Java Constructor

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Index

1. Introduzione
2. Types of Constructor
3. No-Arg Constructor
4. Parameterised Constructor
5. Default Constructor
6. Constructor Overloading
7. How to “call” a constructor

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Introduzione

• A constructor in Java is a “special” method that is invoked when an object of the class is created.
• Inizializzano i campi dello stato iniziale di un oggetto

Important Notes on Java Constructors

• Constructors are invoked implicitly when you instantiate objects.
• The two rules for creating a constructor are:
  1. The name of the constructor should be the same as the class.
  2. A Java constructor must not have a return type.
• If a class doesn’t have a constructor, the Java compiler automatically creates a default constructor during run-time. The default constructor initializes instance variables with default values. For example, the int variable will be initialized to 0

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Types of Constructor

In Java, constructors can be divided into three types:

• No-Arg Constructor
• Parameterised Constructor
• Parameterised Constructor

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No-Arg Constructor

Similar to methods, a Java constructor may or may not have any parameters (arguments).

public class Counter {
	// Costruttore
	public Counter() {
		valore = 0;
	}
}

If a constructor does not accept any parameters, it is known as a no-argument constructor. For example,

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Parameterised Constructor

A Java constructor can also accept one or more parameters.

class Main {
 
  String languages;
 
  // constructor accepting single value
  Main(String lang) {
    languages = lang;
  }
 
  public static void main(String[] args) {
 
    // call constructor by passing a single value
    Main obj1 = new Main("Java");
    Main obj2 = new Main("Python");
    Main obj3 = new Main("C");
  }
}

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Default Constructor

If we do not create any constructor, the Java compiler automatically creates a no-arg constructor during the execution of the program.

class Main {
 
  int a;
  boolean b;
 
  public static void main(String[] args) {
 
    // calls default constructor
    Main obj = new Main();
 
    System.out.println("Default Value:");
    System.out.println("a = " + obj.a);    // output: 0
    System.out.println("b = " + obj.b);    // output: false
  }

The default constructor initialises any uninitialised instance variables with default values.

Type	Default Value
boolean	false
byte	0
short	0
int	0
long	0L
char	\u0000
float	0.0f
double	0.0d
object	Reference null

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Constructor Overloading

Similar to Java method overloading, we can also create two or more constructors with different parameters. This is called constructor overloading.

Esempio più metodi costruttori nella stessa classe:

public Counter(){
	value = 0;
}
 
public Counter(int initaliaValue){
	value = initialValue;
}

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How to “call” a constructor

I costruttori vengono “chiamati” utilizzando new NomeCostruttore()

static public void main(String[] args)
{
	Counter contatore1 = new Counter();
	Counter contatore2 = new Counter(42);
}
 

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