Java Inheritance

Introduced in 2026-03-05-object-oriented-programming-i (Lecture, Week 2) — “things you have because your parents have them”.

Subclasses and superclasses

Inheritance allows creating a new class from an existing class:

• The new class is the subclass (child/derived class).
• The existing class it’s derived from is the superclass (parent/base class).

extends is the keyword used to implement inheritance in Java:

public class Employee {
    private String name;
    private String address;

    public Employee(String name, String address) {
        this.name = name;
        this.address = address;
    }
    public String getName() { return name; }
    public String getAddress() { return address; }
    public void setAddress(String address) { this.address = address; }
    public String printDetails() { return name + " " + address; }
}

public class ContractEmployee extends Employee {
    private double hourlyRate;
    private int hoursWorked;

    public ContractEmployee(String name, String address, double hourlyRate) {
        super(name, address);
        this.hourlyRate = hourlyRate;
        this.hoursWorked = 0;
    }
    public void logHours(int hours) { this.hoursWorked += hours; }
    public double calculateWeeklyPay() { return hourlyRate * hoursWorked; }
}

The super keyword

super is used in a subclass to access superclass members (attributes, constructors, and methods) — e.g. super(name, address) above calls Employee’s constructor to initialise the inherited fields before the subclass’s own constructor body runs.

Method overriding

The subclass inherits the attributes and methods of its superclass. If the same method signature is defined in both, the subclass’s version overrides the superclass’s version:

public class ContractEmployee extends Employee {
    @Override
    public String printDetails() {
        return "Role: Contract Employee, Hourly Rate: $" + hourlyRate + ", Hours Worked: " + hoursWorked;
    }
}

public class FullTimeEmployee extends Employee {
    @Override
    public String printDetails() {
        return "Role: Full-Time Employee, Monthly Salary: $" + monthlySalary;
    }
}

Both the superclass and subclass method MUST share the same method signature (see java-encapsulation) for this to be overriding rather than a separate overload.

super.methodName(...) can be used inside an override to still call the superclass’s version of the method, e.g. to extend rather than replace its behaviour:

public class FullTimeEmployee extends Employee {
    @Override
    public String printDetails() {
        return super.printDetails() + ", Role: Full-Time Employee, Monthly Salary: $" + monthlySalary;
    }
}