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Why an interface doesn't have a constructor whereas an abstract class have a constructor in Java?
A Constructor is to initialize the non-static members of a particular class with respect to an object.
Constructor in an Interface
An Interface in Java doesn't have a constructor because all data members in interfaces are public static final by default, they are constants (assign the values at the time of declaration).
There are no data members in an interface to initialize them through the constructor.
In order to call a method, we need an object, since the methods in the interface don’t have a body there is no need for calling the methods in an interface.
Since we cannot call the methods in the interface, there is no need of creating an object for an interface and there is no need of having a constructor in it.
Example
interface Addition {
int add(int i, int j);
}
public class Test implements Addition {
public int add(int i, int j) {
int k = i+j;
return k;
}
public static void main(String args[]) {
Test t = new Test();
System.out.println("k value is:" + t.add(10,20));
}
}
Output
k value is:30
Constructor in a Class
The purpose of the constructor in a class is used to initialize fields but not to build objects.
When we try to create a new instance of an abstract superclass, the compiler will give an error.
However, we can inherit an abstract class and make use of its constructor by setting its variables.
Example
abstract class Employee {
public String empName;
abstract double calcSalary();
Employee(String name) {
this.empName = name; // Constructor of abstract class
}
}
class Manager extends Employee {
Manager(String name) {
super(name); // setting the name in the constructor of subclass
}
double calcSalary() {
return 50000;
}
}
public class Test {
public static void main(String args[]) {
Employee e = new Manager("Adithya");
System.out.println("Manager Name is:" + e.empName);
System.out.println("Salary is:" + e.calcSalary());
}
}
Output
Manager Name is:Adithya Salary is:50000.0
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