Java Object-oriented programming
last modified January 27, 2024
In this article we cover object-oriented programming in Java. We mention Java objects, object attributes and methods, object constructors, and access modifiers. Furthermore, we talk about the super keyword, constructor chaining, class constants, inheritance, polymorphism, final classes, and private constructors.
There are three widely used programming paradigms: procedural programming, functional programming, and object-oriented programming. Java is principally an object-oriented programming language. Since Java 8, it has some support of the functional programming.
Object-oriented programming
Object-oriented programming (OOP) is a programming paradigm that uses objects and their interactions to design applications and computer programs.
The following are basic programming concepts in OOP:
- Abstraction
- Polymorphism
- Encapsulation
- Inheritance
The abstraction is simplifying complex reality by modeling classes appropriate to the problem. The polymorphism is the process of using an operator or function in different ways for different data input. The encapsulation hides the implementation details of a class from other objects. The inheritance is a way to form new classes using classes that have already been defined.
Java objects
Objects are basic building blocks of a Java OOP program. An object is a combination of data and methods. In an OOP program, we create objects. These objects communicate together through methods. Each object can receive messages, send messages, and process data.
There are two steps in creating an object. First, we define a class. A class is a template for an object. It is a blueprint which describes the state and behavior that the objects of the class all share. A class can be used to create many objects. Objects created at runtime from a class are called instances of that particular class.
package com.zetcode;
class Being {}
public class SimpleObject {
public static void main(String[] args) {
Being b = new Being();
System.out.println(b);
}
}
In our first example, we create a simple object.
class Being {}
This is a simple class definition. The body of the template is empty. It does not have any data or methods.
Being b = new Being();
We create a new instance of the Being class. For this we have the
new keyword. The b variable is the handle to the
created object.
System.out.println(b);
We print the object to the console to get some basic description of the object.
What does it mean, to print an object? When we print an object, we in fact call
its toString method. But we have not defined any method yet. It is
because every object created inherits from the base Object. It has
some elementary functionality which is shared among all objects created. One of
this is the toString method.
$ javac com/zetcode/SimpleObject.java $ ls com/zetcode/ Being.class SimpleObject.class SimpleObject.java
The compiler creates two class files. The SimpleObject.class is
the application class and the Being.class is the custom class
that we work with in the application.
$ java com.zetcode.SimpleObject com.zetcode.Being@125ee71
We get a the name of the class of which the object is an instance, the @ character, and the unsigned hexadecimal representation of the hash code of the object.
Java object attributes
Object attributes is the data bundled in an instance of a class. The object attributes are called instance variables or member fields. An instance variable is a variable defined in a class, for which each object in the class has a separate copy.
package com.zetcode;
class Person {
public String name;
}
public class ObjectAttributes {
public static void main(String[] args) {
Person p1 = new Person();
p1.name = "Jane";
Person p2 = new Person();
p2.name = "Beky";
System.out.println(p1.name);
System.out.println(p2.name);
}
}
In the above Java code, we have a Person class with
one member field.
class Person {
public String name;
}
We declare a name member field. The public keyword specifies that
the member field will be accessible outside the class block.
Person p1 = new Person(); p1.name = "Jane";
We create an instance of the Person class and set the name variable
to "Jane". We use the dot operator to access the attributes of objects.
Person p2 = new Person(); p2.name = "Beky";
We create another instance of the Person class.
Here we set the variable to "Beky".
System.out.println(p1.name); System.out.println(p2.name);
We print the contents of the variables to the console.
$ java com.zetcode.ObjectAttributes Jane Beky
We see the output of the program. Each instance of the Person
class has a separate copy of the name member field.
Java methods
Methods are functions defined inside the body of a class. They are used to perform operations with the attributes of our objects. Methods bring modularity to our programs.
Methods are essential in the encapsulation concept of the OOP paradigm.
For example, we might have a connect method in our
AccessDatabase class. We need not to be informed how exactly the
method connect connects to the database. We only have to know that
it is used to connect to a database. This is essential in dividing
responsibilities in programming, especially in large applications.
Objects group state and behavior. Methods represent the behavioral part of the objects.
package com.zetcode;
class Circle {
private int radius;
public void setRadius(int radius) {
this.radius = radius;
}
public double area() {
return this.radius * this.radius * Math.PI;
}
}
public class Methods {
public static void main(String[] args) {
Circle c = new Circle();
c.setRadius(5);
System.out.println(c.area());
}
}
In the code example, we have a Circle class. In the class, we
define two methods. The setRadius method assigns a value to the
radius member and the area method computes an area of
the circle from the class member and a constant.
private int radius;
We have one member field in the class. It is the radius of the circle. The
private keyword is an access specifier. It tells that the variable
is restricted to the outside world. If we want to modify this variable from the
outside, we must use the publicly available setRadius method. This
way we protect our data.
public void setRadius(int radius) {
this.radius = radius;
}
This is the setRadius method. The this variable is a
special variable which we use to access the member fields from methods. The
this.radius is an instance variable, while the radius
is a local variable, valid only inside the setRadius method.
Circle c = new Circle(); c.setRadius(5);
We create an instance of the Circle class and set its radius by
calling the setRadius method on the object of the circle. The dot
operator is used to call the method.
public double area() {
return this.radius * this.radius * Math.PI;
}
The area method returns the area of a circle. The
Math.PI is a built-in constant.
$ java com.zetcode.Methods 78.53981633974483
Java access modifiers
Access modifiers set the visibility of methods and member fields.
Java has three access modifiers: public, protected, and
private. The public members can be accessed from
anywhere.
The protected members can be accessed only within the
class itself, by inherited classes, and other classes from the same package.
Finally, the private members are limited to the containing type,
e.g. only within its class or interface. If we do not specify an access
modifier, we have a package-private visibility. In such a case, members and
methods are accessible within the same package.
Access modifiers protect data against accidental modifications. They make the programs more robust.
| Class | Package | Subclass (same package) | Subclass (other package) | World | |
|---|---|---|---|---|---|
| public | + | + | + | + | + |
| protected | + | + | + | + | o |
| no modifier | + | + | + | o | o |
| private | + | o | o | o | o |
The above table summarizes Java access modifiers (+ is accessible, o is not accessible).
package com.zetcode;
class Person {
public String name;
private int age;
public int getAge() {
return this.age;
}
public void setAge(int age) {
this.age = age;
}
}
public class AccessModifiers {
public static void main(String[] args) {
Person p = new Person();
p.name = "Jane";
p.setAge(17);
System.out.println(String.format("%s is %d years old",
p.name, p.getAge()));
}
}
In the above program, we have two member fields: public and private.
public int getAge() {
return this.age;
}
If a member field is private, the only way to access it is via methods. If we
want to modify an attribute outside the class, the method must be declared
public. This is an important aspect of data protection.
public void setAge(int age) {
this.age = age;
}
The setAge method enables us to change the private
age variable from outside of the class definition.
Person p = new Person(); p.name = "Jane";
We create a new instance of the Person class. Because the
name attribute is public, we can access it directly.
However, this is not recommended.
p.setAge(17);
The setAge method modifies the age member field. It
cannot be accessed or modified directly, because it is declared
private.
System.out.println(String.format("%s is %d years old",
p.name, p.getAge()));
Finally, we access both members to build a string, which is printed to the console.
$ java com.zetcode.AccessModifiers Jane is 17 years old
Running the example we have this output.
The following program shows how access modifiers influence the way members are inherited by subclasses.
package com.zetcode;
class Base {
public String name = "Base";
protected int id = 5323;
private boolean isDefined = true;
}
class Derived extends Base {
public void info() {
System.out.println("This is Derived class");
System.out.println("Members inherited:");
System.out.println(this.name);
System.out.println(this.id);
// System.out.println(this.isDefined);
}
}
public class ProtectedMember {
public static void main(String[] args) {
Derived drv = new Derived();
drv.info();
}
}
In this program, we have a Derived class which inherits from the
Base class. The Base class has three member fields,
all with different access modifiers. The isDefined
member is not inherited. The private modifier prevents this.
class Derived extends Base {
The Derived class inherits from the Base class. To
inherit from another class, we use the extends keyword.
System.out.println(this.name); System.out.println(this.id); // System.out.println(this.isDefined);
The public and the protected members are inherited by
the Derived class. They can be accessed. The private
member is not inherited. The line accessing the member field is commented. If we
uncommented the line, the code would not compile.
$ java com.zetcode.ProtectedMember This is Derived class Members inherited: Base 5323
Running the program, we receive this output.
Java constructor
A constructor is a special kind of a method. It is automatically called when the
object is created. Constructors do not return values and also do not use the
void keyword. The purpose of the constructor is to initiate the
state of an object. Constructors have the same name as the class. The
constructors are methods, so they can be overloaded too. Constructors cannot be
directly invoked. The new keyword invokes them. Constructors cannot
be declared synchronized, final, abstract, native, or static.
Constructors cannot be inherited. They are called in the order of inheritance. If we do not write any constructor for a class, Java provides an implicit default constructor. If we provide any kind of a constructor, then the default is not supplied.
package com.zetcode;
class Being {
public Being() {
System.out.println("Being is created");
}
public Being(String being) {
System.out.println(String.format("Being %s is created", being));
}
}
public class Constructor {
@SuppressWarnings("ResultOfObjectAllocationIgnored")
public static void main(String[] args) {
new Being();
new Being("Tom");
}
}
We have a Being class. This class has two constructors. The first one does not take parameters, the second one takes one parameter.
public Being() {
System.out.println("Being is created");
}
This constructor does not take any parameters.
public Being(String being) {
System.out.println(String.format("Being %s is created", being));
}
This constructor takes one string parameter.
@SuppressWarnings("ResultOfObjectAllocationIgnored")
This annotation will suppress a warning that we do not assign our created objects to any variables. Normally this would be a suspicious activity.
new Being();
An instance of the Being class is created. The no-argument
constructor is called upon object creation.
new Being("Tom");
Another instance of the Being class is created. This time the
constructor with a parameter is called upon object creation.
$ java com.zetcode.Constructor Being is created Being Tom is created
In the next example, we initiate data members of the class. Initiation of variables is a typical job for constructors.
package com.zetcode;
class User {
private String occupation;
private String name;
public User(String name, String occupation) {
this.name = name;
this.occupation = occupation;
}
public void info() {
System.out.format("%s is a %s\n", this.name, this.occupation);
}
}
public class MemberInit {
public static void main(String[] args) {
String name = "John Doe";
String occupation = "gardener";
User u = new User(name, occupation);
u.info();
}
}
We have a User class with data members and methods.
private String occupation; private String name;
We have two private variables in the class definition.
public User(String name, String occupation) {
this.name = name;
this.occupation = occupation;
}
In the constructor, we initiate the two data members. The this
keyword is a handler used to reference the object variables from methods. When
the names of constructor parameters and the names of members are equal, using
this keyword is required. Otherwise, the usage is optional.
User u = new User(name, occupation); u.info();
We create a MyFriend object with two arguments. Then we call
the info method of the object.
$ java com.zetcode.MemberInit John Doe is a gardener
Java super keyword
The super keyword is a reference variable that is used in
subclasses to refer to the immediate parent class object. It can be use to refer
to the parent's a) instance variable, b) constructor, c) method.
package com.zetcode;
class Shape {
int x = 50;
int y = 50;
}
class Rectangle extends Shape {
int x = 100;
int y = 100;
public void info() {
System.out.println(x);
System.out.println(super.x);
}
}
public class SuperVariable {
public static void main(String[] args) {
Rectangle r = new Rectangle();
r.info();
}
}
In the example, we refer to the parent's variable with the
super keyword.
public void info() {
System.out.println(x);
System.out.println(super.x);
}
Inside the info method, we refer to the parent's instance
variable with the super.x syntax.
If a constructor does not explicitly invoke a superclass constructor, Java automatically inserts a call to the no-argument constructor of the superclass. If the superclass does not have a no-argument constructor, we get a compile-time error.
package com.zetcode;
class Vehicle {
public Vehicle() {
System.out.println("Vehicle created");
}
}
class Bike extends Vehicle {
public Bike() {
// super();
System.out.println("Bike created");
}
}
public class ImplicitSuper {
public static void main(String[] args) {
Bike bike = new Bike();
System.out.println(bike);
}
}
The example demonstrates the implicit call to the parent's constructor.
public Bike() {
// super();
System.out.println("Bike created");
}
We get the same result if we uncomment the line.
$ java com.zetcode.ImplicitSuper Vehicle created Bike created com.zetcode.Bike@15db9742
Two constructors are called when a Bike object is created.
There can be more than one constructor in a class.
package com.zetcode;
class Vehicle {
protected double price;
public Vehicle() {
System.out.println("Vehicle created");
}
public Vehicle(double price) {
this.price = price;
System.out.printf("Vehicle created, price %.2f set%n", price);
}
}
class Bike extends Vehicle {
public Bike() {
super();
System.out.println("Bike created");
}
public Bike(double price) {
super(price);
System.out.printf("Bike created, its price is: %.2f %n", price);
}
}
public class SuperCalls {
public static void main(String[] args) {
Bike bike1 = new Bike();
Bike bike2 = new Bike(45.90);
}
}
The example uses different syntax of super to call different parent
constructors.
super();
Here, we call the parent's no-argument constructor.
super(price);
This syntax calls the parent's constructor that takes one parameter: the bike's price.
$ java com.zetcode.SuperCalls Vehicle created Bike created Vehicle created, price 45.90 set Bike created, its price is: 45.90
Java constructor chaining
Constructor chaining is the ability to call another constructor from a
constructor. To call another constructor from the same class, we use the
this keyword. To call another constructor from a parent class, we
use the super keyword.
package com.zetcode;
class Shape {
private int x;
private int y;
public Shape(int x, int y) {
this.x = x;
this.y = y;
}
protected int getX() {
return this.x;
}
protected int getY() {
return this.y;
}
}
class Circle extends Shape {
private int r;
public Circle(int r, int x, int y) {
super(x, y);
this.r = r;
}
public Circle() {
this(1, 1, 1);
}
@Override
public String toString() {
return String.format("Circle: r:%d, x:%d, y:%d", r, getX(), getY());
}
}
public class ConstructorChaining {
public static void main(String[] args) {
Circle c1 = new Circle(5, 10, 10);
Circle c2 = new Circle();
System.out.println(c1);
System.out.println(c2);
}
}
We have a Circle class. The class has two constructors.
One that takes one parameter and one that does not take any parameters.
class Shape {
private int x;
private int y;
...
}
The Shape class is responsible for dealing with the
x and y coordinates of various shapes.
public Shape(int x, int y) {
this.x = x;
this.y = y;
}
The constructor of the Shape class initiates the x and
y coordinates with the given parameters.
protected int getX() {
return this.x;
}
protected int getY() {
return this.y;
}
We have defined two methods to retrieve the values of the coordinates. The members are private, so the only access possible is through methods.
class Circle extends Shape {
private int r;
...
}
The Circle class inherits from the Shape class. It
defines the radius member which is specific to this shape.
public Circle(int r, int x, int y) {
super(x, y);
this.r = r;
}
The first constructor of the Circle class takes three parameters:
the radius, and the x and y coordinates.
With the super keyword, we call the parent's constructor passing
the coordinates. Note that the super keyword must be the first
statement in the constructor. The second statement initiates the
radius member of the Circle class.
public Circle() {
this(1, 1, 1);
}
The second constructor takes no parameters. In such a case, we provide some
default values. The this keyword is used to call the three-parameter
constructor of the same class, passing three default values.
@Override
public String toString() {
return String.format("Circle: r:%d, x:%d, y:%d", r, getX(), getY());
}
Inside the toString method, we provide a string representation
of the Circle class. To determine the x and y
coordinates, we use the inherited getX and getY methods.
$ java com.zetcode.ConstructorChaining Circle: r:5, x:10, y:10 Circle: r:1, x:1, y:1
Java class constants
It is possible to create class constants. These constants do not belong to a concrete object. They belong to the class. By convention, constants are written in uppercase letters.
package com.zetcode;
class Math {
public static final double PI = 3.14159265359;
}
public class ClassConstant {
public static void main(String[] args) {
System.out.println(Math.PI);
}
}
We have a Math class with a PI constant.
public static final double PI = 3.14159265359;
The final keyword is used to define a constant. The
static keyword enables to refer the member without creating an
instance of the class. The public keyword makes it accessible
outside the body of the class.
$ java com.zetcode.ClassConstant 3.14159265359
Running the example we get the above output.
Java toString method
Each object has the toString method. It returns a human-readable
representation of an object. The default implementation returns the fully
qualified name of the type of the Object. When we call the
System.out.println method with an object as a parameter, the
toString is being called.
package com.zetcode;
class Being {
@Override
public String toString() {
return "This is Being class";
}
}
public class ThetoStringMethod {
public static void main(String[] args) {
Being b = new Being();
Object o = new Object();
System.out.println(o.toString());
System.out.println(b.toString());
System.out.println(b);
}
}
We have a Being class in which we override the default
implementation of the toString method.
@Override
public String toString() {
return "This is Being class";
}
Each class created inherits from the base Object. The
toString method belongs to this object class. The
@Override annotation informs the compiler that the element is meant
to override an element declared in a superclass. The compiler will then check
that we did not create any error.
Being b = new Being(); Object o = new Object();
We create two objects: one custom defined and one built-in.
System.out.println(o.toString()); System.out.println(b.toString());
We call the toString method explicitly on these two objects.
System.out.println(b);
As we have specified earlier, placing an object as a parameter to the
System.out.println will call its toString
method. This time, we have called the method implicitly.
$ java com.zetcode.ThetoStringMethod java.lang.Object@125ee71 This is Being class This is Being class
This is what we get when we run the example.
Inheritance in Java
Inheritance is a way to form new classes using classes that have already been defined. The newly formed classes are called derived classes, the classes that we derive from are called base classes. Important benefits of inheritance are code reuse and reduction of complexity of a program. The derived classes (descendants) override or extend the functionality of base classes (ancestors).
package com.zetcode;
class Being {
public Being() {
System.out.println("Being is created");
}
}
class Human extends Being {
public Human() {
System.out.println("Human is created");
}
}
public class Inheritance {
@SuppressWarnings("ResultOfObjectAllocationIgnored")
public static void main(String[] args) {
new Human();
}
}
In this program, we have two classes: a base Being class
and a derived Human class. The derived class inherits from
the base class.
class Human extends Being {
In Java, we use the extends keyword to create inheritance
relations.
new Human();
We instantiate the derived Human class.
$ java com.zetcode.Inheritance Being is created Human is created
We can see that both constructors were called. First, the constructor of the base class is called, then the constructor of the derived class.
A more complex example follows.
package com.zetcode;
class Being {
static int count = 0;
public Being() {
count++;
System.out.println("Being is created");
}
public void getCount() {
System.out.format("There are %d Beings%n", count);
}
}
class Human extends Being {
public Human() {
System.out.println("Human is created");
}
}
class Animal extends Being {
public Animal() {
System.out.println("Animal is created");
}
}
class Dog extends Animal {
public Dog() {
System.out.println("Dog is created");
}
}
public class Inheritance2 {
@SuppressWarnings("ResultOfObjectAllocationIgnored")
public static void main(String[] args) {
new Human();
Dog dog = new Dog();
dog.getCount();
}
}
With four classes, the inheritance hierarchy is more complicated. The
Human and the Animal classes inherit from the
Being class and the Dog class inherits directly from
the Animal class and indirectly from the
Being class.
static int count = 0;
We define a static variable. Static members are shared by all
instances of a class.
public Being() {
count++;
System.out.println("Being is created");
}
Each time the Being class is instantiated, we increase the count
variable by one. This way we keep track of the number of instances
created.
class Animal extends Being {
...
class Dog extends Animal {
...
The Animal inherits from the Being and the
Dog inherits from the Animal. Indirectly, the
Dog inherits from the Being as well.
new Human(); Dog dog = new Dog(); dog.getCount();
We create instances from the Human and from the
Dog classes. We call the getCount method of
the Dog object.
$ java com.zetcode.Inheritance2 Being is created Human is created Being is created Animal is created Dog is created There are 2 Beings
The Human object calls two constructors. The Dog
object calls three constructors. There are two Beings instantiated.
Java polymorphism
Polymorphism is the process of using an operator or function in different ways for different data input. In practical terms, polymorphism means that if class B inherits from class A, it doesn't have to inherit everything about class A; it can do some of the things that class A does differently.
In general, polymorphism is the ability to appear in different forms. Technically, it is the ability to redefine methods for derived classes. Polymorphism is concerned with the application of specific implementations to an interface or a more generic base class.
Simply put, polymorphism is the ability to redefine methods for derived classes.
package com.zetcode;
abstract class Shape {
protected int x;
protected int y;
public abstract int area();
}
class Rectangle extends Shape {
public Rectangle(int x, int y) {
this.x = x;
this.y = y;
}
@Override
public int area() {
return this.x * this.y;
}
}
class Square extends Shape {
public Square(int x) {
this.x = x;
}
@Override
public int area() {
return this.x * this.x;
}
}
public class Polymorphism {
public static void main(String[] args) {
Shape[] shapes = { new Square(5),
new Rectangle(9, 4), new Square(12) };
for (Shape shape : shapes) {
System.out.println(shape.area());
}
}
}
In the above program, we have an abstract Shape class. This class
morphs into two descendant classes: Rectangle and
Square. Both provide their own implementation of the
area method. Polymorphism brings flexibility and scalability to the
OOP systems.
@Override
public int area() {
return this.x * this.y;
}
...
@Override
public int area() {
return this.x * this.x;
}
The Rectangle and Square classes have their
own implementations of the area method.
Shape[] shapes = { new Square(5),
new Rectangle(9, 4), new Square(12) };
We create an array of three shapes.
for (Shape shape : shapes) {
System.out.println(shape.area());
}
We go through each shape and call the area method on it. The
compiler calls the correct method for each shape. This is the essence of
polymorphism.
Final class, private constructor
A class with a final modifier cannot be subclassed. A class with
a constructor that has a private modifier cannot be instantiated.
package com.zetcode;
final class MyMath {
public static final double PI = 3.14159265358979323846;
// other static members and methods
}
public class FinalClass {
public static void main(String[] args) {
System.out.println(MyMath.PI);
}
}
We have a MyMath class. This class has some static members and
methods. We do not want anyone to inherit from our class; therefore, we declare
it to be final.
Furthermore, we also do not want to allow creation of instances from our class. We decide it to be used only from a static context. Declaring a private constructor, the class cannot be instantiated.
package com.zetcode;
final class MyMath {
private MyMath() {}
public static final double PI = 3.14159265358979323846;
// other static members and methods
}
public class PrivateConstructor {
public static void main(String[] args) {
System.out.println(MyMath.PI);
}
}
Our MyMath class cannot be instantiated and cannot be subclassed.
This is how java.lang.Math is designed in Java language.
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Java Object-Oriented Programming Concepts
In this article we have covered object-oriented programming in Java.
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