Widening

Widening, also known as up-casting / automatic conversion that takes place in the following situations :

• When a small primitive type value is automatically accommodated in a bigger/wider primitive data type.
• When a reference variable of a subclass is automatically accommodated in the reference variable of its super class.

Note : Widening happen when data type are compatible or conversion from lower to bigger type. Widening is automatic process by Java compiler.

Primitive Type Widening Example

Order of Widening conversion

Byte->Short->Int->Long->Float->Double

public class PrimitiveWidening {

	public static void main(String[] args) {
		int i=200;
		float f=i;
		long l=i;

		System.out.println("Int :"+i);
		System.out.println("Float :"+f);
		System.out.println("Long :"+l);
	}
}

Output

Int :200
Float :200.0
Long :200

Here type conversion is from lower or upper that is widening of primitive type data and that’s not required any type of casting explicitly.

Reference Type Widening Example

public class Doctor {
	public void getDetail() {
		System.out.println("Doctor Detail !!!");
	}
}

public class Surgeon extends Doctor {
	public void getDetail() {
		System.out.println("Surgeon Detail !!!");
	}
}

public class ReferenceTypeWidening {

	public static void main(String[] args) {
		Surgeon s=new Surgeon();
        //Reference of Sub class(Surgeon) type is widened to the Super class (Doctor)
		Doctor d=s;
		d.getDetail();
	}
}

Output

Surgeon Detail !!!

In the previous code, we have a class Doctor extended by class Surgeon, hence Doctor is a super class and Surgeon is its subclass. Method getDetail() of super class Doctor is overridden in subclass Surgeon.
We have created an object of subclass Surgeon, which is assigned to its reference, s.
This subclass Surgeon reference, s is widened/up-casted and assigned to super class Doctor reference, d.

Narrowing

Narrowing also known as down-casting/explicit casting is conversion that take place in such situations:

• Narrowing a wider/bigger primitive type value to a smaller primitive type value..
• Narrowing the super class reference to the reference of its subclass, during inheritance.

Note : Narrowing happen when data type are incompatible or conversion from bigger to lower type. That require explicit casting of data. While narrowing of data type some of data values can lost.

Primitive Type Narrowing Example

Order of Narrowing conversion

Double->Float->Long->Int->Short->Byte

public class PrimitiveNarrowing {

	public static void main(String[] args) {
		double d=2567834.38;
		//Narrowing required explicitly type casting
		float f=(float)d;
		long l=(long)d;
		int i=(int)d;
		short s=(short)d;
		byte b=(byte)d;

		System.out.println("Double :"+d);
		System.out.println("Float :"+f);
		System.out.println("Long :"+l);
		System.out.println("Int :"+i);
		System.out.println("Short :"+s);
		System.out.println("Byte :"+b);
	}
}

Output

Double :2567834.38
Float :2567834.5
Long :2567834
Int :2567834
Short :11930
Byte :-102

Here type conversion is from upper or lower type that is narrowing of primitive type data and that’s required explicit type casting and will loss some of values because every smaller type data have limited range.

Reference Type Widening Example

public class Doctor {
	public void getDetail() {
		System.out.println("Doctor Detail !!!");
	}
}

public class Surgeon extends Doctor {
	public void getDetail() {
		System.out.println("Surgeon Detail !!!");
	}
}

public class ReferenceTypeNarrowing {

	public static void main(String[] args) {
		//Object of sub class is reference by reference of super class(Doctor)
		Doctor d=new Surgeon();
        //Reference of Superclass(Doctor) type is narrowing to the Sub class (Surgeon)
		Surgeon s=(Surgeon)d;
		s.getDetail();
	}

}

Output

Surgeon Detail !!!

In the previous code, we have a class Doctor extended by class Surgeon, hence Doctor is a super class and Surgeon is its subclass. Method getDetail() of super class Doctor is overridden in subclass Surgeon.
We have created an object of subclass(Surgeon), which is assigned to the reference of super class(Doctor), d.
This super class(Doctor) reference, d is narrowed/down-casted and assigned to subclass(Surgeon) reference, s.

Now we have understand the concept of Widening and Narrowing of data type/reference type values. These are straight forward case where we can easily understand like that required widening or narrowing of data.
In next part we will see how to handle widening or narrowing of data while executing an expression.

Type Promotion in Expressions

While execution of an expression intermediate value may exceed that range of operands and hence expression value will be promoted. Some conditions for type promotion are:

• Java automatically promotes each byte, short, or char operand to int when evaluating an expression.
• If one operand is a long, float or double the whole expression is promoted to long, float or double respectively.

Example

public class TypePromotionInExpression {

public static void main(String[] args) {
byte b = 52;
char c = 'b';
short s = 1026;
int i = 60000;
float f = 4.57f;
double d = .4231;

// The Expression
double result = (f * b) + (i / c) - (d * s);

//Result after all the promotions are done as double
System.out.println("Expression Result = " + result);

//Explicit Type Casting as Int. As Int is whole number
//that's what above double result will lost precision.
int intResult=(int)result;
System.out.println("Int Result = " + intResult);
}
}

Expression Result = 415.5394146484375
Int Result = 415

As per above type promotion conditions for expression there is one value type is double as bigger type that’s what complete expression result will convert to double type.

As we have learn about narrowing of data required between incompatible type or bigger to lower type conversion. Here expression result is double and want to convert as int that required explicit type conversion and that will lost precision value because int is whole number and not have any precision value.

References

https://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html