Java what is a constructor

I will show an example of a Java constructor declaration with parameters later in this text. The fourth part of a Java constructor declaration is the body of the constructor. In the constructor example above the constructor has no operations inside the constructor body. It is said to be an "empty" constructor. A class can have multiple constructors, as long as their signature the parameters they take are not the same. You can define as many constructors as you need.

When a Java class contains multiple constructors, we say that the constructor is overloaded comes in multiple versions. This is what constructor overloading means, that a Java class contains multiple constructors. The Java class above contains two constructors. The first constructor is a no-arg constructor, meaning it takes no parameters no arguments.

The second constructor takes an int parameter. Inside the constructor body the int parameter value is assigned to a field, meaning the value of the parameter is copied into the field.

The field is thus initialized to the given parameter value. The keyword this in front of the field name this. It just signals to the compiler that it is the field named number that is being referred to.

This is explained in more detail in the section about constructor parameters. You don't have to define a constructor for a class, but if you don't define any constructor, the Java compiler will insert a default, no-argument constructor for you.

Thus, once the class is compiled it will always at least have a no-argument constructor. If you do define a constructor for your class, then the Java compiler will not insert the default no-argument constructor into your class.

As you have already seen, it is possible for a Java constructor to take parameters. These parameters can then be used to initialize the internal state fields of the newly created object. Here is an example:. In this example the Java constructor declaration is marked in bold. As you can see, three parameters are declared: first , last and year. Inside the body of the constructor the values of these three parameters are assigned to the fields of the Employee object.

The line breaks after each parameter are optional. The Java compiler ignores line breaks here. You can also write the parameter declaration in a single line if you want, like this:. To call this constructor that takes three parameters, you would instantiate an Employee object like this:. The parameters are passed to the constructor inside the parentheses after the class name on the right side of the equal sign.

The object is then created, and the constructor executed. 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.

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

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Reference Materials String. Start Learning Java. Explore Java Examples. Related Topics Java enum Constructor. However, once you define your own constructor, the default constructor is no longer used. As the name specifies the no argument constructors of Java does not accept any parameters instead, using these constructors the instance variables of a method will be initialized with fixed values for all objects.

Most often, you will need a constructor that accepts one or more parameters.