When to use a constructor in java?

Answer 1

Private constructors generally have one of two uses.

The first is for implementing the singleton design pattern, in which you only ever want one instance of the class in existence. So you don't want new instances of the class being generated over and over. You do something like:

class Singleton {

// All classes can access this instance of the class and no other.

public static instance = new Singleton();

private Singleton() {

}

}

The second is for limiting the creation of classes which contain only static methods or constants.

Of course, private constructors are useful any time you want to limit the way other classes/packages/programmers interact with your class.

Answer 2

You use constructors on a class for instantiation of an object.

For example:

You have a

class Person()

private String name;

private String address;

private int number;

public Person(String name, String address, int number)

{

/*lines of codes*/

}

in the code you need two Persons object.

Person person1 = new Person("mark","USA", 23)

Person person2 = new Person("anne","Japan" 56)

person1 is a new object instance

person2 is another new object instance which has independent state from person1

Answer 3

What are the importance of constructor in java?

Constructor is automatically called immediately after the object is created , before the new operator complete . Constructor is important because they have no return type , not even void.

Answer 4

Every class, including abstract classes, MUST have a constructor. Hard Code that into your brain. But just because a class must have one, doesn't mean the programmer has to type it. A constructor looks like this:

class Car {

Car() { } // The constructor for the Car class

}

You notice anything missing in the declaration above? There's no return type! Two key points to remember about constructors are that they have no return type and their names must exactly match the class name. Typically, constructors are used to initialize instance variable state, as follows:

class Car {

int size;

String name;

Car(String name, int size) {

this.name = name;

this.size = size;

}

}

In the preceding code example, the Car class does not have a no-arg constructor. That means the following will fail to compile:

Car f = new Car(); // Won't compile, no matching constructor

but the following will compile:

Car f = new Car("Ford", 43); // No problem. Arguments match

// the Car constructor.

So it's very common for a class to have a no-arg constructor, regardless of how many other overloaded constructors are in the class (constructors can be overloaded just like methods). You can't always make that work for your classes; occasionally you have a class where it makes no sense to create an instance without supplying information to the constructor. A java.awt.Color object, for example, can't be created by calling a no-arg constructor, because that would be like saying to the JVM, "Make me a new Color object, and I really don't care what color it is...." Do you seriously want the JVM making your color choices?

Answer 5

You will use Private constructors when you dont want any other class to instantiate your current class using a new keyword.

ex:

Ferrari obj = new Ferrari();

The above line of code will not work if the constructor of the Ferrari class is declared private

Answer 6

No. The constructor is the starting point of creation of any object in Java. Without going through a constructor you cannot create an object

Answer 7

A Private constructor is simply a constructor that can only be used within the class it was declared in. By default java creates you a constructor which is public though. Hope that helps

Answer 8

We use constructor to initialize the instant variable of a particular class that is used for the various operations.

Answer 9

Yes, that's how create SINGleton