Raja PLECI: What is Type Casting in Java - Casting one Class to other class or interface Example

Type casting in Java is to cast one type, a class or interface, into
another type i.e. another class or interface. Since Java is an Object oriented
programming language and supports both Inheritance and Polymorphism, It’s easy that
Super class reference variable is pointing to Sub Class object but catch here
is that there is no way for Java compiler to know that a Super class variable is
pointing to Sub Class object. Which means you can not call method which is
declared on sub class. In order to do that, you first need to cast the Object back into its original
type. This is called type-casting in Java. This will be more clear when
we see an example of type casting in next section. Type casting comes with risk
of ClassCastException in Java, which is quite common with method which
accept Object type and later type cast into more specific type. we will see
when ClassCastException comes during type casting and How to avoid it in
coming section of this article. Another
worth noting point here is that from Java 5 onwards you can use Generics to write type-safe code
to reduce amount of type casting in Java which also reduces risk of java.lang.ClassCastException at
runtime.

What is type casting in Java

From first paragraph, we pretty much know What is type casting in Java. Anyway, In simple words type casting
is process of converting one type, which could be a class or interface to another, But as
per rules of Java programming language only classes or interfaces (collectively
known as Type) from same Type hierarchy can be cast or converted into each
other. If you try to cast two object which doesn't share same type hierarchy,
i.e. there is no parent child relationship between them, you will get compile time error. On the
other hand if you type cast objects from same type hierarchy but the object
which you are casting are not of the same type on which you are casting then it
will throw ClassCastException in Java. Some people may ask
why do you need type casting? well
you need type casting to get access of fields and methods declared on target
type or class. You can not access them with any other type. Let's see a simple
example of type casting in Java with two classes Base and Derived which
shares same type hierarchy.

Type
casting example in Java

In this Example of type casting in Java we have two classes, Base and Derived. Derived class
extends Base i.e. Base is a Super class and Derived is a
Sub class. So there type hierarchy looks like following tree :

Base

Derived

Now look at following code :

Base b = new
Derived();
//reference variable of Base class points object
of Derived class

Derived d = b; //compile
time error, requires casting

Derived d = (Derived)
b; // type
casting Base to Derived

Above code type casting object of Derived class into
Base class and it will throw ClassCastExcepiton if b is not an
object of Derived class. If Base and Derived class are
not related to each other and doesn't part of same type hierarchy, cast will
throw compile time error. for example you can not cast String and StringBuffer, as
they are not from same type hierarchy.

Type-casting and ClassCastExcepiton in Java

As explained in last section type casting can result in ClassCastException in Java. If
the object you are casting is of
different type. ClassCastException is quite common while using
Java collection framework classes e.g. ArrayList, LinkedList or HashSet etc because they
accept object of type java.lang.Object, which allows insertion of any
object into collection. let's a real life example of ClassCastException in Java
during type casting :

ArrayList names = new
ArrayList();

names.add("abcd"); //adding String

names.add(1); //adding
Integer

String name = (String) names.get(0); //OK

name = (String) names.get(1); // throw ClassCastException because you can not convert
Integer to String

In above example we have an ArrayList of String which
stores names. But we also added an incorrect name which is Integer and when
we retrieve Object from collection they are of type java.lang.Object which
needs to be cast on respective for performing operation. This leads into java.lang.ClassCastException when we
try to type cast second object, which is an Integer, into String. This problem
can be avoided by using Generics in Java which we will
see in next section.

Generics and Type Casting in Java

Generics was introduced in Java 5 along with another type-safe feature Enum, which ensures type
safety of code during compile time, So rather you getting ClassCastException during runtime
by type casting you get compile timer error why your code violate type safety.
Use of Generics also removes casting from many places e.g. now while retrieving
object from Collection you don't need to type cast into respective type. Here
is the modified version of same code which is free of ClassCastException because
use of Generics :

ArrayList<String> names = new ArrayList<String>(); //ArrayList of
String only accept String

names.add("abcd");

names.add(1); //compile
time error you can not add Integer into ArrayList of String

String name = names.get(0); // no type casting requires

If you are new to Generics, those angle bracket denotes type. to learn
more about Generics, See How Generics works in Java.

In this Java tutorial we learn What is type casting in Java, how
to cast one class to another class or interface in Java with a simple type casting Example. We
have also seen risk of ClassCastException related to type casting and
How can we reduce that risk by using Generics in Java. In Summary minimize type
casting in Java program and use type-safe Enum and Generics to reduce or
completely eliminate type casting from Java code.

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