Constructor Overloading in Java with examples 构造方法重载 Default constructor 默认构造器 缺省构造器 创建对象 类实例化

Providing Constructors for Your Classes (The Java™ Tutorials > Learning the Java Language > Classes and Objects) https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html

A class contains constructors that are invoked to create objects from the class blueprint. Constructor declarations look like method declarations—except that they use the name of the class and have no return type. For example, Bicycle has one constructor:

Constructor Overloading in Java with examples https://beginnersbook.com/2013/05/constructor-overloading/

Constructor overloading is a concept of having more than one constructor with different parameters list, in such a way so that each constructor performs a different task. For e.g. Vectorclass has 4 types of constructors. If you do not want to specify the initial capacity and capacity increment then you can simply use default constructor of Vector class like this Vector v = new Vector(); however if you need to specify the capacity and increment then you call the parameterized constructor of Vector class with two int arguments like this: Vector v= new Vector(10, 5);

You must have understood the purpose of constructor overloading. Lets see how to overload a constructor with the help of following java program.

Constructor Overloading Example

Here we are creating two objects of class StudentData. One is with default constructor and another one using parameterized constructor. Both the constructors have different initialization code, similarly you can create any number of constructors with different-2 initialization codes for different-2 purposes.
StudentData.java

class StudentData
{
   private int stuID;
   private String stuName;
   private int stuAge;
   StudentData()
   {
       //Default constructor
       stuID = 100;
       stuName = "New Student";
       stuAge = 18;
   }
   StudentData(int num1, String str, int num2)
   {
       //Parameterized constructor
       stuID = num1;
       stuName = str;
       stuAge = num2;
   }
   //Getter and setter methods
   public int getStuID() {
       return stuID;
   }
   public void setStuID(int stuID) {
       this.stuID = stuID;
   }
   public String getStuName() {
       return stuName;
   }
   public void setStuName(String stuName) {
       this.stuName = stuName;
   }
   public int getStuAge() {
       return stuAge;
   }
   public void setStuAge(int stuAge) {
       this.stuAge = stuAge;
   }

   public static void main(String args[])
   {
       //This object creation would call the default constructor
       StudentData myobj = new StudentData();
       System.out.println("Student Name is: "+myobj.getStuName());
       System.out.println("Student Age is: "+myobj.getStuAge());
       System.out.println("Student ID is: "+myobj.getStuID());

       /*This object creation would call the parameterized
        * constructor StudentData(int, String, int)*/
       StudentData myobj2 = new StudentData(555, "Chaitanya", 25);
       System.out.println("Student Name is: "+myobj2.getStuName());
       System.out.println("Student Age is: "+myobj2.getStuAge());
       System.out.println("Student ID is: "+myobj2.getStuID());
  }
}

Output:

Student Name is: New Student
Student Age is: 18
Student ID is: 100
Student Name is: Chaitanya
Student Age is: 25
Student ID is: 555

Let’s understand the role of this () in constructor overloading

public class OverloadingExample2
{
   private int rollNum;
   OverloadingExample2()
   {
      rollNum =100;
   }
   OverloadingExample2(int rnum)
   {
      this();
      /*this() is used for calling the default
       * constructor from parameterized constructor.
       * It should always be the first statement
       * inside constructor body.
       */
      rollNum = rollNum+ rnum;
   }
   public int getRollNum() {
	  return rollNum;
   }
   public void setRollNum(int rollNum) {
	  this.rollNum = rollNum;
   }
   public static void main(String args[])
   {
	   OverloadingExample2 obj = new OverloadingExample2(12);
       System.out.println(obj.getRollNum());
    }
}

Output:

112

As you can see in the above program that we called the parameterized constructor during object creation. Since we have this() placed in parameterized constructor, the default constructor got invoked from it and initialized the variable rollNum.

Test your skills – Guess the output of the following program

public class OverloadingExample2
{
   private int rollNum;
   OverloadingExample2()
   {
      rollNum =100;
   }
   OverloadingExample2(int rnum)
   {

      rollNum = rollNum+ rnum;
      this();
   }
   public int getRollNum() {
      return rollNum;
   }
   public void setRollNum(int rollNum) {
      this.rollNum = rollNum;
   }
   public static void main(String args[])
   {
       OverloadingExample2 obj = new OverloadingExample2(12);
       System.out.println(obj.getRollNum());
    }
}

Output:

Exception in thread "main" java.lang.Error: Unresolved compilation
problem:Constructor call must be the first statement in a constructor

Program gave a compilation error. Reason: this() should be the first statement inside a constructor.

Another Constructor overloading Example

Another important point to note while overloading a constructor is: When we don’t implement any constructor, the java compiler inserts the default constructor into our code during compilation, however if we implement any constructor then compiler doesn’t do it. See the example below.

public class Demo
{
   private int rollNum;
   //We are not defining a no-arg constructor here

   Demo(int rnum)
   {
      rollNum = rollNum+ rnum;
   }
   //Getter and Setter methods

   public static void main(String args[])
   {
      //This statement would invoke no-arg constructor
      Demo obj = new Demo();
   }
}

Output:

Exception in thread "main" java.lang.Error: Unresolved compilation
problem:The constructor Demo() is undefined

Creating Objects

Creating Objects (The Java™ Tutorials > Learning the Java Language > Classes and Objects) https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html

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The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases.

Creating Objects

As you know, a class provides the blueprint for objects; you create an object from a class. Each of the following statements taken from the CreateObjectDemo program creates an object and assigns it to a variable:

Point originOne = new Point(23, 94);
Rectangle rectOne = new Rectangle(originOne, 100, 200);
Rectangle rectTwo = new Rectangle(50, 100);

The first line creates an object of the Point class, and the second and third lines each create an object of the Rectangle class.

Each of these statements has three parts (discussed in detail below):

1. Declaration: The code set in bold are all variable declarations that associate a variable name with an object type.
2. Instantiation: The new keyword is a Java operator that creates the object.
3. Initialization: The new operator is followed by a call to a constructor, which initializes the new object.

Declaring a Variable to Refer to an Object

Previously, you learned that to declare a variable, you write:

type name;

This notifies the compiler that you will use name to refer to data whose type is type. With a primitive variable, this declaration also reserves the proper amount of memory for the variable.

You can also declare a reference variable on its own line. For example:

Point originOne;

If you declare originOne like this, its value will be undetermined until an object is actually created and assigned to it. Simply declaring a reference variable does not create an object. For that, you need to use the new operator, as described in the next section. You must assign an object to originOne before you use it in your code. Otherwise, you will get a compiler error.

A variable in this state, which currently references no object, can be illustrated as follows (the variable name, originOne, plus a reference pointing to nothing):

Instantiating a Class

The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the object constructor.

---

Note: The phrase "instantiating a class" means the same thing as "creating an object." When you create an object, you are creating an "instance" of a class, therefore "instantiating" a class.

---

The new operator requires a single, postfix argument: a call to a constructor. The name of the constructor provides the name of the class to instantiate.

The new operator returns a reference to the object it created. This reference is usually assigned to a variable of the appropriate type, like:

Point originOne = new Point(23, 94);

The reference returned by the new operator does not have to be assigned to a variable. It can also be used directly in an expression. For example:

int height = new Rectangle().height;

This statement will be discussed in the next section.

Initializing an Object

Here's the code for the Point class:

public class Point {
    public int x = 0;
    public int y = 0;
    //constructor
    public Point(int a, int b) {
        x = a;
        y = b;
    }
}

This class contains a single constructor. You can recognize a constructor because its declaration uses the same name as the class and it has no return type. The constructor in the Point class takes two integer arguments, as declared by the code (int a, int b). The following statement provides 23 and 94 as values for those arguments:

Point originOne = new Point(23, 94);

The result of executing this statement can be illustrated in the next figure:

Here's the code for the Rectangle class, which contains four constructors:

public class Rectangle {
    public int width = 0;
    public int height = 0;
    public Point origin;

    // four constructors
    public Rectangle() {
        origin = new Point(0, 0);
    }
    public Rectangle(Point p) {
        origin = p;
    }
    public Rectangle(int w, int h) {
        origin = new Point(0, 0);
        width = w;
        height = h;
    }
    public Rectangle(Point p, int w, int h) {
        origin = p;
        width = w;
        height = h;
    }

    // a method for moving the rectangle
    public void move(int x, int y) {
        origin.x = x;
        origin.y = y;
    }

    // a method for computing the area of the rectangle
    public int getArea() {
        return width * height;
    }
}

Each constructor lets you provide initial values for the rectangle's origin, width, and height, using both primitive and reference types. If a class has multiple constructors, they must have different signatures. The Java compiler differentiates the constructors based on the number and the type of the arguments. When the Java compiler encounters the following code, it knows to call the constructor in the Rectangle class that requires a Point argument followed by two integer arguments:

Rectangle rectOne = new Rectangle(originOne, 100, 200);

This calls one of Rectangle's constructors that initializes origin to originOne. Also, the constructor sets width to 100 and height to 200. Now there are two references to the same Point object—an object can have multiple references to it, as shown in the next figure:

The following line of code calls the Rectangle constructor that requires two integer arguments, which provide the initial values for width and height. If you inspect the code within the constructor, you will see that it creates a new Point object whose x and y values are initialized to 0:

Rectangle rectTwo = new Rectangle(50, 100);

The Rectangle constructor used in the following statement doesn't take any arguments, so it's called a no-argument constructor:

Rectangle rect = new Rectangle();

All classes have at least one constructor. If a class does not explicitly declare any, the Java compiler automatically provides a no-argument constructor, called the default constructor. This default constructor calls the class parent's no-argument constructor, or the Object constructor if the class has no other parent. If the parent has no constructor (Object does have one), the compiler will reject the program.

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