Object Oriented Programming #

In the second major unit of this class, we will focus on programming in Java because it’s an object oriented programming language; C is not. However, the ideas and concepts of the memory model in C will inform both how we think about object oriented programming (or OOP) and kinds of programs we design.

So, what is object oriented programming? #

There is one overarching idea in programming — from small programs, to gigantic systems: Separation of Interface from Implementation i.e. separating what you need to know in order to use a tool from how the tool actually works on the inside. This is behind good coding, it’s behind good system design, it is behind the design of protocols like TCP, HTTP and DNS, it’s behind almost anything that involves computing! This separation allows us to manage the complexity of building big programs. It allows us to reuse in a new program code that was written in some other time and place for some other purpose (and to avoid duplicating code within a program, which is a sin!). It allows us to make changes to some of our code without having to worry about other parts of the program breaking. It makes collaboration easier and less error-prone. It brings many, many other benefits. In short: separating interface from implementation is a really good thing.

In procedural programming, which is what we’ve mostly been doing so far, the language mechanism that supported separating interface from implementation are functions: a function’s prototype (plus some documentation, if we’re lucky) is the interface, its definition is the implementation. Scoping rules actually prohibit us from accessing the local variables and parameters inside the function’s body, so this separation of interface from implementation isn’t just conceptual — it’s actually enforced by the compiler! However, the function mechanism on its own is by no means enough to achieve the nirvana of full separation of Interface from Implementation. Why?

1. It doesn’t help with structs. You have to understand how the designer of code intends to manipulate structs to store data in order to use that code. That means understanding implementation details!

2. You can’t modify or extend the behavior of existing functions or existing collections of functions and structs without messing with the function/struct definitions, which means you have to understand those implementation details!

3. You’re limited to a single implementation for each interface (a single function definition for each prototype), which is a real problem. As a simple analogy, that would be like saying each kind of light-bulb has to have a different socket, which would be … difficult. Instead, we live in a world in which the same socket works for 40watt or 60watt bulbs, for incandescent bulbs or for fluorescent bulbs or LED bulbs or — more interestingly — black-light bulbs. For many kinds of programming problems we want the same thing: many different implementations of the same interface.

Object-oriented programming is a programming paradigm — that means not merely a language but a whole philosophy on what a program is — with the goal of complete separation of interface from implementation. It’s basic ideas are

1. Combine structs and the functions that manipulate those structs into a single entity — such entities are called objects — that offers true separation of the interface to your struct+functions from its implementation. This is said to provide encapsulation and data-hiding. Note: this addresses Point 1 from above.

2. Use a mechanism called inheritance to create new kinds of objects that modify or extend old kinds of objects without having to open up the implementation (in some cases without even having access to the implementation!).
   Note: this addresses Point 2 from above.

3. Use inheritance and a mechanism called polymorphic function calls to allow multiple definitions of the same interface.
   Note: this addresses Point 3 from above.

To sum up: The Procedural Programming paradigm sees a program as functions calling functions, with data being passed around as arguments to or return-values from function calls. The Object-Oriented Programming paradigm sees a program as a bunch of data+function bundles (the objects) that communicate by calling each other’s member functions.

Trying (and failing) to Achieve OOP in C #

To further motivate this, let’s consider the implementation of object-oriented like implementation of an ArrayList in C. From that implementation, which is a procedural program, we can observe where procedural programming falls short and where OOP can help us.

In implementing, we need to first define a struct to hold the data of the ArrayList.

//data elements
typedef struct ArrayList{
  int len; //number of items in the array
  int size; // current size of array
  int * array;
  
}alist_t;

In many ways we can think of this as object, but it is not object-oriented. There are two main reasons for this.

1. How to use this ArrayList struct, as an data storage object, is separate from the definition. To put it another way: while we know what it stores the structure by itself doesn’t expres how to use it.

2. While this may appear to offer a form of encapsulation — it does place related data fields together into a single structure — it does not provide any data-hiding. If I (as a programmer) had access to an alist_t I could modify any part of it directly, such as the len, making all the data incosistent.

Continuing on, with the structure of the ArrayList defined, we can now define an API over the array list, like the below function prototypes.

#define AL_INIT_SIZE 16

//interface elements
alist_t * new_al();
void del_al(alist_t * al);

//return 0/1 success failure if out of bounds
int get(alist_t * al, int idx, int * dest); //store what's got at dest
int set(alist_t * al, int idx, int val); //set al_list[i] to val

//return 0/1 success failure if list is empty
int pop_tail(alist_t * al, int * dest); //store what's poped at dest

//always adds to the end, and expands as neccesary.
void push_tail(alist_t * al, int val); //add val to the tail

//internal function
void _expand(alist_t * al); //expand the array

//useful functions
void print_al(alist_t * al);

One thing you may notice right away, is that each of the functions that operate over ArrayLists must also take in alist_t as its first argument. That’s again because we do not have full encapsulation. The functions are generic but we must specify which ArrayList to operate on. In a object oriented model, the functions implicitly operate over the current instance, as we will see, and thus we do not need to

Another example of an issue with encapsulation is _expand — this function is called when we run out of space in the array. It’s marked, in comments, as an internal function, so normal users shouldn’t call it. But there are no mechanisms to offer that protection.

Finally, perhaps the most challenging issue is that this ArrayList is not extensible. It can only store ints; it only has these features. If we wanted to expand it to have other features, the entire API would need to change.

Essentially, while we can do everything in a procedural way, it’s unpleasant and potentially not as effective. With OOP we can do better.

Why Java? #

For the rest of this class, we’ll be transition to Java programming because Java, by design, is a completely Object Oriented Programming language. Yes. You may already know Java, but we are going to zoom out and then zoom into Java in new ways, particularly focusing in on the features of objects for encapsulation, data-hiding, inheritance, and polymorphism.

We’ll first start by reviewing some of the basic features of Java (as a refresher) and then quickly dive into Java’s object model.

Java Review #

You already know Java! So let’s just quickly go through the basics, as a refresher, just in case you’ve forgotten a few things let’s go over a basic program.

Hello World #

We start where we always start, at the beginning, with “Hello, world!” With Java, each file represents a class. A class is a definition of how to create an object … more on that later! For now, we want to create a new class that will print “Hello, world!” We name that file HelloWorld.java

public class HelloWorld{
    public static void main(Sting args[]){
        System.out.println("Hello, world\n");
    }

}

We compile that program with the java compiler javac

javac HelloWorld.java

This produces a compiled version of our program, a class file, called HelloWorld.class that contains java bytecode. To execute this byte code, we use the Java virtual machine (JVM) which will interpret and execute the program.

$ java HelloWorld
Hello, world!

Looking back at this program, there is a lot of code going on here. Java is quite a bit heavier than other languages, like C, because everything must be part of the object model.

This starts with declaring our hello-world program to be an object, that is, we put it in its own file and say it’s a publicly accessible class: public class HelloWorld. The main method (or function) also has a lot of modifiers:

• public : allow the method to called outside of the class
• static : allow the method to be called without instantiating the containing object — that is, you don’t need to create a new HelloWorld object to call main
• void : it returns no values

And the arguments are also complicated

• String args[] : the main method accepts an array of String types that represent the command line arguments provided to the exection.

Finally, the printing statement System.in.println() uses the . operator as it the method println() operates on the in object/class which is further found as a member of System package. Phew!!!

Basic Types #

People like to say that everything in Java is really an object, but that’s not exactly true. Everything but the basic types are objects. The basic types in Java are the same as those you’d find in C, plus two more

The key difference between basic types and objects in Java is that basic types can be created on the stack, but objects must be allocated using new. For example,

int i = 10; //basic type
MyObj o =  new Myobj(); //object

That’s kind of what makes them basic … As convention, to further distinguish basic types from objects, objects are given names with upper case letters while basic types are given type names with lower case letters.

Operators over basic types #

All the basic types are numeric and so the same set of operators as you might expect work on them. This includes,

• x + y : addition
• x - y : subtraction
• x * y : multiplication
• x / y : divisions
• x % y : modulo

Assignment operators

• x+=y : add x to y and store result in x
• x-=y : subtract y from x and store result in x
• x*=y : multiply x by y and store result in x
• x/=y : divided x by y and store result in x

Then there are unary addition and subtraction for adding/subtracting 1:

• x++ : add one to x and return x
• ++x : add one to x and return x+1
• x-- : subtract one from x and return x
• --x : subtract one from x and return x-1

However, when you are working with an object, to perform an operation on that object you must use the . operator.

String type #

The only exception to the rules above about basic types, operators, and assignments are strings. The String object—which is clearly an object based on the upper case letter “S” in “String—is the only object that can be declared and used much like a basic type, including the operators. This is the only exception, and it was done because strings are so fundamental programming that the shortcut made enormous practical sense.

For example, we can declare and operate on a string as if it was a basic type

String s = "This is a string";
s = "This is a different string"; //reassign
s = "One " + " two " + " three ";  //conctenate strings
s = new String("This is also a string!");

However, strings are still objects. So if you were to do the following below, we’d get that s and t are different strings.

String s = "I am a string";
String t = new String("I am a string");

if( s == t)
  System.out.println("s and t are the *same* string");
else
  System.out.println("s and t are *different* strings");

This is because s and t are objects. The == operator compares the reference to those objects are the same, as in, does s and t reference the same object, or two different objects? In this case, it’s two different objects that represent the same string because of the explicit new.

Instead, we need to use . operator to do the compares, and in this case, we get that the two strings are equal using string .equlas().

String s = "I am a string";
String t = new String("I am a string");

if( s.equals(t))
  System.out.println("s and t are the *same* string");
else
  System.out.println("s and t are *different* strings");

If you want to access individual char’s in a string, we don’t use the [ ] operators (strings are not arrays!), but rather the charAt() method. For example, for the string

Strings s = "I love cs2113!";
System.out.println("The char at index 3 is "+s.charAt(3));

Will print The char at index 3 is o

Java Objects #

Java objects are much like structs in C, they are a way to group related data together into a single structure. Unlike a struct, a Java object also allows you to associate methods (both public and private) with that object to operate over the data.

To review this functionality, let’s build up a working example of defining shapes and lines in the Cartesian plain. Recall that this the x, y coordinate system. For example, perhaps we have three points in the plain.

    y
    ^   p0
    |    * (2.3,4.8)
    |p1
    | * (1.1,3.0)
    |          
    |        p2 * (5.8,0.3)
<---+---------------> x
    |
    v

If we anted to define these points and some operations over them, say the distances between them, we could write the following program:

public class PointDistances{

    public static void main(String args[]){
        
        //                  p0  p1  p2
        double Xpoints[] = {2.3,1.1,5.8};
        double Ypoints[] = {4.8,3.0,0.3};

        System.out.println("dist(p0,p1) = " +
                           Math.sqrt(Math.pow(Xpoints[0] - Xpoints[1],2) +
                                     Math.pow(Ypoints[0] - Ypoints[1],2))
                           );
        System.out.println("dist(p0,p2) = " +
                           Math.sqrt(Math.pow(Xpoints[0] - Xpoints[2],2) +
                                     Math.pow(Ypoints[0] - Ypoints[2],2))
                           );
        System.out.println("dist(p0,p1) = " +
                           Math.sqrt(Math.pow(Xpoints[1] - Xpoints[2],2) +
                                     Math.pow(Ypoints[1] - Ypoints[2],2))
                           );
    }
}

While this provides some structure, it doesn’t quite feel right. Adding another point would require modifying the array and writing a new method. Let’s do better.

Point Object #

Let’s start by defining a new class for storing a point, we’ll place it in a new file called Point.java. A class is a recipe for creating objects. It defines what is stored in the object and what methods operate over.

//Point.java
public class Point{

    double x;
    double y;

    //constructor
    public Point(double X, double Y){
        this.x=X;
        this.y=Y;
    }

    public double dist(Point other){
        return Math.sqrt( Math.pow(this.x - other.x,2) +
                          Math.pow(this.y - other.y,2));
    }
}

This class defines two data members, x and y to store the coordinates. It also defines a constructor for the Point object, and a method dist.

Constructor #

A constructor for an object is a special method that defines how to “builds” an object of the current class. In our case this is rather simple. We only need to the know the x and y coordinate, and we wish to store them.

(Aside: All objects inherent from the Object class, and that constructor is actually called first. So once your constructor is called, the object already exists with a bunch of features of Object – what you are doing is modifying that core object type to be of the class we’re defining. In this case, that’s a Point.)

this #

The this variable is special variable that refers to the current object. Recall that the class is a definition for how to create objects, but within methods, like the constructor, we need a way to differentiate between things that are associated with the class and the current object being operated on.

Within the constructor, the this refers to the new object being constructed. So assigning to this.x or this.y says, store the input values X and Y within the newly created object.

(Aside: You don’t explicly need to use this, its implied due to the scoping. So x=X and y=Y would work just as well. Java will look for the most locally bounded x to use, and in this case that would be the x within the object being constructed. Where this would be strictly necessary is when you have variable shadowing. For example, if the constructor were declared as public Point(double x, double y) — lowercase x and y —then the most locally bound x and y are within the function, not the object. I find this helps disambiguate scoping, so sometimes I use it, but not always.)

dist method #

The dist method is an object that operates over the object, called via the . operator, and another Point object. Via the . operate we get access to the Point object whose method was called, in this case this, and also the Point object passed as an argument. Using both of those we can compute the distance between this point and the other point, returning the result.

main method #

Now we can put it all together, writing a new main method.

public class PointDistances2{

    public static void main(String args[]){

        Point p0 = new Point(2.3,4.8);
        Point p1 = new Point(1.1,3.0);
        Point p2 = new Point(5.8,0.3);

        double d01 = p0.dist(p1);
        double d02 = p0.dist(p2);
        double d12 = p1.dist(p2);
        
        System.out.println("dist(p0,p1) = " + d01);
        System.out.println("dist(p0,p2) = " + d02);
        System.out.println("dist(p1,p2) = " + d12);
        
    }
}

$ java PointDistances2.java
dist(p0,p1) = 2.1633307652783933
dist(p0,p2) = 5.70087712549569
dist(p1,p2) = 5.420332093147061

Note that for each Point we have to create a new instance of it. That act of calling new invokes the constructor with the arguments, returning a new object, whose reference is stored in the variable. Latter, we can use those objects to calculate distances.

Memory Diagrams in Java #

Just like in C we should also consider memory diagramming in Java. Let’s look at the last program above PointDistance2 and draw the memory diagram.

    STACK                            HEAP
     
    .-----------------.             .---------.
    | p0    |   .-----+-----------> | x | 2.2 | (Point Objects)
    |-------+---------.             |---+-----|
    | p1    |   .-----+---.         | y | 4.8 | 
    |-------+---------.    \        '---------'
    | p2    |   .-----+--.  '-----> .---------.
    |-------+---------.   \         | x | 1.1 |
    | d01   | 2.16... |    \        |---+-----|
    |-------+---------.     \       | y | 3.0 |
    | d02   | 5.70... |      \      '---------'
    |-------+---------.       '---> .---------.
    | d12   | 5.42... |             | x | 5.8 |
    '-----------------'             |---+-----|
                                    | y | 0.3 |
                                    '---------'

The basic types, in this case double, is stored on the stack, but the objects are allocated on the heap, via new. That means the variables in main reference those objects and thus pointers.

Note to access members of the objects, we always use the . operator, and never the -> operator in Java. That’s because in Java you can never have a stack instance of an object: all objects are allocated on the heap and all variables reference a heap stored object. There is no need for two operators, and thus, as programmers are lazy, we opt for the simpler . operator.

Encapsulation and Data Hiding #

Java has a strong notion of encapsulation, which means that data and methods within objects should be limited such that they are accessed within scope. For example, the data members x and y are unprotected (default public)

This means, as in the below example, we can use the . operator to both read the values of p.x and p.y as well as modify them.

public class PointDistances3{

    public static void main(String args[]){

        Point p = new Point(2.3,4.8);

        System.out.println("p = ("+p.x+","+p.y+")");

        p.x = 5;
        p.y = 10;

        System.out.println("p = ("+p.x+","+p.y+")");
        
    }
}

However, this can be dangerous and violates the principal of encapsulation where only access data where most appropriate.

Public vs. Private Data Members #

Java uses a notion of public and private declarations on both object data and methods to limit how programmers can use the object. (There is also protected but we’ll get to that later.)

Perhaps we want to better control access to the data members, we can declare x and y as private class members.

public class Point{

   private double x;
   private double y;

   //..

And this will cause a compiler error when accessing x and y directly using the . operator.

PointAccess.java:9: error: x has private access in Point
        p.x = 5;
         ^
PointAccess.java:10: error: y has private access in Point
        p.y = 10;

Getter and Setters #

So if we have private members, how do we access these members? We use getter and setter methods.

public class Point{

    private double x;
    private double y;

    public Point(double X, double Y){
        this.x=X;
        this.y=Y;
    }

    public double getX(){
        return this.x;
    }
    
    public double getY(){
        return this.y;
    }

    public void setX(double X){
        this.x = X;
    }

    public void setY(double Y){
        this.y = Y;
    }
    
    public double dist(Point other){
        return Math.sqrt( Math.pow(this.x - other.x,2) +
                          Math.pow(this.y - other.y,2));
    }
}

And then we call those public methods, rather than directly accessing the data members directly.

public class PointAccess2{

    public static void main(String args[]){

        Point p = new Point(2.3,4.8);

        System.out.println("p = ("+p.getX()+","+p.getY()+")");

        p.setX(5);
        p.setY(10);

        System.out.println("p = ("+p.getX()+","+p.getY()+")");
        
    }
}

Perhaps, we also want a method in Point to set both X and Y and the same time. So we can add that as well

  public void setXY(double X, double Y){
        this.x = X;
        this.y = Y;
    }

And wait, that’s also very much like the constructor, so why not call that there?!?

 public Point(double X, double Y){
        setXY(X,Y);
    }

And now we’re really programming.

toString() #

As we start to develop our Point class, we should still be somewhat unsatisfied with the printing routine.

   System.out.println("p = ("+p.getX()+","+p.getY()+")");

This also seems like a violation of encapsulation because the representation of the Point as “(x,y)” string shouldn’t change regardless of the instance of the Point. Instead, it should be part of the class definition of the object. It would make a lot more sense to have a method that returned the string representation. Even better, it would be great if we could just do the following, like we do with basic types?

System.out.println("p="+p);

If you were to try that, what you’d find is that something does print, but not what you expect

p = Point@5acf9800
p = Point@5acf9800

That string is the Java reference representation. Essentially say that it’s the Point stored at reference 5acf9800. The + operator with a string and a non-string object will automatically convert the non string object to a string by calling a special method toString() on that object. If one isn’t define, then the default toString() method is called inheritted from the Object class.

But, we can overwrite the default toString() to write our own specific for Point.

public String toString(){
        return "("+this.x+","+this.y+")";
    }

Now, we’ve further simplified our main method

public class PointAccess3{

    public static void main(String args[]){

        Point p = new Point(2.3,4.8);

        System.out.println("p = "+p);

        p.setXY(5,10);

        System.out.println("p = "+p);
        
    }
}

More advanced object programming #

Line object #

Suppose now we want to extend on our object model and create a new Line class that builds on our notion of Point. A Line can simply be defined as a combination of two points, a start and and end. This is relatively straightforward to write the length() method and toString() method given our work on Point.

public class Line{

    private Point start;
    private Point end;

    public Line(double x1, double y1, double x2, double y2){
        start = new Point(x1,y1);
        end = new Point(x2,y2);
    }

    public double length(){
        return start.dist(end);
    }

    public String toString(){
        return "["+start+":"+end+"]";
    }

}

Here’s an example main method

public class LineExample1{

    public static void main(String args[]){

        Line l = new Line(4.0,6.0,5.0,7.0);

        System.out.println("l = " + l);
        System.out.println("l.length = " + l.length());
    }
}

Method and Constructor Overloading #

Looking at the constructor for Line, you might wonder — it’s just taking two Points, why not pass two Points? Yes. Totally true, but it would also be nice to keep the other constructor, based on 4 doubles. Good news everyone! You can do both by overloading the constructor.

In our Line class, let’s define a second constructor. It has the same name of the other constructor, but takes different arguments. Here are the two constructors side by side.

    public Line(Point p1, Point p2){
        start = p1;
        end = p2;
    }
    public Line(double x1, double y1, double x2, double y2){
        start = new Point(x1,y1);
        end = new Point(x2,y2);
    }

How does Java choose between these two constructors? It does type inspection on the arguments and matches to the constructor with the right matching types. So you cannot have two constructors (or methods within scope of an object/class) with the same name and the same arguments, otherwise Java will not know which method to call.

Maintaining Encapsulation with Objects References #

The new Line constructor that takes two Points bring up an interesting new problem regarding encapsulation. Consider the getter method for Line that retrieves one of the points:

    public Point getStart(){
        return start;
    }

If this method were called by the user on a Line instance, than altering the Point start would allow the user to alter the data stored within that Line instance, even though it is set to private. Also, consider the new constructor

    public Line(Point p1, Point p2){
        start = p1;
        end = p2;
    }

The user passing the point p1 could still alter it, even after using it in the constructor. To see this, let’s look at a memory diagram, where we can see that we only have one object referenced by start, p1 and s at point **A**. Modification by using . from any of these references modifies the same object. After modifying, it prints [(20,10):(2.1,3.4)] despite no calls to l to modify the underlying data it stores.

public class LineExample2{

    public static void main(String args[]){

        Point p1 = new Point(4.0,8.5);
        Point p2 = new Point(2.1,3.4);
        Line l = new Line(p1,p2); 

        Point s = l.getStart();//**A**

        s.setXY(20.0,10.0);  //**B**
        System.out.println("l = " + l);// [(20,10):(2.1,3.4)]

    }
}

   **A**                                                
   
   STACK                    HEAP                       
                            
                             .--------.       
                   .-------> | x | 4.0| <-----.
  .----------.    /          |--------|       |
  | p1  |  .-+---'    .----> | y | 8.5|       |
  |-----+----|       /       '--------'       |
  | p2  |  .-+----. /                         |
  |-----+----|     X         .--------.       |
  | l   |  .-+--. / \        | x | 4.0| <-----+--.
  |-----+----|   X   '-----> |--------|       |  |
  | s   |  .-+-./ \          | y | 8.5|       |  |
  '----------'     \         '--------'       |  |
                    \                         |  |
                     \       .----------.     |  |
                      '----> |start | .-+-----'  |
                             |------+---|        |
                             |  end | .-+--------'
                             '----------'           

Now — before solving this problem, you might ask is this really a problem? It really depends on the program. In many cases this may be the desired functionality. Here, though, we want to ensure that only via calls to modifiers within Line can we alter the underlying data storage.

Copy Constructor #

In this case, let’s solve this problem by creating a new overloaded constructor for Point that takes another Point instance as it’s argument, constructing a new Point object based on it’s x and y value. Like in the example below

   public Point(Point other){ 
      this.x = other.getX();
      this.y = other.getY();
   }

And in our Line constructor, that takes two Points, we modify that to call this Point constructor to perform a copy.

   public Line(Point p1, Point p2){
     this.start = new Point(p1);
     this.end = new Point(p2);
   }

Similarly, when we call getStart() or getEnd(), we also use this constructor

    public Point getStart(){
        return new Point(this.start);
    }
    public Point getEnd(){
        return new Point(this.end);
    }

Looking at the memory diagram for the main method above, with these modication, we see there are no longer shared references (at **B**).

   **B** 
   
   STACK                      HEAP
                            
                             .--------.          .--------.
                   .-------> | x | 4.0|   .----> | x | 4.0|
  .----------.    /          |--------|   |      |--------|
  | p1  |  .-+---'           | y | 8.5|   |      | y | 8.5|
  |-----+----|               '--------'   |      '--------'
  | p2  |  .-+----.                       |              
  |-----+----|     \         .--------.   |      .--------.
  | l   |  .-+--.   \        | x | 4.0|   | .--> | x | 4.0|
  |-----+----|   \   '-----> |--------|   | |    |--------|
  | s   |  .-+-.  \          | y | 8.5|   | |    | y | 8.5|
  '----------'  \  \         '--------'   | |    '--------'
                 \  \                     | |
                  \  \       .----------. | |    .--------.
                   \  '----> |start | .-+-' | .->| x |20.0|
                    \        |------+---|   | |  |--------|
                     \       |  end | .-+---' |  | y |10.0|
                      \      '----------'     |  '--------'
                       '----------------------'      
                             

Deep vs. Shallow copy #

The new Point constructor is called deep copy constructor because it copies the object to a new object, without any references between them. That is relatively straightforward to do here because Point’s data members are both basic types. In contrast, a shallow copy which creates a new object that shares references.

As an example, consider the following method for create a copy of the current line. (Note, this wouldn’t compile due to start and end as private members.)

  public Line(Line other){
    this.start = other.start;
    this.end = other.end;
  }

Doing so would mean the new Line object (this) and the other Line object (other) would actually share the underlying data storage Points. This is a shallow copy. Alternatively, we can do a deep copy of the line using the facilities already in place.

  public Line(Line other){
    this.start = other.getStart(); //returns a copy of Point start
    this.end = other.getEnd(); //returns a copy of Point end
  }

With that copy constructor in place, we can write a copy() method really easily

  public Line copy(){
    return new Line(this); //construct new object based on this object
  }

Private Methods #

Just like with data, we can also declare a method private if they shouldn’t be called outside the scope of the object. To see an example of this, let’s consider adding a new data member to the Line class, slope.

The slope line is calculated as run over rise. For the example two points below (forming a line), the run is the distance between their x components (-4), and the rise is the distance between their y components (8).

    y
    ^   
    |    * (2,9)
    |    :\
    |  8 : \
    |    :  \    
    |    :...* (6,1)
    |     -4
<---+---------------> x
    |
    v

The slope of a line is run divided by rise, or 8/4 = -2. Importantly, to calculate the slope, we need to order the points by their x component.

Adding Slope #

Let’s start by modifying the List class to add a private member slope, and we can set that slope at construction once start and end is established.

public class Line{

    private Point start;
    private Point end;
    private double slope;
    
    public Line(double x1, double y1, double x2, double y2){
        start = new Point(x1,y1);
        end = new Point(x2,y2);
        slope = calcSlope();
    }

    public Line(Point p1, Point p2){
        start = new Point(p1);
        end = new Point(p2);
        slope = calcSlope();
    }
    
    \\ ...
    
    public getSlope(){
        return slope;
    }
    
    private double calcSlope(){
        double run, rise;
        
        if(end.getX() > start.getX()){
            run = end.getX() - start.getX();
            rise = end.getY() - start.getY();
        }else{
            run = start.getX() - end.getX();
            rise = start.getY() - end.getY();
        }
        
        return rise/run;
    }

Looking at the method calcSlope(), it will never need to be called anywhere but in the constructor for the Line. It only needs to be called once, and if the user wishes to learn the slope, they can call getSlope(). As a result, this method doesn’t need to be public, and so it is declared private.

Additionally, we could simplify our calcSlope() with an additional private method that reorders the points during construction such that the start point always has the lower x-value compared to the end point.

   private void orderPoints(){
        if(end.getX() < start.getX()){
            Point tmp = start;
            start = end;
            end = start;
        }
    }

Again we want this to be a private method; the user shouldn’t call it directly. Instead, it’s called at construction to get the points in order, simplifying the calcSlope() method in the process.

public Line(double x1, double y1, double x2, double y2){
        start = new Point(x1,y1);
        end = new Point(x2,y2);
        orderPoints();
        slope = calcSlope();
    }

    public Line(Point p1, Point p2){
        start = new Point(p1);
        end = new Point(p2);
        orderPoints();
        slope = calcSlope();
    }

    
    private double calcSlope(){
        double run = end.getX() - start.getX();
        double rise = end.getY() - start.getY();
        return rise/run;
    }

    private void orderPoints(){
        if(end.getX() < start.getX()){
            Point tmp = start;
            start = end;
            end = start;
        }
    }

Static Methods #

The static declarations for classes allow functions and data to be accessible within the class without needing to instantiate an object instance of that class.

The classic example of static is the main method. It’s declared static, that’s because when you execute the main method of a class, you’re not instantiating an instance of that class as an object, and then calling main. Instead you’re calling main directly, or statically, without an instance.

Within the object oriented model, we use static to provide utility functions or constants that don’t require object instantiating. You are probably familiar with Integer.parseInt(). The Integer class is an object version of the basic type, and it is common to need to create an int by parsing a String, like:

  int a = Integer.parseInt("5"); //a gets then integer 5

You’ll notice that we don’t create a new Integer object to call the member function parseInt(). We call parseInt() directly and statically.

Static Slope Calculation #

Calculating the slope of a line is a fairly useful utility that we can attach to our Line class as a static method. Doing so means a user doesn’t have to create a Line in order to calculate the slope of the line that connects two points.

We can write that static method like so, and note that it’s also public because it is called outside the context of the class.

   public static double calcSlope(Point p1, Point p2){
        if(p1.getX() > p2.getX())
            return (p1.getY() - p2.getY()) / (p1.getX() - p2.getX());
        else
            return (p2.getY() - p1.getY()) / (p2.getX() - p1.getX());
    }

Additionally, we can’t rely on the points to be properly ordered, since we’re not instantiating a Line. So we have to use if/else. Now we can write a main method taking advantage of the new static method.

public class staticSlope{

    public static void main(String args[]){
        Point p1 = new Point(2,9);
        Point p2 = new Point(6,1);

        //Also, java has printf :)
        System.out.printf("p1: %s p2: %s dist(p1,p2): %.2f slope: %.2f\n",
                          p1,p2,
                          p1.dist(p2),
                          Line.calcSlope(p1,p2));
        //%.2f -- says print two decimal points 
        
    }

}

And just for fun — Java also have printf() :) — which, in some cases is easier to use than println() or print().

Arrays #

Just like in C, Java has arrays that store aligned data that is indexable with the [] operator. Unlike C, though, Arrays are actually objects, an Array object created with a new call. Like other objects, they are dynamically allocated.

Arrays of Basic Types #

To create an array of a basic type, we must first declare the variable to be an array object. This is done by adding [] to the variable name at deceleration.

  int intArray[];

Then we need to allocate the array object of that type. This works much like calloc() call in C. Java determines the size of each element based on the type (in this case int) and the number of items based on the allocation (in this case 10). As this is a dynamic allocation, we need the new operator.

  intArray = new int[10];

And just like calloc(), the new array is zero’ed out. The memory diagram looks like the following.

  STACK                    HEAP
  
  .--------------.        .---.
  | intArray | .-+----->  | 0 |  intArray[0]
  '--------------'        |---|
                          | 0 |  intArray[1]
                          |---|
                          :   :
                          :   :
                          |---|
                          | 0 |  intArray[9]
                          '---'

For all basic types, the initial value of the array is always the zero value. For bool, this would be false.

Arrays of Objects #

Arrays of Objects are declare in the same way. For example, an array of Points.

    Point pArray[] =  new Points[10]

And just like with basic types, the new array is zero’ed out. But recall that the value of a object variable is a memory reference. So the zero’ed out value is null. You still have to instantiate each of the Point objects of the array.

   for(int i=0;i<10;i++)
      pArray[i] = new Point(i,i);

As a memory diagram, we would have the following

  STACK                    HEAP
                                 .-------.
  .------------.        .---. .->| x | 0 |
  | pArray | .-+----->  | .-+-'  |-------| pArray[0]
  '------------'        |---|    | y | 0 |
                        | .-+-.  '-------' 
                        |---| |  .-------.
                        :   : '->| x | 1 |
                        :   :    |-------| pArray[1]
                        |---|    | y | 1 |
                        | .-+-.  '-------'
                        '---' |     :
                Array Object  |     :
                              |  .-------.
                              '->| x | 9 |
                                 |-------| pArray[9]
                                 | y | 9 |
                                 '-------'
                                 
                                 Point Objects

Using {} to allocate an array #

You can also allocate arrays without a new call by specifying directly using {} and the initial values. This works for both basic array types and object array types.

  int intArray[] = {0,1,2,3,4};
  Point pArray[] = {new Point(0,0), new Point(1,1), new Point(2,2,)};

Even when using initial value allocation, it’s still an allocation. Under the covers, Java calls new, allocates the array, and assigns the initial values for you.

Iterating over an Array #

Arrays, as objects, they carry with them data and methods to support different operation. Most immediate is that an array object stores its length, which you can use to assure you do not access beyond the range of the array. If you do, this throws an exception, and if not caught, your program halts.

Using the length member, we can simply iterate over the array using indexes. Here’s a routine that prints the distance between consecutive points in the array.

    Point lastPoint = null;
    for(int i=0;i<pArray.length;i++){
       if ( lastPoint != null){
          System.out.printf("dist(%s,%s)=%.2f\n",
                             lastPoint,
                             pArray[i],
                             lastPoint.dist(pArray[i]));
       }
       lastPoint = pArray[i];
   }

However, you may notice that this is a bit wasteful because i is only used for indexing. Instead, it would be nice to use a for-each semantic, where we say “for each point in the array” without the index. Java provides a way to simplify iteration when the “for-each” semantic is desired.

    Point lastPoint = null;
    for(Point p : pArray){
       if ( lastPoint != null){
          System.out.printf("dist(%s,%s)=%.2f\n",
                             lastPoint,
                             p,
                             lastPoint.dist(p));
       }
       lastPoint = p;
   }

At each step the iteration, p gets assigned the next Point in the array, without having to use the index feature. Any object that implements Iterable can be used in this syntax.

Material on this page adopted with permission from USNA courses ic211, taught by Nate Chambers, Gavin Taylor, Chris Brown, and many others. Thank you.