Question

    // Casting of class
    ArrayList a1 = new ArrayList<Integer>();
    Number n1 = (Number)a1; // clause 1 Error 
    AdjustmentListener c1 = (AdjustmentListener)a1; // Clause 2 No error
    EventListener c3 = (EventListener)a1; // Clause 3 No error

Based on the above, clause 1 has error. Logical. There is no relation between those 2 classes. For clause 2, there is no relation between an arrayList (a class) and the adjustmentlistener interface. Why is there no error?

EDIT 1:

    String str1 = new String();
    AdjustmentListener c2 = (AdjustmentListener)str1; // clause 4 compiler error

Answer 1

In order for a cast to succeed, there must be some possible implementer of both arguments. Take, for example, the following imaginary class declarations:

5: class ComparableArrayList extends ArrayList implements Comparable
7: class NumberList extends Number implements List
8: class ComparableList implements Comparable, List

An instance of each would allow the cast to succeed. For clause 4, in order for the cast to work, you would need

class NumberArrayList extends ArrayList, Number

and Java does not allow multiple inheritance.

EDIT for edited question:

In clause 1, there is no possible relationship between the classes, because you cannot have a class extend both ArrayList and Number. In clauses 2 and 3, it is possible to create a relationship by creating a new class that extends ArrayList and implements AdjustmentListener. Then set a1 to an instance of that class.