Java Tutorial

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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:

Create a constructor:

Try it Yourself »

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:

You can have as many parameters as you want:

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Java constructors or constructors in Java is a terminology used to construct something in our programs. 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.

What are Constructors in Java?

In Java, Constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling the constructor, memory for the object is allocated in the memory. It is a special type of method that is used to initialize the object. Every time an object is created using the new() keyword, at least one constructor is called.

Note: It is not necessary to write a constructor for a class. It is because the java compiler creates a default constructor (constructor with no arguments) if your class doesn’t have any.

How Java Constructors are Different From Java Methods?

  • Constructors must have the same name as the class within which it is defined it is not necessary for the method in Java.
  • Constructors do not return any type while method(s) have the return type or void if does not return any value.
  • Constructors are called only once at the time of Object creation while method(s) can be called any number of times.

Now let us come up with the syntax for the constructor being invoked at the time of object or instance creation.

The first line of a constructor is a call to super() or this(), (a call to a constructor of a super-class or an overloaded constructor), if you don’t type in the call to super in your constructor the compiler will provide you with a non-argument call to super at the first line of your code, the super constructor must be called to create an object:

If you think your class is not a subclass it actually is, every class in Java is the subclass of a class object even if you don’t say extends object in your class definition.

Need of Constructors in Java

Think of a Box. If we talk about a box class then it will have some class variables (say length, breadth, and height). But when it comes to creating its object(i.e Box will now exist in the computer’s memory), then can a box be there with no value defined for its dimensions? The answer is N o .  So constructors are used to assign values to the class variables at the time of object creation, either explicitly done by the programmer or by Java itself (default constructor).

When Constructor is called?

Each time an object is created using a new() keyword, at least one constructor (it could be the default constructor) is invoked to assign initial values to the data members of the same class. Rules for writing constructors are as follows:

  • The constructor(s) of a class must have the same name as the class name in which it resides.
  • A constructor in Java can not be abstract, final, static, or Synchronized.
  • Access modifiers can be used in constructor declaration to control its access i.e which other class can call the constructor.

So by far, we have learned constructors are used to initialize the object’s state. Like methods , a constructor also contains a collection of statements(i.e. instructions) that are executed at the time of Object creation.

Types of Constructors in Java

Now is the correct time to discuss the types of the constructor, so primarily there are three types of constructors in Java are mentioned below:

  • Default Constructor
  • Parameterized Constructor
  • Copy Constructor

1. Default Constructor in Java

A constructor that has no parameters is known as default the constructor. A default constructor is invisible. And if we write a constructor with no arguments, the compiler does not create a default constructor. It is taken out. It is being overloaded and called a parameterized constructor. The default constructor changed into the parameterized constructor. But Parameterized constructor can’t change the default constructor.

Note: Default constructor provides the default values to the object like 0, null, etc. depending on the type.

2. Parameterized Constructor in Java

A constructor that has parameters is known as parameterized constructor. If we want to initialize fields of the class with our own values, then use a parameterized constructor.

Remember: Does constructor return any value? There are no “return value” statements in the constructor, but the constructor returns the current class instance. We can write ‘return’ inside a constructor.

Now the most important topic that comes into play is the strong incorporation of OOPS with constructors known as constructor overloading. Just like methods, we can overload constructors for creating objects in different ways. The compiler differentiates constructors on the basis of the number of parameters, types of parameters, and order of the parameters. 

3. Copy Constructor in Java

Unlike other constructors copy constructor is passed with another object which copies the data available from the passed object to the newly created object.

To know deep down about constructors there are two concepts been widely used as listed below: 

  • Constructor Chaining

Frequently Asked Questions(FAQs)

Q1. what is a constructor in java.

A constructor in Java is a special method used to initialize objects.

Q2. Can a Java constructor be private?

Yes, a constructor can be declared private. A private constructor is used in restricting object creation.

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Java Constructors

  • What is a Constructor?

A constructor in Java is similar to a method that is invoked when an object of the class is created.

Unlike Java methods , a constructor has the same name as that of the class and does not have any return type. For example,

Here, Test() is a constructor. It has the same name as that of the class and doesn't have a return type.

Recommended Reading: Why do constructors not return values

Example 1: Java Constructor

In the above example, we have created a constructor named Main() . Inside the constructor, we are initializing the value of the name variable.

Notice the statement of creating an object of the Main class.

Here, when the object is created, the Main() constructor is called. And, the value of the name variable is initialized.

Hence, the program prints the value of the name variables as Programiz .

Types of Constructor

In Java, constructors can be divided into 3 types:

  • No-Arg Constructor
  • Parameterized Constructor
  • Default Constructor

1. Java No-Arg Constructors

Similar to methods, a Java constructor may or may not have any parameters (arguments).

If a constructor does not accept any parameters, it is known as a no-argument constructor. For example,

Example 2: Java private no-arg constructor

In the above example, we have created a constructor Main() . Here, the constructor does not accept any parameters. Hence, it is known as a no-arg constructor.

Notice that we have declared the constructor as private.

Once a constructor is declared private , it cannot be accessed from outside the class. So, creating objects from outside the class is prohibited using the private constructor.

Here, we are creating the object inside the same class. Hence, the program is able to access the constructor. To learn more, visit Java Implement Private Constructor .

However, if we want to create objects outside the class, then we need to declare the constructor as public .

Example 3: Java public no-arg constructors

Recommended Reading: Java Access Modifier

2. Java Parameterized Constructor

A Java constructor can also accept one or more parameters. Such constructors are known as parameterized constructors (constructor with parameters).

Example 4: Parameterized constructor

In the above example, we have created a constructor named Main() . Here, the constructor takes a single parameter. Notice the expression,

Here, we are passing the single value to the constructor. Based on the argument passed, the language variable is initialized inside the constructor.

3. Java Default Constructor

If we do not create any constructor, the Java compiler automatically create a no-arg constructor during the execution of the program. This constructor is called default constructor.

Example 5: Default Constructor

Here, we haven't created any constructors. Hence, the Java compiler automatically creates the default constructor.

The default constructor initializes any uninitialized instance variables with default values.

In the above program, the variables a and b are initialized with default value 0 and false respectively.

The above program is equivalent to:

The output of the program is the same as Example 5.

  • Important Notes on Java Constructors
  • Constructors are invoked implicitly when you instantiate objects.
  • The two rules for creating a constructor are: The name of the constructor should be the same as the class. A Java constructor must not have a return type.
  • If a class doesn't have a constructor, the Java compiler automatically creates a default constructor during run-time. The default constructor initializes instance variables with default values. For example, the int variable will be initialized to 0
  • Constructor types: No-Arg Constructor - a constructor that does not accept any arguments Parameterized constructor - a constructor that accepts arguments Default Constructor - a constructor that is automatically created by the Java compiler if it is not explicitly defined.
  • A constructor cannot be abstract or static or final .
  • A constructor can be overloaded but can not be overridden.
  • Constructors Overloading in Java

Similar to Java method overloading , we can also create two or more constructors with different parameters. This is called constructors overloading.

Example 6: Java Constructor Overloading

In the above example, we have two constructors: Main() and Main(String language) . Here, both the constructor initialize the value of the variable language with different values.

Based on the parameter passed during object creation, different constructors are called and different values are assigned.

It is also possible to call one constructor from another constructor. To learn more, visit Java Call One Constructor from Another .

Note : We have used this keyword to specify the variable of the class. To know more about this keyword, visit Java this keyword .

Table of Contents

  • Example: Java Constructor
  • Java No-Arg Constructors
  • Java Parameterized Constructor
  • Java Default Constructor

Sorry about that.

Related Tutorials

Java Tutorial

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

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:

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

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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Default constructor vs. inline field initialization

What's the difference between a default constructor and just initializing an object's fields directly?

What reasons are there to prefer one of the following examples over the other?

  • constructor

Barry Brown's user avatar

  • 11 According to this answer , your Example 1 is actually transformed into your Example 2 under the hood. –  Daniel Jan 7, 2015 at 17:46

5 Answers 5

Initialisers are executed before constructor bodies. (Which has implications if you have both initialisers and constructors, the constructor code executes second and overrides an initialised value)

Initialisers are good when you always need the same initial value (like in your example, an array of given size, or integer of specific value), but it can work in your favour or against you:

If you have many constructors that initialise variables differently (i.e. with different values), then initialisers are useless because the changes will be overridden, and wasteful.

On the other hand, if you have many constructors that initialise with the same value then you can save lines of code (and make your code slightly more maintainable) by keeping initialisation in one place.

Like Michael said, there's a matter of taste involved as well - you might like to keep code in one place. Although if you have many constructors your code isn't in one place in any case, so I would favour initialisers.

davin's user avatar

The reason to prefer Example one is that it's the same functionality for less code (which is always good).

Apart from that, no difference.

However, if you do have explicit constructors, I'd prefer to put all initialization code into those (and chain them) rather than splitting it up between constructors and field initializers.

Michael Borgwardt's user avatar

Should we favor field initializer or constructor to give a default value to a field?

I will not consider exceptions that may rise during field instantiation and field lazy/eager instantiation that touch other concerns than readability and maintainability concerns. For two codes that perform the same logic and produce the same result, the way with the best readability and maintainability should be favored.

choosing the first or the second option is before all a question of code organization , readability and maintainability .

keep a consistency in the way of choosing (it makes overall application code clearer)

don't hesitate to use field initializers to instantiate Collection fields to prevent NullPointerException

don't use field initializers for fields that may be overwritten by constructors

in classes with a single constructor, the field initializer way is generally more readable and less verbose

in classes with multiple constructors where constructors have no or very few coupling between them , the field initializer way is generally more readable and less verbose

in classes with multiple constructors where constructors have coupling between them , none of the two ways is really better but whatever the chosen way, combining it with the chaining constructor is the way (see use case 1).

OP Question

With a very simple code, the assignment during the field declaration seems better and it is.

This is less verbose and more straight :

than the constructor way :

In real classes with so real specificities, things are different. In fact, according to specificities encountered, a way, the other one or anyone of them should be favored.

More elaborated examples to illustrate

Study case 1

I will start from a simple Car class that I will update to illustrate these points. Car declare 4 fields and some constructors that have relation between them.

1.Giving a default value in field intializers for all fields is undesirable

Default values specified in the fields declaration are not all reliable. Only name and origin fields have really default values.

nbSeat and color fields are first valued in their declaration, then these may be overwritten in the constructors with arguments. It is error-prone and besides with this way of valuing fields, the class decreases its reliability level. How could rely on any default value assigned during the fields declaration while it has proven to be not reliable for two fields ?

2.Using constructor to value all fields and relying on constructors chaining is fine

This solution is really fine as it doesn't create duplication, it gathers all the logic at a place : the constructor with the maximum number of parameters. It has a single drawback : the requirement to chain the call to another constructor. But is it a drawback ?

3.Giving a default value in field intializers for fields which constructors don't assign to them a new value is better but has still duplication issues

By not valuing nbSeat and color fields in their declaration, we distinguish clearly fields with default values and fields without.

This solution is rather fine but it repeats the instantiation logic in each Car constructor contrary to the previous solution with constructor chaining.

In this simple example, we could start to understand the duplication issue but it seems only a little annoying. In real cases, the duplication may be much important as the constructor may perform computation and validation. Having a single constructor performing the instantiation logic becomes so very helpful.

So finally the assignment in the fields declaration will not always spare the constructor to delegate to another constructor.

Here is an improved version.

4.Giving a default value in field intializers for fields which constructors don't assign to them a new value and relying on constructors chaining is fine

Study case 2

We will modify the original Car class. Now, Car declare 5 fields and 3 constructors that have no relation between them.

1.Using constructor to value fields with default values is undesirable

As we don't value name and origin fields in their declaration and we have not a common constructor naturally invoked by other constructors, we are forced to introduce a initDefaultValues() method and invoke it in each constructor. So we have not to forget to call this method. Note that we could inline initDefaultValues() body in the no arg constructor but invoking this() with no arg from the other constructor is not necessary natural and may be easily forgotten :

2.Giving a default value in field initializers for fields which constructors don't assign to them a new value is fine

Here we don't need to have a initDefaultValues() method or a no arg constructor to call. Field initializers is perfect.

In any cases) Valuing fields in field initializers should not be performed for all fields but only for those that cannot be overwritten by a constructor.

Use case 1) In case of multiple constructors with common processing between them, it is primarily opinion-based. Solution 2 ( Using constructor to value all fields and relying on constructors chaining ) and solution 4 ( Giving a default value in field intializers for fields which constructors don't assign to them a new value and relying on constructors chaining ) appear as the most readable, maintainable and robust solutions.

Use case 2) In case of multiple constructors with no common processing/relation between them as in the single constructor case, solution 2 ( Giving a default value in field intializers for fields which constructors don't assign to them a new value ) looks better.

davidxxx's user avatar

I prefer field initializers and resort to a default constructor when there is complex initialization logic to perform (e.g. populate a map, one ivar depends on another through a series of heuristic steps to execute, etc).

@Michael B said:

... I'd prefer to put all initialization code into those (and chain them) rather than splitting it up between constructors and field initializers.

MichaelB (I bow to the 71+ K rep) makes perfect sense, but my tendency is to keep the simple initializations in the inline final initializers and do the complex part of the initializations in the constructor.

Bert F's user avatar

The only difference that I can think of is that if you were to add another constructor

then in the first example, you would no longer have a default constructor whereas in the 2nd example, you would still have the default constructor (and could even make a call to it from inside our new constructor if we wanted to)

Ojonugwa Jude Ochalifu's user avatar

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