Constructor in interface ?

Can we write constructor inside interface in java?

• No. Interfaces does not allow constructors.
• Why interface does not have constructor? The variables inside interfaces are static final variables means constants and we can not create object fro interface so there is no need of constructor in interface that is the reason interface doesn't allow us to create constructor.
• what happens when a constructor is defined for an interface

What will happens if we try to create constructor inside interfaces in java

• If we try to create constructor in interface compile time error will come.
• Error description: Interfaces cannot have constructors.

1. public interface sample{
2.  
3.  int a=10;
4. 
   
5. sample(){//Interfaces cannot have constructors.
6. } 
7.  
8. }

Interfaces in Java 8:

• Before java 8 interfaces allows only public abstract methods.
• If we declare any method in interface with default it will be treated as public abstract method.
• Interface methods doesn't have body. The class which implements interfaces are responsible for implementing unimplemented methods of interface.
• But in Java 8 static and default methods added.

Default methods:

• Defaults methods are also  known as defender methods or virtual extension methods
• Default methods will help us to avoid utility classes.
• We can define utility methods inside the interface and use it in all classes which is implementing.
• One of the major reason to introduce this default methods in java 8 is to support lambda expressions in collections API and to enhance.

1. package com.instanceofjava;
2. interface Java8InterfaceDemo{
3.  
4. abstract void print();
5.   
6. default void display(){
7.  
8. System.out.println("default method of interface");
9.  
10. }
11.  
12. }

1. package com.instanceofjava;
2. class Sample implements Java8InterfaceDemo{
3.  
4. void print(){
5. System.out.print("overridden method ")
6.  }
7. public static void main(String[] args){
8.   
9. Sample obj= new Sample();
10.  
11. obj.print(); // calling implemented method
12. obj.display(); // calling inherited method
13. Java8InterfaceDemo.display(); calling using interface name
14.  
15. }
16.  
17. }

Output:

1. overridden method
2. default method of interface
3. default method of interface

Static methods in Java 8:

• These static method will act as helper methods.
• These methods are the parts of interface not belongs to implementation class objects.

1. package com.interfacesinJava8;
2. interface StaticInterface{
3.  
4. Static void print(String str){
5.  
6. System.out.println("Static method of interface:"+str);
7.  
8. }
9. }

1. package com.instanceofjava;
2. class Demo implements StaticInterface{
3.  
4. public static void main(String[] args){
5.   
6.  StaticInterface.print("Java 8")
7.  
8. }
9.  
10. }

Output:

1. Static method of interface: Java 8