Type casting Overview

Type casting is the process of converting one data type into another. In Java, type casting can be divided into two main types:

1. Data-type casting
2. Class-type casting

1. Data-type Casting

Converting one data type to another data type is called as data type casting. Data-type Casting is classified into two types

Widening ( Implicit Casting )
Narrowing ( Explicit Casting )

Widening (Implicit Casting):

When a smaller data type is converted into a larger data type, it is called widening or implicit casting. This happens automatically as there is no risk of data loss.

Syntax:

// Implicit conversion from int to double
double x = 25;  
System.out.println(x); // Output: 25.0

Narrowing (Explicit Casting):

Casting a larger data type to a smaller data type is called narrowing or explicit casting. Since there is a possibility of data loss, it must be done explicitly by the programmer.

Syntax:

// Explicit conversion from double to int
int y = (int) 75.25;
System.out.println(y); // Output: 75

Example:

package com.quipoin;

public class Demo1 {
	public static void main(String[] args) {
	
		double x=125;                       // Widening happens implicitly
		//integer type is converted to double type
		System.out.println("Value of x:"+x);
		System.out.println("------------------");

		int y=(int) 8.5;              // Narrowing happens explicitly
		//double type is converted to integer type
		System.out.println("Value of y:"+y);
	}
}

Output:

Value of x:125.0
------------------
Value of y:8

Example: here we see both 'widening' and 'narrowing'

package com.quipoin;

public class Calculator {
	static void square(int n) {
		int res;
		res=n*n;
		System.out.println("Square of "+n+" is : "+res);
	}
	static void square(double n) {
		double res = n*n;
		System.out.println("Square of "+n+" is : "+res);
	}
	public static void main(String[] args) {
		
		System.out.println("---------------------------------");

		System.out.println("Performing Narrowing Operation!!");
		Calculator.square(15);
		Calculator.square((int) 25.5);        // Narrowing explicitly
		System.out.println("--------------------------------");

		System.out.println("Performing Widening Operation!!");
		Calculator.square(5.5);
		Calculator.square(10);                 // Widening implicitly
	}
}

Output:

---------------------------------
Performing Narrowing Operation!!
Square of 15 is : 225
Square of 25 is : 625
--------------------------------
Performing Widening Operation!!
Square of 5.5 is : 30.25
Square of 10 is : 100

2. Class-type Casting

Class-type casting involves converting one object type to another within an inheritance hierarchy. It is divided into:

Upcasting
Downcasting

Upcasting

Converting the subclass type to the superclass type is known as upcasting in other words creating an object of sub-class type and storing its address into super class type.

Syntax :

Superclass obj = new Subclass();

Key Point

Upcasting will be performed either implicitly or explicitly.
If the compiler performs up-casting on its own then it is implicit up-casting, and if casting is performed in the code by the user then it is called explicit up-casting
To achieve up-casting inheritance is a must.

Example:

package com.quipoin;

class Demo1 {
	int x=10;
	void test() {
		System.out.println("Running test method!!");
	}
}
class Demo2 extends Demo1{
	int y=20;
	void disp() {
		System.out.println("Running disp method!!");
	}
}
public class MainD1D2 {
	public static void main(String[] args) {
		Demo1 d1=new Demo2();//up casting
		System.out.println("Value of x : "+d1.x);
		d1.test();
	}
}

Output:

Value of x : 10
Running test method!!

NOTE: When we perform upcasting inherited members/properties get hidden, only we can fetch superclass members.

Downcasting

Converting a superclass object back to a subclass type is called downcasting. This must be done explicitly using type casting.

Syntax :

Subclass obj = (Subclass) superclassReference;

Key Point

Downcasting should be performed explicitly in the code by the user.
To achieve down casting up casting is a must, in other words down casting should be performed only on the upcasted objects otherwise JVM throws ClassCastException.
When we create an object of subclass it contains its own properties with super class properties, hence we can fetch superclass properties from down casted reference.

Example:

package com.quipoin;

class Demo1 {
	int x=10;
	void test() {
		System.out.println("Running test method!!");
	}
}
class Demo2 extends Demo1{
	int y=20;
	void disp() {
		System.out.println("Running disp method!!");
	}
}
public class MainD1D2 {
	public static void main(String[] args) {
		Demo1 d1=new Demo2();//up casting
		Demo2 d2=(Demo2) d1;//down casting
		System.out.println("Value of x : "+d2.x);
		d2.test();
		System.out.println("Value of y : "+d2.y);
		d2.disp();
	}
}

Output:

Value of x : 10
Running test method!!
Value of y : 20
Running disp method!!

Two Minute Drill

Widening (Implicit Casting): Converts smaller data types to larger ones automatically.
Narrowing (Explicit Casting): Converts larger data types to smaller ones with explicit instructions.
Upcasting: Converts a subclass object to a superclass type.
Downcasting: Converts a superclass object back to a subclass type but requires explicit casting.