Why constructors 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. They are arranged in a way that each constructor performs a different task. 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.

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Computer Organization. Most often, you will need a constructor that accepts one or more parameters. Parameters are added to a constructor in the same way that they are added to a method, just declare them inside the parentheses after the constructor's name.

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