How do constructors work in java

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.

After execution of the above constructor, the fields initialized by the constructor will have the values of the parameters passed to the constructor. A Java constructor parameter can have the same name as a field. If a constructor parameter has the same name as a field, the Java compiler has problems knowing which you refer to.

By default, if a parameter or local variable has the same name as a field in the same class, the parameter or local variable "shadows" for the field. Look at this constructor example:. Inside the constructor of the Employee class the firstName , lastName and birthYear identifiers now refer to the constructor parameters, not to the Employee fields with the same names.

Thus, the constructor now just sets the parameters equal to themselves. The Employee fields are never initialized. To signal to the Java compiler that you mean the fields of the Employee class and not the parameters, put the this keyword and a dot in front of the field name. Here is how the Java constructor declaration from before looks with that change:. You call a constructor when you create a new instance of the class containing the constructor.

Here is a Java constructor call example:. This example invokes calls the no-argument constructor for MyClass as defined earlier in this text. In case you want to pass parameters to the constructor, you include the parameters between the parentheses after the class name, like this:.

This example passes one parameter to the MyClass constructor that takes an int as parameter. In Java it is possible to call a constructor from inside another constructor. When you call a constructor from inside another constructor, you use the this keyword to refer to the constructor. They are differentiated by the compiler by the number of parameters in the list and their types. There is no copy constructor in Java. In this example, we are going to copy the values of one object into another using Java constructor.

We can copy the values of one object into another by assigning the objects values to another object. In this case, there is no need to create the constructor. Yes, like object creation, starting a thread, calling a method, etc. You can perform any operation in the constructor as you perform in the method. Java provides a Constructor class which can be used to get the internal information of a constructor in the class.

It is found in the java. JavaTpoint offers too many high quality services. Mail us on [email protected] , to get more information about given services. Please mail your requirement at [email protected] Duration: 1 week to 2 week. Java Training Basics of Java. Abstract class Interface Abstract vs Interface. Package Access Modifiers Encapsulation.

Copying the values of one object into another Does constructor perform other tasks instead of the initialization. Next Topic static keyword in java. Reinforcement Learning. R Programming. React Native. Python Design Patterns. Python Pillow. Python Turtle. Verbal Ability. Interview Questions. It's easy to create a MockQueue and MockDatabase to test our OrderProcessor class, consider what would happen if this class is calling a Singleton or Service method to get its dependency.

It would be difficult to test that class. Since object creation is a fundamental concept, every Java developer should know how Constructor works, how they initialize an object, how a super class constructor is called and so on. In Next section, we will see how that happens. How Constructor Works in Java? Constructor is special, they contain a block of code, which executed when you create an object using new operator. If your class has a superclass or parent class then its constructor will be executed before your class.

Similarly, if you have more than one constructor in your class, you can call them from your constructor. This is known as constructor chaining and you use this and super to call constructor from same class and parent class respectively. You can use public, private, protected access modifiers with constructor or can even leave them without any parameter, in that case, it will use default access, which is at package-private level.

Private constructors are special, because if you make your constructor private, then no one can call it from outside that class, which means no external way to create instance of that class. This also prevents a class from being subclasses because by default first line of constructor has a call to super , no argument constructor of parent class, if you make that private, it will not be accessible on child class and compiler will throw error.

Private constructor has another special use, in singleton design pattern , where goal is to keep just one instance of that class. Singleton creates instance by itself, caches it and provides a getInstance method to make that instance available to outside world.

Also, you cannot make constructor abstract, synchronized or final, those are illegal keyword for constructor and using them there will be error at compile time.

Some facts about Constructor in Java There are lot facts about constructor you as a Java developer should know, this will help you to read and understand existing Java code in your organization or from any open source library. Constructor can be overloaded This means you can have more than one constructor in your class all with the same name until they have different method signature, which comprises type of argument and order type of argument.