Java quiz 20: Convert from a class to interfaces

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In this quiz InterfaceA extends InterfaceB. The class MyClass implements InterfaceA.
The question is whether it is allowed to convert from MyClass to both interfaces
and what the consequences would be.

Java quiz 20: Convert from a class to interfaces

Level: Normal

Note: The class MyClass and the interfaces InterfaceA and InterfaceB are three separate files in one package.

What is written to the standard output as the result of executing the following code?

InfB.java

public interface InfB
{
  int x = 3;
  
  int myMethod();
}

InfA.java

public interface InfA extends InfB
{
  int x = 7;
}

MyClass.java

public class MyClass implements InfA
{
  int x = 5;
  
  public int myMethod()
  {
    return x;
  }
  public static void main(String[] args)
  {
    InfA infA = new MyClass();
    InfB infB = new MyClass();
    MyClass mc = new MyClass();
    System.out.print(mc.x);
    System.out.print(infA.x);
    System.out.print(infB.x);
    System.out.print(mc.myMethod());
    System.out.print(infA.myMethod());
    System.out.print(infB.myMethod());
  }
}

Author: Sar Maroof

Select the correct answer.


Answer explanation

  1. By converting from a class to interfaces, the interface can acces the overridden methods.
  2. The overridden method myMethod returns the variable x of the class MyClass. Therefore the methods infB.myMethod(), infA.myMethod() and infB.myMethod()
    return 5, which is the value of the variable x of MyClass.

The correct answer is: c.

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Sar Maroof is a professional software development teacher, gives master classes and publishes technical articles. He is also an expert software developer and worked for several big as well as small companies and later as a freelancer.

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