Inheritance

Inheritance is inheriting the properties of a class and adding some additional fields and methods.
If there are multiple entities with some common fields/states as well as some specific fields/states, then we can define a common class for both of them and specific classes can be extended from the common class. Specfic fields/states can be added to the specific classes.

e.g If we take the example of a bank then we can have two entities Employee and Customer, and both are person. So both entities inherit the attributes of a person.

Lets have a look at the below class Person that represents a person:

package oops.inheritance;

public class Person {
	String name;
	String address;

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getAddress() {
		return address;
	}

	public void setAddress(String address) {
		this.address = address;
	}
}

Employee class extending Person class :

package oops.inheritance;

public class Employee extends Person {
	int employeeId;
	String department;

	public int getEmployeeId() {
		return employeeId;
	}

	public void setEmployeeId(int employeeId) {
		this.employeeId = employeeId;
	}

	public String getDepartment() {
		return department;
	}

	public void setDepartment(String department) {
		this.department = department;
	}
}

Here Employee class is inheriting the name and address fields from Person class.
And on top of name/address property adding it's own fields employeeId and department.

Customer class extending Person class :

package oops.inheritance;

public class Customer extends Person {
	String customerId;
	long accountNumber;

	public String getCustomerId() {
		return customerId;
	}

	public void setCustomerId(String customerId) {
		this.customerId = customerId;
	}

	public long getAccountNumber() {
		return accountNumber;
	}

	public void setAccountNumber(long accountNumber) {
		this.accountNumber = accountNumber;
	}
}

Here Customer class is inheriting the name and address fields from Person class.
And on top of name/address property adding it's own fields customerId and accountNumber.

Important Rules of Inheritance

A parent can hold reference to its child class objects and invoke only those methods of child whose are inherited/overridden from parent class. Because at compile time, parent class knows only those methods which are defined as part of itself, and overridden methods are identified at run time.
So even if you try to call a method that is not inherited/overridden from parent class, it will fail at compile time.