Java- Object Type Casting / Chapter - 10 / OOP's Concept

Now that we have learned that,

We can use parent reference to hold child object.

example:-     Object  o   =  new String ("javabychetan");

We can use interface reference to hold child object implemented class object.

example:-     Runnable      r   =  new Thread ( );

let's understand the syntax of "How to do Type-Casting".

Before Doing type casting we have to apply or check all these three rules otherwise, we won't able to perform type casting.

Rule 1. (Compile-Time Checking 1)

The type of 'd'  and 'c' must have some relation (either child to parent or parent to child or some other type).Otherwise, we will get Compile time error saying  "incompatible type found:d type required c type.

example 1:-

Object            o           = new      String ("javabychetan");

StringBuffer   sb      = new       (StringBuffer) o ;                 valid type casting

example 2:-

String          s           = new     String ("javabychetan");

StringBuffer   sb      = new      (StringBuffer) s ;

Compile time error saying  "incompatible type found :java.lang.string  required type java.lang.StringBuffer.

Rule 2. (Compile Time Checking 2)

'c' must be either same type or derived type 'A'.Otherwise, we will get Compile time error saying  "incompatible type found:C type required A type.

example 1:-

Object            o           = new      String ("javabychetan");

StringBuffer   sb      = new       (StringBuffer) o ;                 valid type casting

example 2:-

String          s           = new     String ("javabychetan");

StringBuffer   sb      = new      (String) s ;

Compile time error saying  "incompatible type found :java.lang.string  required type java.lang.StringBuffer.

Rule 3. (Run Time Checking )

Runtime object type of' must be either same type or derived the type of 'c' type. Otherwise, we will get runtime exception saying"ClassCastException".

example 1:-

Object            o           = new      String ("javabychetan");

StringBuffer   sb      = new       (StringBuffer) o ;

Runtime Exception Saying"ClassCastException".

example 2:-

Object            o           = new      String ("javabychetan");

String   sb      = new       (String) o ;

this is valid type casting.

Note: Before Doing type casting we have to apply or check all these three rules otherwise, we won't able to perform type casting.