Why constructor is used

How constructors are different from methods in Java? Constructors must have the same name as the class within which it is defined while 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 time. Now let us come up with the syntax for the constructor been invoked at the time of object or instance creation.

A constructor in Java can not be abstract, final, static and Synchronized. Access modifiers can be used in constructor declaration to control its access i. Like methods , a constructor also contains a collection of statements i. Need of Constructor? 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. The answer is no. So constructors are used to assigning values to the class variables at the time of object creation, either explicitly done by the programmer or by Java itself default constructor.

When is a Constructor 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. Now is the correct time to discuss types of the constructor, so primarily there are two types of constructors in java: No-argument constructor Parameterized Constructor Type 1: No-argument constructor No-argument constructor: A constructor that has no parameter is known as the default constructor.

And if we write a constructor with arguments or no-arguments then the compiler does not create a default constructor. Default constructor provides the default values to the object like 0, null, etc. If we want to initialize fields of the class with our own values, then use a parameterized constructor. Remember: Does constructor return any value? Now the most important topic that comes into play is the strong incorporation of OOPS with constructors known as constructor overloading.

JustLike methods, we can overload constructors for creating objects in different ways. Compiler differentiates constructors on the basis of numbers of parameters, types of the parameters, and order of the parameters. If you like GeeksforGeeks and would like to contribute, you can also write an article using write. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Connect and share knowledge within a single location that is structured and easy to search. This is a very basic question, I searched it but i just want to ask this community that we have both constructors and methods. But normally we use constructor to initialize the variables instead of the methods. I think that both can be used to initialize variables. So what is the basic difference between both. Is there any solid reason?

The most important difference: When you instantiate an object it's constructor will be invoked whereas calling a method is always optional. You therefore might forget to call your initialization method and fail to initialize everything correctly. Or if you have mandatory arguments, don't define a default constructor and instead require construction with parameters:.

This has the advantage of requiring the arguments before you even instantiate the class. So you're forced to do:. So whenever possible, always err on the side of doing your initialization in a constructor. There are a few cases where a constructor might not be viable. For example, the only way to fail a constructor is by using an exception. The failure to construct may be "routine" and not truly exceptional.

Also exceptions might be expensive on some architectures. Another case might be when you want to ensure that virtual methods are fully bound, which is guaranteed to be true only after construction.

They can't both be used to initialize member variables. It is the job of the constructor to initialize members, and it is called automatically whenever you create a new instance. Constructors are called automatically, so there's no need to worry whether the user has invoked an initialization method yet. However, the Google style guide does have something to say about constructors :. Google's recommendation is to have straightforward initiation in a constructor, and non-trivial initiation in a separate method.

The constructor is invoked automatically when you create the object unlike a function. Also, the constructor is used to indicate the intent that this is for the initialization of the object.

Imagine you have a member variable that has no empty constructor. Then you have no other option but to initialize it in the initializer list of your constructor. Also the constructor is called when you allocate an array with the new operator, while initialization with methods will make the code way more complicated. Along with this, we will also see the types of the constructor.

In Java, the constructor is similar to the method. The property of the constructor is that it must have the same name as the class name. It has no return type. We do not require to call the constructor manually.

It automatically invokes implicitly during the instantiation. In other words, a constructor is a method that is called at runtime during the object creation by using the new operator.

The JVM calls it automatically when we create an object. When we do not define a constructor in the class, the default constructor is always invisibly present in the class. There are the following reasons to use constructors:. In short, we use the constructor to initialize the instance variable of the class. As the name suggests, it accepts arguments parameters. The parameterized constructor is used if we want to dynamically initialize the instance variables with the specified values at the time of instantiation.

The default constructor does not accept any parameter. It is used if we want to initialize the instance variables with certain values. Every Java class has a default constructor, invisibly. So, we need not to define it, separately. Remember that the default constructor is removed from the class when we create a parameterized constructor. In the Employee class, we have created two constructors one is the default constructor and the other is the parameterized constructor.

The Employee class has two private variables namely, name and age. In the main method, we have instantiated the class and used both the constructors. JavaTpoint offers too many high quality services.