}
}