CODEWITHSUNDEEP
javainheritance
user3317186
user3317186

Reputation: 115

Java Upcast versus Downcast

I have the following code

    class X{} 
class Y extends X{} 
class Z extends X{} 

public class RunTimeCastDemo{ 
 public static void main(String args[]){ 
 X x = new X(); 
 Y y = new Y(); 
 Z z = new Z(); 


 X x1 = y; // compiles ok (y is subclass of X), upcast 
 X x2 = z; // compiles ok (z is subclass of X), upcast

The code above was given to me in a lecture. I know that X is the base class of both Y and Z. x is a reference to an X type object, y is a reference to an Y type object, and z is a reference to a Z type object. The part that is confusing me is the last two lines of the code. From my understanding, the reference x1 of type X is assigned the same reference as y which is type Y. Since x1 is assigned to the same reference as y, that means it goes from type X to Y which would be downcasting. Am I reading the code wrong?

Upvotes: 11

Views: 5331

Answers (4)

Kevin Bowersox
Kevin Bowersox

Reputation: 94469

When an instance of type Y or Z (subclass) is treated as the base type (superclass) X this is an upcast.

Upcasts are implicit(hidden) and cause a derived type to be treated as a super type.

This is an example of an upcast:

X x1 = y;

The cast is implicit (hidden) but could be thought of as:

X x1 = (X) y;

A downcast is from a super type to a derived type. So to downcast x1 to the type Y :

X x1 = y;
Y y1 = (Y) x1;

Downcasts are not implicit and must be explicitly declared. They create the potential for a ClassCastException if the instance we are casting is not of the type we are casting to.

Upvotes: 3

Ale Zalazar
Ale Zalazar

Reputation: 1980

It is upcasting, because you have an instance of type Y which is referenced by a variable of type X, and X is ´up´ (above) in the class hierarchy.

Upvotes: 0

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279990

Your class hierarchy

Object
  |
  X
 / \
Y   Z

From my understanding, the reference x1 of type X is assigned the same reference as y which is type Y. Since x1 is assigned to the same reference as y, that means it goes from type X to Y which would be downcasting. Am I reading the code wrong?

X x1 = y; // compiles ok (y is subclass of X), upcast

You're assigning y to x1. You're assigning a reference of type Y to a reference of type X. Looking at the hierarchy, you're going upwards, thus upcast.

Upvotes: 8

pedromss
pedromss

Reputation: 2453

Y extends X means that the objects you create with new Y() will be of type Y and consequently of type X. Because one derives from the other and X is the super class, any object that is of type Y is also of type X.

For example all classes in java derive from the Object class, hence, a String is a string and also an Object.

Upvotes: 0

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