Week 6 [Mon, Sep 18th] - Topics

Given below is an extract from the -- Java Tutorial, with some adaptations.

public class BoxForIntegers {
    private Integer x;

    public void set(Integer x) {
        this.x = x;
    }
    public Integer get() {
        return x;
    }
}

    

    


    
    

public class BoxForString {
    private String x;

    public void set(String x) {
        this.x = x;
    }
    public String get() {
        return x;
    }
}

    

    


    
    

public class Box {
    private Object x;

    public void set(Object x) {
        this.x = x;
    }
    public Object get() {
        return x;
    }
}

    

    


    
    

This section includes extract from the -- Java Tutorial, with some adaptations.

The definition of a generic class includes a type parameter section, delimited by angle brackets (<>). It specifies the type parameters (also called type variables) T1, T2, ..., and Tn. A generic class is defined with the following format:

class name<T1, T2, ..., Tn> { /* ... */ }

    

    


    
    

public class Box {
    private Object x;

    public void set(Object x) {
        this.x = x;
    }

    public Object get() {
        return x;
    }
}

    

    


    
    

public class Box<T> {
    private T x;

    public void set(T x) {
        this.x = x;
    }

    public T get() {
        return x;
    }
}

    

    


    
    

To reference the generic Box class from within your code, you must perform a generic type invocation, which replaces T with some concrete value, such as Integer. It is similar to an ordinary method invocation, but instead of passing an argument to a method, you are passing a type argument enclosed within angle brackets — e.g., <Integer> or <String, Integer> — to the generic class itself. Note that in some cases you can omit the type parameter i.e., <> if the type parameter can be inferred from the context.

Box<Integer> integerBox;
integerBox = new Box<>(); // type parameter omitted as it can be inferred
integerBox.set(Integer.valueOf(4));
Integer i = integerBox.get(); // returns an Integer

    

    


    
    

The compiler is able to check for type errors when using generic code.

Box<String> stringBox = new Box<>();
stringBox.set(Double.valueOf(5.0)); //compile error!

    

    


    
    

A generic class can have multiple type parameters.

public interface Pair<K, V> {
    public K getKey();
    public V getValue();
}

    

    


    
    

public class OrderedPair<K, V> implements Pair<K, V> {

    private K key;
    private V value;

    public OrderedPair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey()	{ return key; }
    public V getValue() { return value; }
}

    

    


    
    

Pair<String, Integer> p1 = new OrderedPair<>("Even", 8);
Pair<String, String>  p2 = new OrderedPair<>("hello", "world");

    

    


    
    

A type variable can be any non-primitive type you specify: any class type, any interface type, any array type, or even another type variable.

By convention, type parameter names are single, uppercase letters. The most commonly used type parameter names are:

• E - Element (used extensively by the Java Collections Framework)
• K - Key
• N - Number
• T - Type
• V - Value
• S, U, V etc. - 2nd, 3rd, 4th types

This section uses extracts from the -- Java Tutorial, with some adaptations.

A collection — sometimes called a container — is simply an object that groups multiple elements into a single unit. Collections are used to store, retrieve, manipulate, and communicate aggregate data.

The collections framework is a unified architecture for representing and manipulating collections. It contains the following:

• Interfaces: These are abstract data types that represent collections. Interfaces allow collections to be manipulated independently of the details of their representation.
  Example: the List<E> interface can be used to manipulate list-like collections which may be implemented in different ways such as ArrayList<E> or LinkedList<E>.
  

• Implementations: These are the concrete implementations of the collection interfaces. In essence, they are reusable data structures.
  Example: the ArrayList<E> class implements the List<E> interface while the HashMap<K, V> class implements the Map<K, V> interface.
  

• Algorithms: These are the methods that perform useful computations, such as searching and sorting, on objects that implement collection interfaces. The algorithms are said to be polymorphic: that is, the same method can be used on many different implementations of the appropriate collection interface.
  Example: the sort(List<E>) method can sort a collection that implements the List<E> interface.

A well-known example of collections frameworks is the C++ Standard Template Library (STL). Although both are collections frameworks and the syntax look similar, note that there are important philosophical and implementation differences between the two.

The following list describes the core collection interfaces:

• Collection — the root of the collection hierarchy. A collection represents a group of objects known as its elements. The Collection interface is the least common denominator that all collections implement and is used to pass collections around and to manipulate them when maximum generality is desired. Some types of collections allow duplicate elements, and others do not. Some are ordered and others are unordered. The Java platform doesn't provide any direct implementations of this interface but provides implementations of more specific subinterfaces, such as Set and List. Also see the Collection API.

• Set — a collection that cannot contain duplicate elements. This interface models the mathematical set abstraction and is used to represent sets, such as the cards comprising a poker hand, the courses making up a student's schedule, or the processes running on a machine. Also see the Set API.

• List — an ordered collection (sometimes called a sequence). Lists can contain duplicate elements. The user of a List generally has precise control over where in the list each element is inserted and can access elements by their integer index (position). Also see the List API.

• Queue — a collection used to hold multiple elements prior to processing. Besides basic Collection operations, a Queue provides additional insertion, extraction, and inspection operations. Also see the Queue API.

• Map — an object that maps keys to values. A Map cannot contain duplicate keys; each key can map to at most one value. Also see the Map API.

• Others: Deque, SortedSet, SortedMap

The ArrayList class is a resizable-array implementation of the List interface. Unlike a normal array, an ArrayList can grow in size as you add more items to it. The example below illustrates some of the useful methods of the ArrayList class using an ArrayList of String objects.

import java.util.ArrayList;

public class ArrayListDemo {

    public static void main(String args[]) {
        ArrayList<String> items = new ArrayList<>();

        System.out.println("Before adding any items:" + items);

        items.add("Apple");
        items.add("Box");
        items.add("Cup");
        items.add("Dart");
        print("After adding four items: " + items);

        items.remove("Box"); // remove item "Box"
        print("After removing Box: " + items);

        items.add(1, "Banana"); // add "Banana" at index 1
        print("After adding Banana: " + items);

        items.add("Egg"); // add "Egg", will be added to the end
        items.add("Cup"); // add another "Cup"
        print("After adding Egg: " + items);

        print("Number of items: " + items.size());

        print("Index of Cup: " + items.indexOf("Cup"));
        print("Index of Zebra: " + items.indexOf("Zebra"));

        print("Item at index 3 is: " + items.get(2));

        print("Do we have a Box?: " + items.contains("Box"));
        print("Do we have an Apple?: " + items.contains("Apple"));

        items.clear();
        print("After clearing: " + items);
    }

    private static void print(String text) {
        System.out.println(text);
    }
}

    

    


    
    

Before adding any items:[]
After adding four items: [Apple, Box, Cup, Dart]
After removing Box: [Apple, Cup, Dart]
After adding Banana: [Apple, Banana, Cup, Dart]
After adding Egg: [Apple, Banana, Cup, Dart, Egg, Cup]
Number of items: 6
Index of Cup: 2
Index of Zebra: -1
Item at index 3 is: Cup
Do we have a Box?: false
Do we have an Apple?: true
After clearing: []

    

    


    
    

[Try the above code on Repl.it]

HashMap is an implementation of the Map interface. It allows you to store a collection of key-value pairs. The example below illustrates how to use a HashMap<String, Point> to maintain a list of coordinates and their identifiers e.g., the identifier x1 is used to identify the point 0,0 where x1 is the key and 0,0 is the value.

import java.awt.Point;
import java.util.HashMap;
import java.util.Map;

public class HashMapDemo {
    public static void main(String[] args) {
        HashMap<String, Point> points = new HashMap<>();

        // put the key-value pairs in the HashMap
        points.put("x1", new Point(0, 0));
        points.put("x2", new Point(0, 5));
        points.put("x3", new Point(5, 5));
        points.put("x4", new Point(5, 0));

        // retrieve a value for a key using the get method
        print("Coordinates of x1: " + pointAsString(points.get("x1")));

        // check if a key or a value exists
        print("Key x1 exists? " + points.containsKey("x1"));
        print("Key x1 exists? " + points.containsKey("y1"));
        print("Value (0,0) exists? " + points.containsValue(new Point(0, 0)));
        print("Value (1,2) exists? " + points.containsValue(new Point(1, 2)));

        // update the value of a key to a new value
        points.put("x1", new Point(-1,-1));

        // iterate over the entries
        for (Map.Entry<String, Point> entry : points.entrySet()) {
            print(entry.getKey() + " = " + pointAsString(entry.getValue()));
        }

        print("Number of keys: " + points.size());
        points.clear();
        print("Number of keys after clearing: " + points.size());

    }

    public static String pointAsString(Point p) {
        return "[" + p.x + "," + p.y + "]";
    }

    public static void print(String s) {
        System.out.println(s);
    }
}

    

    


    
    

Coordinates of x1: [0,0]
Key x1 exists? true
Key x1 exists? false
Value (0,0) exists? true
Value (1,2) exists? false
x1 = [-1,-1]
x2 = [0,5]
x3 = [5,5]
x4 = [5,0]
Number of keys: 4
Number of keys after clearing: 0

    

    


    
    

[Try the above code on Repl.it]

A model is a representation of something else.

A model provides a simpler view of a complex entity because a model captures only a selected aspect. This omission of some aspects implies models are abstractions.

Multiple models of the same entity may be needed to capture it fully.

In software development, models are useful in several ways:

a) To analyze a complex entity related to software development.

b) To communicate information among stakeholders. Models can be used as a visual aid in discussions and documentation.

c) As a blueprint for creating software. Models can be used as instructions for building software.

An OO solution is basically a network of objects interacting with each other. Therefore, it is useful to be able to model how the relevant objects are 'networked' together inside a software i.e. how the objects are connected together.

Note that these object structures within the same software can change over time.

However, object structures do not change at random; they change based on a set of rules set by the designer of that software. Those rules that object structures need to follow can be illustrated as a class structure i.e. a structure that exists among the relevant classes.

UML Object Diagrams model object structures. UML Class Diagrams model class structures.

Contents related to UML diagrams in the panels given below belong to a different chapter (i.e., the chapter dedicated to UML); they have been embedded here for convenience.

Classes form the basis of class diagrams.

Associations are the main connections among the classes in a class diagram.

The most basic class diagram is a bunch of classes with some solid lines among them to represent associations, such as this one.

An example class diagram showing associations between classes.

In addition, associations can show additional decorations such as association labels, association roles, multiplicity and navigability to add more information to a class diagram.

Here is the same class diagram shown earlier but with some additional information included:

Object diagrams can be used to complement class diagrams. For example, you can use object diagrams to model different object structures that can result from a design represented by a given class diagram.

Compared to the notation for class diagrams, object diagrams differ in the following ways:

• Show objects instead of classes:
  • Instance name may be shown
  • There is a : before the class name
  • Instance and class names are underlined
• Methods are omitted
• Multiplicities are omitted. Reason: an association line in an object diagram represents a connection to exactly one object (i.e., the multiplicity is always 1).

Furthermore, multiple object diagrams can correspond to a single class diagram.

Both object diagrams are derived from the same class diagram shown earlier. In other words, each of these object diagrams shows ‘an instance of’ the same class diagram.

When the class diagram has an inheritance relationship, the object diagram should show either an object of the parent class or the child class, but not both.

Suppose Employee is a child class of the Person class. The class diagram will be as follows:

Now, how do you show an Employee object named jake?

• This is not correct, as there should be only one object.

• This is OK.

• This is OK, as jake is a Person too. That is, we can show the parent class instead of the child class if the child class doesn't matter to the purpose of the diagram (i.e., the reader of this diagram will not need to know that jake is in fact an Employee).

Association labels/roles can be omitted unless they add value (e.g., showing them is useful if there are multiple associations between the two classes in concern -- otherwise you wouldn't know which association the object diagram is showing)

Consider this class diagram and the object diagram:

We can clearly see that both Adam and Eve lives in hall h1 (i.e., OK to omit the association label lives in) but we can't see if History is Adam's major or his minor (i.e., the diagram should have included either an association label or a role there). In contrast, we can see Eve is an English major.