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

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

Constructor in the interface

• Interfacein Java does not have aconstructor, because all data members in the interface are by defaultpublic 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 constructoris 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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