OO

Encapsulation

Access Levels

Default Access

aka Package access level

If no OOP#Access Specifiers are used with a member, it becomes default.

OOP#Public

OOP#Protected

OOP#Private

OOP#Access specifiers applied to classes

• Top-level classes other than the main public class can have only default (unspecified) package level access in Java.

Sealed Classes

Sealed classes restrict which classes can inherit them, enhancing encapsulation and giving more control.

Introduced in Version#17.

public abstract sealed class Shape permits Circle, Rectangle {
	// Common methods and fields
}

public final class Circle extends Shape {
	// Circle-specific implementation
}

public final class Rectangle extends Shape {
	// Rectangle-specific implementation
}

Inheritance

• Implemented by extends keyword

Types of Inheritance in Java

• OOP#Single Inheritance
• OOP#Multilevel Inheritance
• OOP#Hierarchical Inheritance
• OOP#Hybrid Inheritance

OOP#Multiple inheritance not allowed

Inheritance in case of #Default Access

• Are fields and methods with default access inherited?

Only if the subclass is located in the same package as the superclass.

• How is multiple inheritance handled in case of methods with default access?

In case both parent interfaces have a default method with same method signature, the implementing class should explicitly tell which one its trying to use or it should override the default method.

//If I1 and I2 both have a fun() as default method
class C implements I1, I2{
	I1.super.fun(); //Use I1's fun() method
}

Polymorphism

Overriding

Overriding by return types

Before Java 5 it wasn't possible to override a method based on its return type. Now, we can override a method if its return types are in the same direction as the subclass, that is, if the methods have OOP#Covariant Return Types.

Features

OOP#Object Context Qualifiers

• this - OOP#Self-reference
• super - OOP#Super

Inner Class

Anonymous Inner Class

main(){
	A obj = new A();{
				func(){
					Syso("This is an anonymous class")
			}
	}
}

OR (post Java 8)

main(){
	A obj = () -> Syso("anonymous class")
}

Member Inner Class

If class B is inside class A => B is a member and an inner class

main(){
	A obj = new A();
	A.B obj1 = obj.new B();
}

//If j is member of class B
obj1.j=5;

.class files generated in this case:

• A.class
• A$B.class

'Qualified this' construct

#^064042

Member classes have an implicit reference to an instance of the enclosing (outer) class. Qualified This refers to this instance.

If the outer class is A and inner class is B, you can refer to this instance of A from B as A.this.

Since we are using a qualifier along with this, to form a fully qualified identifier, it is called "Qualified" this.

Reference

• What does "qualified this" construct mean in java?

In Effective Java inside the item "Item 22: Favor static member classes over nonstatic" Josh Bloch says:

Each instance of a nonstatic member class is implicitly associated with an enclosing instance of its containing class. Within instance methods of a nonstatic member class, you can invoke methods on the enclosing instance or obtain a reference to the enclosing instance using the qualified this construct. ^064042