Java OOP principles with examples.

Abstraction.java

// Abstraction - separating something out of the class and reusing it anywhere else.
public class Person {
    String firstName;
    String lastName;
    short age;
    Address address; // Taking address specific fields out into another class
}

// This now can be used anywhere else
class Address {    
    String streetName;
    short streetNumber;
    short flatNumber;
}

class Employee {
    // some fields ...
    Address address;
}

Encapsulation.java

// 1) Encapsulation - keep fields private and provide access using public methods.
public class Person {

    private String firstName;
    private String lastName;
    private short age;
    private float weight;

    public Person() {
        super();
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public short getAge() {
        return age;
    }

    public void setAge(short age) {
        this.age = age;
    }

    public float getWeight() {
        return weight;
    }

    public void setWeight(float weight) {
        this.weight = weight;
    }
}

Inheritance.java

// Inheritance - lets you create new classes that shares some of the attributes of existing class
public class Vehicle {
    float speed;
    double weight;
    // etc.
}

class Car extends Vehicle {
    short wheels;
    // etc.
}

class Plane extends Vehicle {
    short wings;
    float wingLength;
    // etc.
}

Polymorphism.java

// Polymorphism - lets programmers use the same word to mean different things in different contexts.
//
// One form of polymorphism in Java is method overloading. That’s when different meanings are implied
// by the code itself. The other form is method overriding. That’s when the different meanings are
// implied by the values of the supplied variables.
public class Person {    
    void walk() {
        System.out.println("Running...");
    }
}

class Employee extends Person {    
    void walk() {
        System.out.println("Running fast...");
    }
    
    public static void main(String arg[]) {
        Person p = new Employee();
        p.walk();
    }
}