Week 1 [Mon, Jan 9th] - Topics

The following description of the Joys of the Programming Craft was taken from Chapter 1 of the famous book The Mythical Man-Month, by Frederick P. Brooks.

Why is programming fun? What delights may its practitioner expect as his reward?

First is the sheer joy of making things. As the child delights in his mud pie, so the adult enjoys building things, especially things of his own design. I think this delight must be an image of God's delight in making things, a delight shown in the distinctness and newness of each leaf and each snowflake.

Second is the pleasure of making things that are useful to other people. Deep within, you want others to use your work and to find it helpful. In this respect the programming system is not essentially different from the child's first clay pencil holder "for Daddy's office."

Third is the fascination of fashioning complex puzzle-like objects of interlocking moving parts and watching them work in subtle cycles, playing out the consequences of principles built in from the beginning. The programmed computer has all the fascination of the pinball machine or the jukebox mechanism, carried to the ultimate.

Fourth is the joy of always learning, which springs from the nonrepeating nature of the task. In one way or another the problem is ever new, and its solver learns something: sometimes practical, sometimes theoretical, and sometimes both.

Finally, there is the delight of working in such a tractable medium. The programmer, like the poet, works only slightly removed from pure thought-stuff. He builds his castles in the air, from air, creating by the exertion of the imagination. Few media of creation are so flexible, so easy to polish and rework, so readily capable of realizing grand conceptual structures....

Yet the program construct, unlike the poet's words, is real in the sense that it moves and works, producing visible outputs separate from the construct itself. It prints results, draws pictures, produces sounds, moves arms. The magic of myth and legend has come true in our time. One types the correct incantation on a keyboard, and a display screen comes to life, showing things that never were nor could be.

Programming then is fun because it gratifies creative longings built deep within us and delights sensibilities you have in common with all men.

Not all is delight, however, and knowing the inherent woes makes it easier to bear them when they appear.

First, one must perform perfectly. The computer resembles the magic of legend in this respect, too. If one character, one pause, of the incantation is not strictly in proper form, the magic doesn't work. Human beings are not accustomed to being perfect, and few areas of human activity demand it. Adjusting to the requirement for perfection is, I think, the most difficult part of learning to program.

Next, other people set one's objectives, provide one's resources, and furnish one's information. One rarely controls the circumstances of his work, or even its goal. In management terms, one's authority is not sufficient for his responsibility. It seems that in all fields, however, the jobs where things get done never have formal authority commensurate with responsibility. In practice, actual (as opposed to formal) authority is acquired from the very momentum of accomplishment.

The dependence upon others has a particular case that is especially painful for the system programmer. He depends upon other people's programs. These are often maldesigned, poorly implemented, incompletely delivered (no source code or test cases), and poorly documented. So he must spend hours studying and fixing things that in an ideal world would be complete, available, and usable.

The next woe is that designing grand concepts is fun; finding nitty little bugs is just work. With any creative activity come dreary hours of tedious, painstaking labor, and programming is no exception.

Next, one finds that debugging has a linear convergence, or worse, where one somehow expects a quadratic sort of approach to the end. So testing drags on and on, the last difficult bugs taking more time to find than the first.

The last woe, and sometimes the last straw, is that the product over which one has labored so long appears to be obsolete upon (or before) completion. Already colleagues and competitors are in hot pursuit of new and better ideas. Already the displacement of one's thought-child is not only conceived, but scheduled.

This always seems worse than it really is. The new and better product is generally not available when one completes his own; it is only talked about. It, too, will require months of development. The real tiger is never a match for the paper one, unless actual use is wanted. Then the virtues of reality have a satisfaction all their own.

Of course the technological base on which one builds is always advancing. As soon as one freezes a design, it becomes obsolete in terms of its concepts. But implementation of real products demands phasing and quantizing. The obsolescence of an implementation must be measured against other existing implementations, not against unrealized concepts. The challenge and the mission are to find real solutions to real problems on actual schedules with available resources.

This then is programming, both a tar pit in which many efforts have floundered and a creative activity with joys and woes all its own. For many, the joys far outweigh the woes....

This book chapter assumes you are familiar with basic C++ programming. It provides a crash course to help you migrate from C++ to Java.

This chapter borrows heavily from the excellent book ThinkJava by Allen Downey and Chris Mayfield. As required by the terms of reuse of that book, this chapter is released under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License and not under the MIT license as the rest of this book.

Some conventions used in this chapter:

Other resources used:

• C++ and Java Syntax Differences Cheat Sheet by Alex Allain
• Java Tutorial provided by Oracle. Extracts from this resource are marked as -- Java Tutorial

Java was conceived by James Gosling and his team at Sun Microsystems in 1991.

Java is directly related to both C and C++. Java inherits its syntax from C. Its object model is adapted from C++. --Java: A Beginner’s Guide, by Oracle

Oracle became the owner of Java in 2010, when it acquired Sun Microsystems.

Java has remained the most popular language in the world for several years now (as at July 2018), according to the TIOBE index.

Java is both and . Instead of translating programs directly into machine language, the Java compiler generates byte code. Byte code is portable, so it is possible to compile a Java program on one machine, transfer the byte code to another machine, and run the byte code on the other machine. That’s why Java is considered a platform independent technology, aka WORA (Write Once Run Anywhere). The interpreter that runs byte code is called a “Java Virtual Machine” (JVM).

Java technology is both a programming language and a platform. The Java programming language is a high-level object-oriented language that has a particular syntax and style. A Java platform is a particular environment in which Java programming language applications run. --Oracle

To run Java programs, you only need to have a recent version of the Java Runtime Environment (JRE) installed in your device.

If you want to develop applications for Java, download and install a recent version of the Java Development Kit (JDK), which includes the JRE as well as additional resources needed to develop Java applications.

In Java, the HelloWorld program looks like this:

public class HelloWorld {

    public static void main(String[] args) {
        // generate some simple output
        System.out.println("Hello, World!");
    }
}

    

    


    
    

For reference, the equivalent C++ code is given below:

#include <iostream>
using namespace std;

int main() {
    // generate some simple output
    cout << "Hello, World!";
    return 0;
}

    

    


    
    

This HelloWorld Java program defines one method named main: public static void main(String[] args)

System.out.println() displays a given text on the screen.

Some similarities:

• Java programs consists of statements, grouped , which are then grouped into classes.
• Java is “case-sensitive”, which means SYSTEM is different from System.
• public is an access modifier that indicates the method is accessible from outside this class. Similarly, private access modifier indicates that a method/attribute is not accessible outside the class.
• static indicates this method is defined as a class-level member. Do not worry if you don’t know what that means. It will be explained later.
• void indicates that the method does not return anything.
• The name and format of the main method is special as it is the method that Java executes when you run a Java program.
• A class is a collection of methods. This program defines a class named HelloWorld.
• Java uses squiggly braces ({ and }) to group things together.
• The line starting with // is a comment. You can use // for single line comments and /* ... */ for multi-line comments in Java code.

Some differences:

• Java use the term method instead of function. In particular, Java doesn’t have stand-alone functions. Every method should belong to a class. The main method will not work unless it is inside the HelloWorld class.
• A Java class definition does not end with a semicolon, but most Java statements do.
• In most cases (i.e., there are exceptions), the name of the class has to match the name of the file it is in, so this class has to be in a file named HelloWorld.java.
• There is no need for the HelloWorld code to have something like #include <iostream>. The library files needed by the HelloWorld code is available by default without having to "include" them explicitly.
• There is no need to return 0 at the end of the main method to indicate the execution was successful. It is considered as a successful execution unless an error is signalled specifically.

To compile the HelloWorld program, open a command console, navigate to the folder containing the file, and run the following command.

>_ javac HelloWorld.java

If the compilation is successful, you should see a file HelloWorld.class. That file contains the byte code for your program. If the compilation is unsuccessful, you will be notified of the compile-time errors.

Notes:

• javac is the java compiler that you get when you install the JDK.
• For the above command to work, your console program should be able to find the javac executable (e.g., In Windows, the location of the javac.exe should be in the PATH system variable).
  This page shows how to set PATH in different OS'es.

To run the HelloWorld program, in a command console, run the following command from the folder containing HelloWorld.class file.

>_ java HelloWorld

Notes:

• java in the command above refers to the Java interpreter installed in your computer.
• Similar to javac, your console should be able to find the java executable.

When you run a Java program, you can encounter a . These errors are also called "exceptions" because they usually indicate that something exceptional (and bad) has happened. When a run-time error occurs, the interpreter displays an error message that explains what happened and where.

System.out.println(5/0);

    

    


    
    

Exception in thread "main" java.lang.ArithmeticException: / by zero
    at Hello.main(Hello.java:5)

    

    


    
    

Integrated Development Environments (IDEs) can automate the intermediate step of compiling. They usually have a Run button which compiles the code first and then runs it.

Example IDEs:

• Intellij IDEA
• Eclipse
• NetBeans

Java has a number of primitive data types, as given below:

• byte: an integer in the range -128 to 127 (inclusive).
• short: an integer in the range -32,768 to 32,767 (inclusive).
• int: an integer in the range -2³¹ to 2³¹-1.
• long: An integer in the range -2⁶³ to 2⁶³-1.
• float: a single-precision 32-bit IEEE 754 floating point. This data type should never be used for precise values, such as currency. For that, you will need to use the java.math.BigDecimal class instead.
• double: a double-precision 64-bit IEEE 754 floating point. For decimal values, this data type is generally the default choice. This data type should never be used for precise values, such as currency.
• boolean: has only two possible values: true and false.
• char: The char data type is a single 16-bit Unicode character. It has a minimum value of '\u0000' (or 0) and a maximum value of '\uffff' (or 65,535 inclusive).

The String type (a peek)

Java has a built-in type called String to represent strings. While String is not a primitive type, strings are used often. String values are demarcated by enclosing in a pair of double quotes (e.g., "Hello"). You can use the + operator to concatenate strings (e.g., "Hello " + "!").

You’ll learn more about strings in a later section.

Java is a statically-typed language in that variables have a fixed type. Here are some examples of declaring variables and assigning values to them.

int x;
x = 5;
int hour = 11;
boolean isCorrect = true;
char capitalC = 'C';
byte b = 100;
short s = 10000;
int i = 100000;

    

    


    
    

You can use any name starting with a letter, underscore, or $ as a variable name but you cannot use Java keywords as variables names. You can display the value of a variable using System.out.print or System.out.println (the latter goes to the next line after printing). To output multiple values on the same line, it’s common to use several print statements followed by println at the end.

int hour = 11;
int minute = 59;
System.out.print("The current time is ");
System.out.print(hour);
System.out.print(":");
System.out.print(minute);
System.out.println("."); //use println here to complete the line
System.out.println("done");

    

    


    
    

The current time is 11:59.
done

    

    


    
    

Use the keyword final to indicate that the variable value, once assigned, should not be allowed to change later i.e., act like a ‘constant’. By convention, names for constants are all uppercase, with the underscore character (_) between words.

final double CM_PER_INCH = 2.54;

    

    


    
    

Java supports the usual arithmetic operators, given below.

The following program uses some operators as part of an expression hour * 60 + minute:

int hour = 11;
int minute = 59;
System.out.print("Number of minutes since midnight: ");
System.out.println(hour * 60 + minute);

    

    


    
    

Number of minutes since midnight: 719

    

    


    
    

When an expression has multiple operators, normal operator precedence rules apply. Furthermore, you can use parentheses to specify a precise precedence.

Java does not allow .

The unary operators require only one operand; they perform various operations such as incrementing/decrementing a value by one, negating an expression, or inverting the value of a boolean.-- Java Tutorial

Relational operators are used to check conditions like whether two values are equal, or whether one is greater than the other. The following expressions show how they are used:

The result of a relational operator is a boolean value.

Java has three conditional operators that are used to operate on boolean values.

Arrays are indicated using square brackets ([]). To create the array itself, you have to use the new operator. Here are some example array declarations:

int[] counts;
counts = new int[4]; // create an int array of size 4

int size = 5;
double[] values;
values = new double[size]; //use a variable for the size

double[] prices = new double[size]; // declare and create at the same time

    

    


    
    

Alternatively, you can use the shortcut syntax to create and initialize an array:

int[] values = {1, 2, 3, 4, 5, 6};

int[] anArray = {
    100, 200, 300,
    400, 500, 600,
    700, 800, 900, 1000
};

    

    


    
    

-- Java Tutorial

The [] operator selects elements from an array. Array elements .

int[] counts = new int[4];

System.out.println("The first element is " + counts[0]);

counts[0] = 7; // set the element at index 0 to be 7
counts[1] = counts[0] * 2;
counts[2]++; // increment value at index 2

    

    


    
    

A Java array is aware of its size. A Java array prevents a programmer from indexing the array out of bounds. If the index is negative or not present in the array, the result is an error named ArrayIndexOutOfBoundsException.

int[] scores = new int[4];
System.out.println(scores.length) // prints 4
scores[5] = 0; // causes an exception

    

    


    
    

4
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 5
	at Main.main(Main.java:6)

    

    


    
    

It is also possible to create arrays of more than one dimension:

String[][] names = {
    {"Mr. ", "Mrs. ", "Ms. "},
    {"Smith", "Jones"}
};

System.out.println(names[0][0] + names[1][0]); // Mr. Smith
System.out.println(names[0][2] + names[1][1]); // Ms. Jones

    

    


    
    

-- Java Tutorial

Passing arguments to a program

The args parameter of the main method is an array of Strings containing command line arguments supplied (if any) when running the program.

public class Foo{
    public static void main(String[] args) {
        System.out.println(args[0]);
    }
}

    

    


    
    

You can run this program (after compiling it first) from the command line by typing:

>_ java Foo abc

abc

if-else statements

if (x > 0) {
    System.out.println("x is positive");
}

    

    


    
    

if (x % 2 == 0) {
    System.out.println("x is even");
} else {
    System.out.println("x is odd");
}

    

    


    
    

if (x > 0) {
    System.out.println("x is positive");
} else if (x < 0) {
    System.out.println("x is negative");
} else {
    System.out.println("x is zero");
}

    

    


    
    

if (x == 0) {
    System.out.println("x is zero");
} else {
    if (x > 0) {
        System.out.println("x is positive");
    } else {
        System.out.println("x is negative");
    }
}

    

    


    
    

if (x % 2 == 0)
    System.out.println("x is even");
else
    System.out.println("x is odd");

    

    


    
    

switch statements

public class SwitchDemo {
    public static void main(String[] args) {

        int month = 8;
        String monthString;
        switch (month) {
        case 1:  monthString = "January";
            break;
        case 2:  monthString = "February";
            break;
        case 3:  monthString = "March";
            break;
        case 4:  monthString = "April";
            break;
        case 5:  monthString = "May";
            break;
        case 6:  monthString = "June";
            break;
        case 7:  monthString = "July";
            break;
        case 8:  monthString = "August";
            break;
        case 9:  monthString = "September";
            break;
        case 10: monthString = "October";
            break;
        case 11: monthString = "November";
            break;
        case 12: monthString = "December";
            break;
        default: monthString = "Invalid month";
            break;
        }
        System.out.println(monthString);
    }
}

    

    


    
    

Defining methods

public class PrintTwice {

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

    public static void main(String[] args) {
        String sentence = “Polly likes crackers”
        printTwice(sentence);

    }
}


    

    


    
    

Polly likes crackers
Polly likes crackers

    

    


    
    

return statements

public static void printLogarithm(double x) {
    if (x <= 0.0) {
        System.out.println("Error: x must be positive.");
        return;
    }
    double result = Math.log(x);
    System.out.println("The log of x is " + result);
}

    

    


    
    

public class AreaCalculator{

    public static double calculateArea(double radius) {
        double result = 3.14 * radius * radius;
        return result;
    }

    public static void main(String[] args) {
        double area = calculateArea(12.5);
        System.out.println(area);
    }
}

    

    


    
    

Overloading

public static double calculateArea(double radius) {
    //...
}

public static double calculateArea(double height, double width) {
    //...
}

    

    


    
    

Recursion

public static void nLines(int n) {
    if (n > 0) {
        System.out.println();
        nLines(n - 1);
    }
}

    

    


    
    

Java has while and for constructs for looping.

while loops

public static void countdown(int n) {
    while (n > 0) {
        System.out.println(n);
        n = n - 1;
    }
    System.out.println("Blastoff!");
}

    

    


    
    

for loops

for (initializer; condition; update) {
    statement(s);
}

    

    


    
    

public static void printTable(int rows) {
    for (int i = 1; i <= rows; i = i + 1) {
        printRow(i, rows);
    }
}

    

    


    
    

do-while loops

class DoWhileDemo {
    public static void main(String[] args){
        int count = 1;
        do {
            System.out.println("Count is: " + count);
            count++;
        } while (count < 11);
    }
}

    

    


    
    

break and continue

class Main {
    public static void main(String[] args) {
        int[] numbers = new int[] { 1, 2, 3, 0, 4, 5, 0 };
        for (int i = 0; i < numbers.length; i++) {
            if (numbers[i] == 0) {
                break;
            }
            System.out.print(numbers[i]);
        }
    }
}

    

    


    
    

123

    

    


    
    

public static void main(String[] args) {
    int[] numbers = new int[] { 1, 2, 3, 0, 4, 5, 0 };
    for (int i = 0; i < numbers.length; i++) {
        if (numbers[i] == 0) {
            continue;
        }
        System.out.print(numbers[i]);
    }
}

    

    


    
    

12345

    

    


    
    

Enhanced for loops

for (int i = 0; i < values.length; i++) {
    int value = values[i];
    System.out.println(value);
}

    

    


    
    

for (int value : values) {
    System.out.println(value);
}