Create constructors in java

Constructors in Java – A Complete Guide

Constructor is a block of code that initializes the newly created object. A constructor resembles an instance method in java but it’s not a method as it doesn’t have a return type. In short constructor and method are different(More on this at the end of this guide). People often refer constructor as special type of method in Java.

Constructor has same name as the class and looks like this in a java code.

Note that the constructor name matches with the class name and it doesn’t have a return type.

How does a constructor work

To understand the working of constructor, lets take an example. lets say we have a class MyClass .
When we create the object of MyClass like this:

The new keyword here creates the object of class MyClass and invokes the constructor to initialize this newly created object.

You may get a little lost here as I have not shown you any initialization example, lets have a look at the code below:

A simple constructor program in java

Here we have created an object obj of class Hello and then we displayed the instance variable name of the object. As you can see that the output is BeginnersBook.com which is what we have passed to the name during initialization in constructor. This shows that when we created the object obj the constructor got invoked. In this example we have used this keyword, which refers to the current object, object obj in this example. We will cover this keyword in detail in the next tutorial.

public class Hello < String name; //Constructor Hello()< this.name = "BeginnersBook.com"; >public static void main(String[] args) < Hello obj = new Hello(); System.out.println(obj.name); >>

Types of Constructors

There are three types of constructors: Default, No-arg constructor and Parameterized.

Default constructor

If you do not implement any constructor in your class, Java compiler inserts a default constructor into your code on your behalf. This constructor is known as default constructor. You would not find it in your source code(the java file) as it would be inserted into the code during compilation and exists in .class file. This process is shown in the diagram below:

If you implement any constructor then you no longer receive a default constructor from Java compiler.

no-arg constructor:

Constructor with no arguments is known as no-arg constructor. The signature is same as default constructor, however body can have any code unlike default constructor where the body of the constructor is empty.

Although you may see some people claim that that default and no-arg constructor is same but in fact they are not, even if you write public Demo() in your class Demo it cannot be called default constructor since you have written the code of it.

Example: no-arg constructor

class Demo < public Demo() < System.out.println("This is a no argument constructor"); >public static void main(String args[]) < new Demo(); >>

Output:
This is a no argument constructor

Parameterized constructor

Constructor with arguments(or you can say parameters) is known as Parameterized constructor.

Example: parameterized constructor

In this example we have a parameterized constructor with two parameters id and name . While creating the objects obj1 and obj2 I have passed two arguments so that this constructor gets invoked after creation of obj1 and obj2.

public class Employee < int empId; String empName; //parameterized constructor with two parameters Employee(int id, String name)< this.empId = id; this.empName = name; >void info() < System.out.println("Id: "+empId+" Name: "+empName); >public static void main(String args[]) < Employee obj1 = new Employee(10245,"Chaitanya"); Employee obj2 = new Employee(92232,"Negan"); obj1.info(); obj2.info(); >>

Id: 10245 Name: Chaitanya Id: 92232 Name: Negan

Example2: parameterized constructor

In this example, we have two constructors, a default constructor and a parameterized constructor. When we do not pass any parameter while creating the object using new keyword then default constructor is invoked, however when you pass a parameter then parameterized constructor that matches with the passed parameters list gets invoked.

class Example2 < private int var; //default constructor public Example2() < this.var = 10; >//parameterized constructor public Example2(int num) < this.var = num; >public int getValue() < return var; >public static void main(String args[]) < Example2 obj = new Example2(); Example2 obj2 = new Example2(100); System.out.println("var is: "+obj.getValue()); System.out.println("var is: "+obj2.getValue()); >>

What if you implement only parameterized constructor in class

class Example3 < private int var; public Example3(int num) < var=num; >public int getValue() < return var; >public static void main(String args[]) < Example3 myobj = new Example3(); System.out.println("value of var is: "+myobj.getValue()); >>

Output: It will throw a compilation error. The reason is, the statement Example3 myobj = new Example3() is invoking a default constructor which we don’t have in our program. when you don’t implement any constructor in your class, compiler inserts the default constructor into your code, however when you implement any constructor (in above example I have implemented parameterized constructor with int parameter), then you don’t receive the default constructor by compiler into your code.

If we remove the parameterized constructor from the above code then the program would run fine, because then compiler would insert the default constructor into your code.

Constructor Chaining

When A constructor calls another constructor of same class then this is called constructor chaining. Read more about it here.

Super()

Whenever a child class constructor gets invoked it implicitly invokes the constructor of parent class. You can also say that the compiler inserts a super(); statement at the beginning of child class constructor.

class MyParentClass < MyParentClass()< System.out.println("MyParentClass Constructor"); >> class MyChildClass extends MyParentClass < MyChildClass() < System.out.println("MyChildClass Constructor"); >public static void main(String args[]) < new MyChildClass(); >>

MyParentClass Constructor MyChildClass Constructor

Constructor Overloading

Constructor overloading is a concept of having more than one constructor with different parameters list, in such a way so that each constructor performs a different task.

Refer constructor overloading with example for more details with example.

Java Copy Constructor

A copy constructor is used for copying the values of one object to another object. Java doesn’t have a concept of copy constructor like C++, however there are certain ways by which you can copy the values of one object to another object:

• By creating a parameterized constructor with class reference as parameter.
• By using clone() method of the Object class.
• By assigning the values from one object to another object.

class JavaExample < String web; JavaExample(String w)< web = w; >/* This is the Copy Constructor, it * copies the values of one object * to the another object (the object * that invokes this constructor) */ JavaExample(JavaExample je) < web = je.web; >void disp() < System.out.println("Website: "+web); >public static void main(String args[]) < JavaExample obj1 = new JavaExample("BeginnersBook"); /* Passing the object as an argument to the constructor * This will invoke the copy constructor */ JavaExample obj2 = new JavaExample(obj1); obj1.disp(); obj2.disp(); >>

Website: BeginnersBook Website: BeginnersBook

Quick Recap

1. Every class has a constructor whether it’s a normal class or a abstract class.
2. Constructors are not methods and they don’t have any return type.
3. Constructor name should match with class name .
4. Constructor can use any access specifier, they can be declared as private also. Private constructors are possible in java but there scope is within the class only.
5. Like constructors method can also have name same as class name, but still they have return type, though which we can identify them that they are methods not constructors.
6. If you don’t implement any constructor within the class, compiler will do it for.
7. this() and super() should be the first statement in the constructor code. If you don’t mention them, compiler does it for you accordingly.
8. Constructor overloading is possible but overriding is not possible. Which means we can have overloaded constructor in our class but we can’t override a constructor.
9. Constructors can not be inherited.
10. If Super class doesn’t have a no-arg(default) constructor then compiler would not insert a default constructor in child class as it does in normal scenario.
11. Interfaces do not have constructors.
12. Abstract class can have constructor and it gets invoked when a class, which implements interface, is instantiated. (i.e. object creation of concrete class).
13. A constructor can also invoke another constructor of the same class – By using this(). If you want to invoke a parameterized constructor then do it like this: this(parameter list).

More on Constructor:

Difference between Constructor and Method

I know I should have mentioned it at the beginning of this guide but I wanted to cover everything in a flow. Hope you don’t mind 🙂

1. The purpose of constructor is to initialize the object of a class while the purpose of a method is to perform a task by executing java code.
2. Constructors cannot be abstract, final, static and synchronised while methods can be.
3. Constructors do not have return types while methods do.

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About the Author

I have 15 years of experience in the IT industry, working with renowned multinational corporations. Additionally, I have dedicated over a decade to teaching, allowing me to refine my skills in delivering information in a simple and easily understandable manner.

Comments

concepts explained in a simple and effective manner, its very nice as all small minute discussions are also provided and explained in a systematic way with examples ,this site really helps beginners to have good knowledge of java .

Awesome tutorial you have explained all the concept with practically step by step:- keep it good work

hi sir,it’s very useful.easy to learn.can you explain more clearly about method overriding in java with example and it’s usage! please!!

What happens when super class only has parameterized constructors and child constructor does not evoke super(parameter) explicitly?

Great, pls I am a graphic designer and beginning to learn programming, can you pls help me understand the basics such as objects, methods

Hii,Chaitanya please provide next page option at the bottom.so that it will be easy for the readers.
Thank you

I am working on it. I am updating the old guides and tutorials with the latest info and diagrams. Along with this I am adding next and prev links at the end of each guide. It will take some time. Thanks for sharing the concern. Happy Learning!

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Providing Constructors for Your Classes

A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, Bicycle has one constructor:

public Bicycle(int startCadence, int startSpeed, int startGear)

To create a new Bicycle object called myBike , a constructor is called by the new operator:

Bicycle myBike = new Bicycle(30, 0, 8);

new Bicycle(30, 0, 8) creates space in memory for the object and initializes its fields.

Although Bicycle only has one constructor, it could have others, including a no-argument constructor:

Bicycle yourBike = new Bicycle(); invokes the no-argument constructor to create a new Bicycle object called yourBike .

Both constructors could have been declared in Bicycle because they have different argument lists. As with methods, the Java platform differentiates constructors on the basis of the number of arguments in the list and their types. You cannot write two constructors that have the same number and type of arguments for the same class, because the platform would not be able to tell them apart. Doing so causes a compile-time error.

You don’t have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn’t have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object , which does have a no-argument constructor.

You can use a superclass constructor yourself. The MountainBike class at the beginning of this lesson did just that. This will be discussed later, in the lesson on interfaces and inheritance.

You can use access modifiers in a constructor’s declaration to control which other classes can call the constructor.

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Java Constructors

A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes:

Example

// Create a Main class public class Main < int x; // Create a class attribute // Create a class constructor for the Main class public Main() < x = 5; // Set the initial value for the class attribute x >public static void main(String[] args) < Main myObj = new Main(); // Create an object of class Main (This will call the constructor) System.out.println(myObj.x); // Print the value of x > > // Outputs 5 

Note that the constructor name must match the class name, and it cannot have a return type (like void ).

Also note that the constructor is called when the object is created.

All classes have constructors by default: if you do not create a class constructor yourself, Java creates one for you. However, then you are not able to set initial values for object attributes.

Constructor Parameters

Constructors can also take parameters, which is used to initialize attributes.

The following example adds an int y parameter to the constructor. Inside the constructor we set x to y (x=y). When we call the constructor, we pass a parameter to the constructor (5), which will set the value of x to 5:

Example

public class Main < int x; public Main(int y) < x = y; >public static void main(String[] args) < Main myObj = new Main(5); System.out.println(myObj.x); >> // Outputs 5 

You can have as many parameters as you want:

Example

public class Main < int modelYear; String modelName; public Main(int year, String name) < modelYear = year; modelName = name; >public static void main(String[] args) < Main myCar = new Main(1969, "Mustang"); System.out.println(myCar.modelYear + " " + myCar.modelName); >> // Outputs 1969 Mustang 

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