While overriding we cannot reduce scope of access modifier. Hence, we should declared child class method as public. Parent class method return type is void and hence child class method return type should be void but not Object.

Every variable present inside interface is always public, static and final and we cannot declare instance variables. But abstract class can contain instance variables.

Both interface and abstract class can contain abstract methods.

Both interface and abstract class can contain static methods.

Constructors concept applicable only for classes but not for interfaces.

From Java 8 version onwards, we can write static methods inside interface including main method. Hence the above code compiles and runs fine. It prints Interface Main Method to the console.

For every interface variable, we should perform initialization at the time of declaration only. We cannot write final methods inside interface. For interface static methods, compulsory we should provide implementation.

Every method present in interface is by default public. Hence at the time of overriding/implementing that method, compulsory we should declare that method as public. We didn't declare method as public at Line-4 and Line-6. Hence we will get compile time errors.

For abstract class methods, if we don't want to provide implementation, then compulsory we should declare that method as abstract, otherwise we will get compile time error.

Every variable present in interface is by default public, static and final whether we are declaring or not. Hence private and protected are not allowed.

We can declare methods with default keyword only in interface but not in class. Hence we will get compile time error.

Test.java:10: error: modifier default not allowed here

default void m1()

Inheritance concept applicable only for methods but not for constructors. Hence Parent class no-arg constructor is not available to child class. Due to this, the following line causes compile time error:

Parent p = new Child();

While resolving method calls, compiler will always gives the precedence in the following order.

1. Exact Match

2. Widening

3. Auto-Boxing

4. var-arg

new Test('a');==>Exact Match is not there and hence by widening double-arg constructor will get chance.