Interfaces & Inner-Classes

3.5 Interfaces & Inner-Classes

3.5.1 Questions
1. What is the difference between an Interface and an Abstract class?
Explain when it is best to use Interfaces and when abstract classes.

2. Write a short code snippet to define a new interface I3 that
inherits two existing interfaces I1 and I2. What happens if the definitions of
the I1 and I2 are:

public interface I1
public interface I2

What about if the two interfaces are:
public interface I1
public interface I2


3. Point any errors in the code snippet below and explain them:
interface Shape

public class Tile
void setHeight(int height)
void setWidth(int width)
}

public class TileShape extends Tile implements Shape
public static void main(String[] args)
}


4. What are the access modifiers of the fields and methods inside the
interface? Is it possible to declare an interface as being static? What
about private?

5. What are the differences between inner classes and nested classes?

6. Provide a short code snippet to define an inner anonymous class:
a. That implements a given interface I1 with one method: void f();
b. That extends a given class C1
c. That extends a given class C2 with no default constructor and a
constructor with two string parameters.

7. Which of the following are true when talking about inner classes
(non-anonymous):
a. Can be declared inside other inner classes
b. Can be declared inside methods and blocks of code
c. Can access private members from the outer class(es)
d. Can have more than one constructor
e. Defines an IS-A relationship between the outer and the inner class
f. Can be further extended by other classes

8. Given the code below, how can you access from the method f() the x
member of the outer class?
.
public class A
}
}


9. Given the following interface definition:

public interface I1
public void f1();
}

Which of the following definitions are correct (and explain
1.
public class A implements I1 ;
}

2.
public class A implements I1 ;
}
public void f1() ;
}

3.
public class A2 implements I2 {
public void f2()
}

4.
public class A2 implements I1.I2

10. Given the following class, how can you create a new instance of the
classes A and B in the main method():

public class Test
static class B
}
class MyTest {
public static void main(String[] args)
}

3.5.2 Problems
1. Modify the class below (using an inner class) to allow using it as
an Iterator (make an example call of the method foo).
public interface Iterator
public class Map ;
}
.
public void foo(Iterator it)

2. Given the CDPlayer class with the definition below, extend the
CDPlayer class and add the necessary changes so it can be used as iterator.
(hint: create an method inside the derived class that returns an
Iterator and uses an anonymous inner class do to this)
public interface Iterator
public class CDPlayer ;
private int currentTrack;

public void next()
public void play()
public void stop()
public void prev()
}
public void foo(Iterator it)



3.8 Error handling with exceptions
3.8.1 Questions:
1. What information you can print about an exception?
2. What is the class name of the standard Java exception that does not
have to be handled?
3. What is the exception restriction related to method overriding? Do
constructors have the same restriction? Why?
4.
a. Is the exception specification part of the method signature? Prove
your answer by writing a small piece of code to justify your answer;
comment your proof.
b. Which parts of the method declaration go in the method signature?
5. Knowing that
System.out.print
does not throw any exception, consider the following of code:

private static int getI()
catch (Throwable t)
}
finally
}
What strings are printed to the console, and what is value getI
returns?

3.8.2 Problems
1. Follow the steps below to create a small Java program with 2
classes:
a. create a class RandomPlay with a method that generates two random
numbers between 1 and 10 (see java.lang.Math.random() for generating
random numbers)
b. if the difference of the 2 numbers is higher than 3, throw an
exception you'll specially define for this case
c. add a main() method to class RandomPlay that repeatedly calls the
first method until it throws no exception
d. exit the program by printing the number of times the first method
threw your exception
2. Consider the following class definitions:

public class Exception1 extends Exception
public class Exception2 extends Exception

and method:

public throwException() throws Exception1, Exception2

and the following 2 blocks of code:
block1:

try
catch (Exception1 e)
catch (Exception2 e)

block2:
try
catch (Exception e)

Replace the commented line in block 2 with your code so that block 2
has the same functionality as block 1.


3.10 Detecting types
3.10.1 Questions
1. What is the difference between upcasting and downcasting?
2. What is the difference between "traditional" RTTI and reflection?
3. Let there be the following code fragment:
public class Circle
public class Shape extends Circle
public class Test
}
4. Pretend you are an implementation of the Java Virtual Machine and
try to evaluate (to a boolean value) the following expressions:
a. c.isInstance Circle)
b. c instanceof Shape
c. c.getClass ) == Shape.class
d. c.isInstance Shape)
e. c.getClass ) == Circle.class

5. Is it possible to build a valid line of Java code in a string and
then execute it at run-time (Like in the sample below)? Write a short
program to back-up your answer.
String line = "someClass.method )";
run(line);

3.10.2 Problems:
1. Write a very simple graphical editor that allows you to draw objects
deriving from the Shape class. Read the available classes of shapes
from a configuration file, and present to the user a simple menu that
allows him to select the shape to be drawn.
public interface Shape
Note: Write the simplest thing that meets the requirements. The
dimensions of the shapes will be hardcoded (for ex.: every Circle drawn will have the same radius)

2. Write a program that by means of reflection creates an instance of the following class:
public class Unknown
}
3. Write a test program using reflection to print out the value of the hidden attribute after an instance of the Unknown class has been
created in the previous exercise. If you have succeeded, try to double the
value of the hidden attribute of this instance. Hint: check-out the
java.lang.reflect package.