Constructors in java

• Constructors will be executed when the object is created.
• Constructors should have same name of class name.
• Executed once per object.
• Basically used to assign instance variables
• We can overload constructors.
• We can call super class constructor form sub class constructor.

1. package instanceofjava;
2. class A{
3. A(){
4. }
5. }
6. }

• while creating object constructor will be executed so we can assign instance variables to some default values.

1. package instanceofjava;
2. class A{
3. int a,b
4. A(int x, int y){
5. a=x;
6. b=y;
7. }
8.  
9. public static void main(String[] args){
10.  A a=new A(10,20);
11. 
    
12. }
13. }

Types of constructors:

• There are two types of constructors 
• Default constructors
• Parameterized constructor.

Default constructor:

•  Default constructor will not have any arguments.
• If we not defined any constructor in our class. JVM automatically defines a default constructor to our class.

1. package instanceofjava;
2. class Sample{
3. int a,b
4. Sample(){
5. a=37;
6. b=46;
7. }
8.  
9. public static void main(String[] args){
10.  
11.  Sample obj=new Sample();
12.   System.out.println(obj.a);
13.   System.out.println(obj.b);
14.  
15. }
16. }

Output:

1. 37
2. 46

Parameterized constructor

1. package instanceofjava;
2. class Sample{
3. int a,b
4. Sample(int x, int y){
5. a=x;
6. b=y;
7. }
8.  
9. public static void main(String[] args){
10.  
11.  Sample obj=new Sample(37,46);
12.   System.out.println(obj.a);
13.   System.out.println(obj.b);
14.  
15. }
16. }

Output:

1. 37
2. 46

Constructor overloading:

1. package instanceofjava;
2. class Sample{
3. int a,b 
4. Sample(){
5. this(1,2);
6.  System.out.println("Default constructor");
7.  
8. }
9. Sample(int x , int y){
10.  
11. this(1,2,3); 
12. a=x;
13. b=y;
14.  System.out.println("Two argument constructor");
15.  
16. }
17.  
18. Sample(int a , int b,int c){
19.  System.out.println("Three argument constructor")
20. }
21. public static void main(String[] args){
22.  
23.  Sample obj=new Sample();
24.   System.out.println(obj.a);
25.   System.out.println(obj.b);
26.  
27. }
28. }

Output:

1. Three argument constructor
2. Two argument constructor
3. Default argument constructor
4. 1
5. 2