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

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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