When and why would somebody do the following:
doSomething( (MyClass) null );
Have you ever done this? Could you please share your experience?
You can cast null to any reference type without getting any exception. The println method does not throw null pointer because it first checks whether the object is null or not. If null then it simply prints the string "null" . Otherwise it will call the toString method of that object.
Unfortunately the compiler is not capable of deducing that the statement boolean var1=(var=null); would always lead to the invalid assignment boolean var1=null .
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 tojava.lang.String
for a varargs call
cast tojava.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);
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