Java Constructors

5 Key excerpts on "Java Constructors"

• 

  eBook - PDF

  Developing Java Software

  • Russel Winder, Graham Roberts(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  A constructor is a special kind of method that is used to explicitly control initialization, and is guaranteed to be called when an object is created. Intention The Java language will not allow an object to be created without being initialized, meaning that all its instance variables will be given a known value. Initialization of instance variables can be performed in various different ways (see Section 19.8.2, page 642) with assignment within a constructor method being one of them. In addition, constructors allow object initialization to be parameterized and are useful for any other kind of initialization an object may require, such as reading data from a file or opening a network connection. Syntax A constructor is declared like a method and has a parameter list. However, a constructor declaration has no return type and the constructor name must be the same as the class it is declared in (which is how constructors are identified): constructorName ( parameterList ) { statementSequence } A constructor can optionally be declared public, protected or private. No other modifiers are allowed. A constructor can also throw an exception, requiring a throws clause: constructorName ( parameterList ) throws typeNameList { statementSequence } 21.2 Classes 715 Description Whenever an object is created, a constructor will be called automatically. This is always guaranteed to happen and cannot be avoided, regardless of how the instance variables may be initialized. A class may declare one or more constructors, with constructor overloading being allowed provided each has a different parameter list. A constructor with an empty parameter list is known as a default constructor . If a class does not explicitly declare any constructors the Java compiler will automatically create a public default constructor—one that has an empty parameter list.

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  eBook - ePub

  JAVA Programming Simplified

  From Novice to Professional - Start at the Beginning and Learn the World of Java

  • Dr. Muneer Ahmad Dar(Author)
  • 2020(Publication Date)
  • BPB Publications

    (Publisher)

  setData( ) , it would be uncomplicated and extrabrief to include all of the arrangementsfinished at the instance the object is initiallyproduced. Since the necessity for initialization is so frequent, Java permits objects to initialize themselves while they are formed. This automatic initialization is carried outfrom beginning to end by the application of a constructor.

  A constructor initializes an object instantly upon formation. It has the identical name as the class in which it is located and is similar to a method but we call it a special member function as it has some special features like the constructor is automatically called straight away after the object is formed, prior to the new operator completes. Constructors appear a slightweird and wonderful because they have no return type, not even void. This is because the inherent return type of a class' constructor is the class type itself.

  Being a special member function of a class it has a set of specialized features as under:

  • Constructor has a same name as that of a class name. For example if we want to create a class Student, its constructor will have a same name as Student(); for example the below class has two constructors with the same name as class name Employee: class Employee { // Default Constructor Employee( ) { // Body of the Constructor } // Parameterized Constructor Employee( int d1, int d2) { // Body of the Constructor }
  • Constructor does not return any value, not even void. As in the above example the constructor has no return type.
  • Constructors are invoked/called automatically when the object is created. If we have created a Box class and we are creating an object as Box b = new Box(); then it will call the Box()constructor automatically, if it exists. The above constructors are automatically invoked by the following statements: Employee emp1= new Employee( ); // Will invoke the default constructor Employee emp2= new Employee(2, 200 ); // Will invoke the constructor with arguments

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  eBook - PDF

  Programming Logic & Design, Comprehensive

  • Joyce Farrell(Author)
  • 2017(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  C H A P T E R 11 More Object-Oriented Programming Concepts Upon completion of this chapter, you will be able to: Create constructors Create destructors Understand composition Describe inheritance Understand GUI objects Describe exception handling Appreciate the advantages of object-oriented programming Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 Understanding Constructors In Chapter 10, you learned that you can create classes to encapsulate data and methods, and that you can instantiate objects from the classes you define. For example, you can create an Employee class that contains fields such as lastName , hourlyWage , and weeklyPay , and methods that set and return values for those fields. When you instantiate an object with a statement that uses the class type and an object identifier, such as Employee chauffeur , you are actually calling a method named Employee() . A method that has the same name as a class and that establishes an object is a constructor method, or more simply, a constructor . Constructors fall into two broad categories: • A default constructor is one that requires no arguments. • A non-default constructor or a parameterized constructor requires arguments. All non-default constructors are written by a programmer, but there are two categories of default constructors: • An automatically-created default constructor exists in a class in which the programmer has not explicitly written any constructors. • A programmer-written default constructor can reside in any class and replaces the automatically-created one. In object-oriented programming (OOP) languages, you can write both default and non-default constructors for a class; if so, the constructors are overloaded. In some programming languages, such as Visual Basic and C 11 , you do not need to use the constructor name when declaring an object, but the constructor is called nevertheless.

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  eBook - ePub

  Introduction to Java Programming, 2nd Edition

  • Prof. Sham Tickoo(Author)
  • 2017(Publication Date)
  • CADCIM Technologies

    (Publisher)

  void . The syntax for defining a constructor is given next:

    class_name (list of parameters) {

  body of the constructor;

  }  

  In this syntax, class_name is the name of the constructor which can have list of parameters.

    There are three types of constructors in Java. They are as follows:   1.Default Constructor 2.Parameterized Constructor 3.Copy Constructor   Default Constructor

  Constructor which has no parameter is known as default constructor. It is used to provide default values to variables like 0, null, and so on, depending upon the type of data. It is also known as no-argument constructor.

    For example:   class Demo {

  int i, j;

  Demo( )

  {

  i = 0;

  j = 0;

  }

  ----------;

  ----------;

  }  

  In this example, both the class and constructor have the same name, Demo . The parameter list is empty. Now, whenever an object of the Demo class will be created, a call will be made to the Demo( ) constructor, as given next:

    Demo obj1 = new Demo( );  

  In this statement, obj1 is declared as an object of the Demo class. Here, a call is made to the Demo( ) constructor and the i and j instance variables of the obj1 object are initialized to zero.

      

  Note

  If no constructor is defined inside a class, the default constructor is created by the compiler automatically.  

  Example 11

  The following example illustrates the use of the constructors. The program will perform the addition, subtraction, multiplication, and division operations on the given values and display the resultant values on the screen.

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  eBook - PDF

  An Object-Oriented Approach to Programming Logic and Design

  • Joyce Farrell(Author)
  • 2012(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  C H A P T E R 8 More Object Concepts In this chapter, you will learn about: Constructors Destructors The concept of inheritance Inheritance terminology Accessing private members of a parent class Overriding base class methods How constructors are called during inheritance The concept that a derived class object “ is an ” instance of the base class Using inheritance to achieve good software design Copyright 2012 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. An Introduction to Constructors In Chapter 7, you learned that you can create classes to encapsulate data and methods, and that you can instantiate objects from the classes you define. For example, you can create an Employee class that contains fields such as lastName and hourlyWage and methods that set and return values for those fields, as shown in Figure 8-1. In Chapter 7, you also learned that you can use a class such as Employee to instantiate an object with a statement such as the following: Employee chauffeur When you instantiate an Employee object, you are actually calling a method named Employee() that is provided automatically by the compiler of the object-oriented language you are using. A constructor is a method that establishes an object, reserving enough memory space for it and providing its name. In OO languages, a constructor is created automatically by the compiler for every class you write. When you declare an Employee object, the prewritten constructor is called for the Employee class, and it establishes one Employee instance.

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