How do you call constructor?

Answer 1

Constructors in java cannot be invoked explicitly. They are invoked automatically when an object is created. In java if u dont write any code for constructor, by default compiler inserts a zero argument constructor. If required, it can be overrided.

Answer 2

In your program, it should be the first thing you implement. For example, let's say you have a class called Car. The Car has private instance variables of private String carName, private int carYear, and private double price. In a constructor, you must initialize all these variables.

In a default constructor (if you are required to do so...):

public Car

{

carName = "";

carYear = 0;

price = 0;

}

Then you can make the constructor:

public Car(String name, int year, double thePrice)

{

carName = name;

carYear = year;

price = thePrice;

}

Answer 3

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 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.

Answer 5

A constructor is not a function, so we don't invoke them as we would a function. Constructors are similar to functions in that they both accept arguments and can be overloaded to provide a choice of construction methods. However, unlike a function, there is no return value (not even void).

We invoke a constructor by instantiating an object of the class. The name of the class serves as the class constructor. Where there is a choice of constructor methods, the specific arguments we pass will determine which constructor is invoked.

Unless the class specifies otherwise, objects can be constructed with or without the new operator. We use the new operator only when we wish to construct an anonymous object on the heap. The new operator will return a pointer to the anonymous object (or a nullptr if object construction fails).

Consider the following class definition:

struct my_class {

my_class ();

my_class (const my_class&);

my_class (my_class&&);

// ...

};

This class declares three constructors, a default constructor, a copy constructor and a move constructor. We can invoke these different constructors as follows:

void f() {

my_class a; // invokes default constructor

my_class b (a); // invokes copy constructor

my_class c (std::move (a)); // invokes move constructor

my_class* p = new my_class; // invokes default constructor

my_class* q = new my_class (c); // invokes copy constructor

my_class* r = new my_class (std::move (*p)); // invokes move constructor

delete r; //

delete q; // release anonymous objects from heap

delete p; //

}

Note that when we invoke a constructor with arguments, we use parenthesis just as we would a function. However, the default constructor does not require parenthesis as there are no arguments. Where a default constructor requires arguments, each argument must have a default value, giving us the option of passing our own arguments or accepting the defaults. If any argument has no default value, then it is not a default constructor and we must pass arguments to it. There can be only one default constructor and it is generally a good idea to provide one whenever possible.

There is one other way to invoke a constructor and that is through derivation:

struct base {

base (int x=0): m_data (x) {}

virtual ~base ();

private:

int m_data;

};

struct derived : base {

derived (int x=0): base (x) {}

};

Here the derived class default constructor explicitly invokes the base class default constructor. Had we not done this we would not have been able to initialise the base class member data (it is private and therefore not accessible to the derived class).

Answer 6

You use the new keyword, and then call the appropriate constructor method. Object obj = new Object(); String str = new String("str");

Answer 7

When we want to create an Object for a class at that time we will write constructor.

Answer 8

Constructors are invoked by instantiating an object of the class.

Answer 9

You can invoke the constructor of a class by using the NEW keyword

Ex: ClassName obj = new ClassName();

Answer 10

you cannot call a constructor since it initialises the object when created