Abstraction

Note

Objects in an OOP language provide an abstraction that hides the internal implementation details.

Constructors

Constrcutors are methods that construct objects, a constructor’s function name equals its Class name and has no return type.

package com.codewithmosh;

public class Employee {
    private int baseSalary;
    private int hourlyRate;
    
    // constructor
    public Employee(int baseSalary, int hourlyRate) {
        this.baseSalary = baseSalary;
        this.hourlyRate = hourlyRate;
    }
}

Use constructors in Main:

package com.codewithmosh;

public class Main {

    public static void main(String[] args) {
        Employee employee = new Employee(50_000, 20);
    }
}

Caution

If there is no explicit constructor, Java will create a default constructor that set all primitive types to 0 (or false) and set all reference types to null.

Getters and setters

Create a getter and a setter for a field:

// we can set this field private then
private int baseSalary;

// setter
public void setBaseSalary(int baseSalary) {
    if (baseSalary <= 0)
        throw new IllegalArgumentException("Salary cannot be 0 or less.");
    this.baseSalary = baseSalary;
}

// getter
public int getBaseSalary() {
    return baseSalary;
}

Coupling

Coupling is about how much a class is dependent upon or coupled to another class.

We want to reduce the coupling between classes.

Method overloading

Creating different implementations with different parameters.

public int calculateWage(int extraHours) {
    return baseSalary + (extraHours * hourlyRate);
}

public int calculateWage() {
    return calculateWage(0);
}

Constructor overloading:

public Employee(int baseSalary, int hourlyRate) {
    setBaseSalary(baseSalary);
    setHourlyRate(hourlyRate);
}

public Employee(int baseSalary) {
    this(baseSalary, 0);
}

Caution

Remember overloading a method too many times will make your application hard to maintain.

Static members

A class can have two types of members.

• instance members: belong to instances or objects. we can access through the . operator(e.g. employee.calculateWage())

• static members: or class members. belong to a class, not objects(e.g. Employee.numberOfEmployees). When we want to present a concept that should be in a single place, the value is independent of objects, and we want to share it between all objects, set it static.

Employee:

package com.codewithmosh;

public class Employee {
    private int baseSalary;
    private int hourlyRate;
    // static field
    public static int numberOfEmployees;
    // static method
    public static void printNumberOfEmployees() {
        System.out.println(numberOfEmployees);
    }

    public Employee(int baseSalary, int hourlyRate) {
        setBaseSalary(baseSalary);
        setHourlyRate(hourlyRate);
        numberOfEmployees++;
    }
}

Main:

package com.codewithmosh;

public class Main {

    public static void main(String[] args) {
        Employee employee = new Employee(50_000, 20);
        // use static field
        System.out.println(Employee.numberOfEmployees);
        // use static methods
        Employee.printNumberOfEmployees();
    }
}

Note

Static methods can only see the other static methods in this class.
main method should be static, this enables the java runtime to directly call the method without having to create a new object.