OOP- Lab Session #7: Generic classes and methods Solved

Get familiar with generic types Given the following class

public class MyPair<T, U> { public final T Fst; public final U Snd; 

public MyPair(T fst, U snd) { this.Fst = fst; this.Snd = snd; 

} 

public String toString() { return "(" + Fst + ", " + Snd + ")"; } 

} 

a.       In a new source file, write a program that includes this declaration and also a class with an empty Main method. Compile it to check that the program is well-formed.

b.       Declare a variable of type MyPair<String, Integer> and create some values, for instance new MyPair<String, Integer>("Anders", 13), and assign them to the variable.

c.       Declare a variable of type MyPair<String, Double>. Create a value such as new MyPair<String, Double>("Phoenix", 39.7) and assign it to the variable.

d.       Can you assign a value of type MyPair<String, Double> to a variable of type MyPair<String, Integer>? Should this be allowed?

e.       Declare a variable grades of type MyPair<String, Integer>[], create an array of length 5 with element type MyPair and assign it to the variable. Create a few MyPairs and store them into grades[0], grades[1] and grades[2].

f.        Use the foreach statement to iterate over grades and print all its elements. What are the values of those array elements you did not assign anything to?

g.       Declare a variable appointment of type

MyPair<MyPair<Integer, Integer>, String>

and create a value of this type and assign it to the variable.

What is the type of appointment.Fst.Snd? This shows that a type argument may itself be a constructed type.  

h.       Declare a method Swap() in MyPair<T, U> that returns a new value of type MyPair in which the components have been swapped.

II.            Differences between Object, generic and generic raw types

In JAVA, there is a Map data structure that helps developers to link a key to a value.

Read about Map in

-          https://www.geeksforgeeks.org/map-interface-java-examples/

-          https://www.geeksforgeeks.org/java-util-hashmap-in-java/

-          https://www.tutorialspoint.com/java/java_hashmap_class.htm

As we want to reimplement the Map class, implement a class named MyMap to manage (store and get back) any object by its ID.

-          User can put an object obj to a Map m by calling

m.put(obj.getID(), obj);

-          User can get back an object from the map m by invoking

m.get(id);
1.       Implement the MyMap in two different ways:

a.       Use Object as the type for both the Key and the Value parameters of the put and get methods

b.       Use generic type

2.       With your implementations, write a main function to

a.       Test these two implementations

b.       To show advantage of generic type over Object

c.       To show advantage of parameterized type over generic raw type

Reference: textbook “Java How to Program”, chapter 20