Java转换问题

Question

I don't understand why I need to cast member variables to the proper type when their types are already declared. For example:

public class SomeClass extends SomethingElse {

  private Funky mFunkyVar;
  // a whole bunch of other variables and methods

  public void needToCast() {
    mFunkyVar = (Funky) new FunkySubClass();
  }

}

Am I making some newbie mistake or is this indeed idiomatic Java? In the above I'm assuming that FunkySubClass is indeed a subclass of Funky.

Answer 1

Actually you don't have to cast it!

Am I making some newbie mistake

Yeap, but don't worry... after this you wont do it again ;)

See:

$cat Funky.java  
class Funky {
}
class FunkySubClass extends Funky { 
}
class SomethingElse { 
}
class SomeClass extends SomethingElse { 
    private Funky aFunkyVar;
    //...
    public void noNeedToCast() { 
        aFunkyVar = new FunkySubClass();
    }
}

$javac Funky.java 
$

No compilation errors.

EDIT

You can cast primitives and object references.

For primitives you "have" to cast, if you want to narrow the value.

For instance:

int i = 255;
byte b = ( byte ) i;

If you don't cast the compiler will warn you, "hey, you don't really want to do that"

...
byte b = i;
...
Funky.java:18: possible loss of precision
found   : int
required: byte
        byte d = i;

Casting is like telling the compiler: "hey don't worry, I know what I'm doing, ok?"

Of course, if you cast, and the value didn't fit, you'll get strange results:

int i = 2550;
byte b = ( byte ) i;
System.out.println( b );

prints:

-10 

You don't need to cast when the type is wider than the other type:

byte b = 255;
int i = b;// no cast needed

For references works in a similar fashion.

You need to cast, when you want to go down into the class hierarchy ( narrow ) and you don't need to cast when you want to go upper in the hierarchy ( like in your sample ).

So if we have:

   java.lang.Object 
   |
   + -- Funky
        |
        +-- FunkySubclass

You only have to cast, when you have a type upper in the hierarchy ( Object or Funky ) in this case. And you don't have to, if you have a type lower in the hierarchy:

void other( Object o ) { 
    // Cast needed:
    Funky f = ( Funky ) o;
    FunkySubClass fsc = ( FunkySubClass ) o;
    FunkySubClass fscII = ( FunkySubClass ) f;
    // Cast not needed:
    Object fobj = f; 
    Object fscobj = fsc; 
    Funky fparent = fsc;
}

With primitives the JVM know how to truncate the value, but with reference no. So, if the casted value is not of the target type, you'll get a java.lang.ClassCastException at runtime. That's the price you have to pay, when you tell the compiler "I know what I'm doing" and you don't.

I hope this is clear enough and don't confuse you.

:)

Answer 2

If FunkySubClass is really a subclass of Funky you don't need the cast.

If you show us the error you're getting we could help more.

Answer 3

无需“ (Funky) ”，作为FunkySubClass 是，一个 Funky 。

Answer 4

You don't need to explicitly cast to the base class. For example, it is actually more idiomatic not to cast - a common case is:

List<String> somelist=new ArrayList<String>();

Answer 5

You would only need to cast if FunkySubClass was (improperly named and) in fact the parent class of Funky. Assigning a child class to a parent object is fine.

Answer 6

new FunkySubClass() returns an object of type FunkySubClass, not Funky. Hence you should explicitly tell Java to treat it as a Funky object

It is the right-hand side which is being converted

Answer 7

You don't need to cast it. What makes you think you do?