Why do interfaces not have constructors in Java, but abstract classes have constructors?

Constructor is used to initialize non-static members of a specific class relative to the object.

Constructor in the interface

• Interface in Java does not have a constructor, because all data members in the interface are by default public static final , they are constants (assigned at declaration time).
• There are no data members in the interface that can be initialized through the constructor.
• In order to call a method, we need an object. Because the method in the interface has no body, there is no need to call the method in the interface.
• Since we cannot call methods in the interface, there is no need to create an object for the interface and there is no need to include a constructor in it.

Example 1

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 class

• Class中The purpose of the constructor is to initialize fields, but not to build objects.
• When we try to create a new instance of abstract super class, the compiler gives an error.
• However, we can inherit an abstract class and use its constructor to control it by setting its variables.

Example 2

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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