Why null cast a parameter?

If doSomething is overloaded, you need to cast the null explicitly to MyClass so the right overload is chosen:

public void doSomething(MyClass c) {
    // ...
}

public void doSomething(MyOtherClass c) {
    // ...
}

A non-contrived situation where you need to cast is when you call a varargs function:

class Example {
    static void test(String code, String... s) {
        System.out.println("code: " + code);
        if(s == null) {
            System.out.println("array is null");
            return;
        }
        for(String str: s) {
            if(str != null) {
                System.out.println(str);
            } else {
                System.out.println("element is null");
            }
        }
        System.out.println("---");
    }

    public static void main(String... args) {
        /* the array will contain two elements */
        test("numbers", "one", "two");
        /* the array will contain zero elements */
        test("nothing");
        /* the array will be null in test */
        test("null-array", (String[])null); 
        /* first argument of the array is null */
        test("one-null-element", (String)null); 
        /* will produce a warning. passes a null array */
        test("warning", null);
    }
}

The last line will produce the following warning:

Example.java:26: warning: non-varargs call of varargs method with inexact argument type for last parameter;
cast to java.lang.String for a varargs call
cast to java.lang.String[] for a non-varargs call and to suppress this warning

Let's say you have these two functions, and assume that they accept null as a valid value for the second parameters.

void ShowMessage(String msg, Control parent);
void ShowMessage(String msg, MyDelegate callBack);

These two methods differ only by the type of their second parameters. If you want to use one of them with a null as the second parameter, you must cast the null to the type of second argument of the corresponding function, so that compiler can decide which function to call.

To call the first function: ShowMessage("Test", (Control) null);
For the second: ShowMessage("Test2", (MyDelegate) null);