Week 2 [Fri, Aug 14th] - Topics

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 generates machine code from source code before executing the programcompiled and the interpreter executes the program directly, one statement at a timeinterpreted. 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

According to the Official Java documentation, there are four platforms of the Java programming language:

• Java Platform, Standard Edition (Java SE): Contains the core functionality of the Java programming language.

• Java Platform, Enterprise Edition (Java EE): For developing and running large-scale enterprise applications. Built on top of Java SE.

• Java Platform, Micro Edition (Java ME): For Java programming language applications meant for small devices, like mobile phones. A subset of Java SE.

• JavaFX: For creating applications with graphical user interfaces. Can work with the other three above.

This book chapter uses the Java SE edition unless stated otherwise.

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 A method is a named sequence of statementsmethods, 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 so-called because it does not appear until after the program has started runningrun-time error. 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 changing the meaning of operatorsoperator overloading.

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 i.e., the index of the first element is 0, not 1indices start from 0.

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);
}

Object-Oriented Programming (OOP) is a programming paradigm. A programming paradigm guides programmers to analyze programming problems, and structure programming solutions, in a specific way.

Programming languages have traditionally divided the world into two parts—data and operations on data. Data is static and immutable, except as the operations may change it. The procedures and functions that operate on data have no lasting state of their own; they’re useful only in their ability to affect data.

This division is, of course, grounded in the way computers work, so it’s not one that you can easily ignore or push aside. Like the equally pervasive distinctions between matter and energy and between nouns and verbs, it forms the background against which you work. At some point, all programmers—even object-oriented programmers—must lay out the data structures that their programs will use and define the functions that will act on the data.

With a procedural programming language like C, that’s about all there is to it. The language may offer various kinds of support for organizing data and functions, but it won’t divide the world any differently. Functions and data structures are the basic elements of design.

Object-oriented programming doesn’t so much dispute this view of the world as restructure it at a higher level. It groups operations and data into modular units called objects and lets you combine objects into structured networks to form a complete program. In an object-oriented programming language, objects and object interactions are the basic elements of design.

_{-- Object-Oriented Programming with Objective-C, Apple}

Some other examples of programming paradigms are:

Some programming languages support multiple paradigms.

Every object has both state (data) and behavior (operations on data). In that, they’re not much different from ordinary physical objects. It’s easy to see how a mechanical device, such as a pocket watch or a piano, embodies both state and behavior. But almost anything that’s designed to do a job does, too. Even simple things with no moving parts such as an ordinary bottle combine state (how full the bottle is, whether or not it’s open, how warm its contents are) with behavior (the ability to dispense its contents at various flow rates, to be opened or closed, to withstand high or low temperatures).

It’s this resemblance to real things that gives objects much of their power and appeal. They can not only model components of real systems, but equally as well fulfill assigned roles as components in software systems.

_{-- Object-Oriented Programming with Objective-C, Apple}

Object Oriented Programming (OOP) views the world as a network of interacting objects.

OOP solutions try to create a similar object network inside the computer’s memory – a sort of virtual simulation of the corresponding real world scenario – so that a similar result can be achieved programmatically.

OOP does not demand that the virtual world object network follow the real world exactly.

Every object has both state (data) and behavior (operations on data).

Every object has an interface and an implementation.

Every real world object has:

• an interface through which other objects can interact with it
• an implementation that supports the interface but may not be accessible to the other object

Similarly, every object in the virtual world has an interface and an implementation.

Objects interact by sending messages. Both real world and virtual world object interactions can be viewed as objects sending messages to each other. The message can result in the sender object receiving a response and/or the receiver object’s state being changed. Furthermore, the result can vary based on which object received the message, even if the message is identical (see rows 1 and 2 in the example below).

The concept of Objects in OOP is an abstraction mechanism because it allows us to abstract away the lower level details and work with bigger granularity entities i.e. ignore details of data formats and the method implementation details and work at the level of objects.

Encapsulation protects an implementation from unintended actions and from inadvertent access.
_{-- Object-Oriented Programming with Objective-C, Apple}

An object is an encapsulation of some data and related behavior in terms of two aspects:

1. The packaging aspect: An object packages data and related behavior together into one self-contained unit.

2. The information hiding aspect: The data in an object is hidden from the outside world and are only accessible using the object's interface.

Writing an OOP program is essentially writing instructions that the computer will use to,

1. create the virtual world of the object network, and
2. provide it the inputs to produce the outcome you want.

A class contains instructions for creating a specific kind of objects. It turns out sometimes multiple objects keep the same type of data and have the same behavior because they are of the same kind. Instructions for creating a 'kind' (or ‘class’) of objects can be done once and those same instructions can be used to i.e. create instances ofinstantiate objects of that kind. You call such instructions a Class.

Java is an "object-oriented" language, which means that it uses objects to represent data and provide methods related to them. Object types are called classes e.g., you can use String objects in Java and those objects belong to the String class.

importing

import java.awt.Point;

public class Main{
    public static void main(String[] args) {
        Point spot = new Point(3, 4);
        int x = spot.x;
        System.out.println(x);
   }
}

new operator

Point spot = new Point(3, 4);

Variables that belong to an object are called attributes (or fields).

To access an attribute of an object, Java uses dot notation.

Point spot = new Point(3, 4);
int sum = spot.x * spot.x + spot.y * spot.y;
System.out.println(spot.x + ", " + spot.y + ", " + sum);

3, 4, 25

You can i.e., change/modifymutate an object by assigning a different value to its attributes.

Point spot = new Point(3, 4);
spot.x = 5;
System.out.println(spot.x + ", " + spot.y);

5, 4

Java uses the dot notation to invoke methods on an object too.

Point spot = new Point(3, 4);
System.out.println(spot.x + ", " + spot.y);
spot.translate(5,5);
System.out.println(spot.x + ", " + spot.y);

3, 4
8, 9

You can pass objects as parameters to a method in the usual way.

public static void printPoint(Point p) {
    System.out.println("(" + p.x + ", " + p.y + ")");
}

public static void main(String[] args) {
    Point spot = new Point(3, 4);
    printPoint(spot);
}

3, 4

You can return an object from a method too.

public static Point findCenter(Rectangle box) {
    int x = box.x + box.width / 2;
    int y = box.y + box.height / 2;
    return new Point(x, y);
}

null and NullPointerException

Point spot = null;
int x = spot.x;          // NullPointerException
spot.translate(50, 50);  // NullPointerException

public static Point createCopy(Point p) {
    if (p == null) {
        return null; // return null if p is null
    }

    // create a new object with same x,y values
    return new Point(p.x, p.y);
}

Point result = createCopy(null);
System.out.println(result);

null

It is possible to have multiple variables that refer to the same object.

Point p1 = new Point(0,0);
Point p2 = p1;
System.out.println("p1: " + p1.x + ", " + p1.y);
System.out.println("p2: " + p2.x + ", " + p2.y);
p1.x = 1;
p2.y = 2;
System.out.println("p1: " + p1.x + ", " + p1.y);
System.out.println("p2: " + p2.x + ", " + p2.y);

p1: 0, 0
p2: 0, 0
p1: 1, 2
p2: 1, 2

Java does not have explicit pointers (and other related things such as pointer de-referencing and pointer arithmetic). When an object is passed into a method as an argument, the method gains access to the original object. If the method changes the object it received, the changes are retained in the object even after the method has completed.

public static void swapCoordinates(Point p){
    int temp = p.x;
    p.x = p.y;
    p.y = temp;
}

public static void main(String[] args) {
    Point p3 = new Point(2,3);
    System.out.println("p3: " + p3.x + ", " + p3.y);
    swapCoordinates(p3);
    System.out.println("p3: " + p3.x + ", " + p3.y);
}

p3: 2, 3
p3: 3, 2

What happens when no variables refer to an object?

Point spot = new Point(3, 4);
spot = null;

The first line creates a new Point object and makes spot refer to it. The second line changes spot so that instead of referring to the object, it refers to nothing. If there are no references to an object, there is no way to access its attributes or invoke a method on it. From the programmer’s view, it ceases to exist. However, it’s still present in the computer’s memory, taking up space.

In Java, you don’t have to delete objects you create when they are no longer needed. As your program runs, the system automatically looks for stranded objects and reclaims them; then the space can be reused for new objects. This process is called garbage collection. You don’t have to do anything to make garbage collection happen, and in general don’t have to be aware of it. But in high-performance applications, you may notice a slight delay every now and then when Java reclaims space from discarded objects.

As you know,

• Defining a class introduces a new object type.
• Every object belongs to some object type; that is, it is an instance of some class.
• A class definition is like a template for objects: it specifies what attributes the objects have and what methods can operate on them.
• The new operator instantiates objects, that is, it creates new instances of a class.
• The methods that operate on an object type are defined in the class for that object.

public class Time {
    private int hour;
    private int minute;
    private int second;
}

You can give a class any name you like. The Java convention is to use e.g., MyHelloWord rather than myHelloWorld or myhelloword or my_hello_worldPascalCase format for class names.

The code is there are exceptions to this ruleusually placed in a file whose name matches the class e.g., the Time class should be in a file named Time.java.

When a class is public (e.g., the Time class in the above example) it can be used in other classes. But the Attributes are also called instance variables, because each instance has its own variables.instance variables that are private (e.g., the hour, minute and second attributes of the Time class) can only be accessed from inside the Time class.

Constructors

public Time() {
    hour = 0;
    minute = 0;
    second = 0;
}

public Time(int h, int m, int s) {
    hour = h;
    minute = m;
    second = s;
}

this keyword

public Time(int hour, int minute, int second) {
    this.hour = hour;
    this.minute = minute;
    this.second = second;
}

public Time() {
    this(0, 0, 0); // call the overloaded constructor
}

public Time(int hour, int minute, int second) {
    // ...
}

public int secondsSinceMidnight() {
    return hour*60*60 + minute*60 + second;
}

Time t = new Time(0, 2, 5);
System.out.println(t.secondsSinceMidnight() + " seconds since midnight!");

As the instance variables of Time are private, you can access them from within the Time class only. To compensate, you can provide methods to access attributes:

public int getHour() {
    return hour;
}

public int getMinute() {
    return minute;
}

public int getSecond() {
    return second;
}

Methods like these are formally called “accessors”, but more commonly referred to as getters. By convention, the method that gets a variable named something is called getSomething.

Similarly, you can provide setter methods to modify attributes of a Time object:

public void setHour(int hour) {
    this.hour = hour;
}

public void setMinute(int minute) {
    this.minute = minute;
}

public void setSecond(int second) {
    this.second = second;
}

Revision control software are also known as Version Control Software (VCS), and by a few other names.

Let's take your first few steps in your Git (with GitHub) journey.

1. The first step is to install SourceTree, which is Git + a GUI for Git. If you prefer to use Git via the command line (i.e., without a GUI), you can install Git instead.

2. Next, initialize a repository. Let us assume you want to version control content in a specific directory. In that case, you need to initialize a Git repository in that directory. Here are the steps:

Create a directory for the repo (e.g., a directory named things).

Windows: Click File → Clone/New…. Click on Create button.
Mac: New... → Create New Repository.

Enter the location of the directory (Windows version shown below) and click Create.

Go to the things folder and observe how a hidden folder .git has been created.

Windows: you might have to configure Windows Explorer to show hidden files.

Open a Git Bash Terminal.

If you installed SourceTree, you can click the Terminal button to open a GitBash terminal.

Navigate to the things directory.

Use the command git init which should initialize the repo.

$ git init
Initialized empty Git repository in c:/repos/things/.git/

You can use the command ls -a to view all files, which should show the .git directory that was created by the previous command.

$ ls -a
.  ..  .git

You can also use the git status command to check the status of the newly-created repo. It should respond with something like the following:

git status

# On branch master
#
# Initial commit
#
nothing to commit (create/copy files and use "git add" to track)

Tracking and ignoring

In a repo, you can specify which files to track and which files to ignore. Some files such as temporary log files created during the build/test process should not be revision-controlled.

Staging and committing

After initializing a repository, Git can help you with revision controlling files inside the working directory. However, it is not automatic. It is up to you to tell Git which of your changes (aka revisions) should be committed to its memory for later use. Saving changes into Git's memory in that way is often called committing and a change saved to the revision history is called a commit.

Here are the steps you can follow to learn how to work with Git commits:

1. Do some changes to the content inside the working directory e.g., create a file named fruits.txt in the things directory and add some dummy text to it.

2. Observe how the file is detected by Git.

The file is shown as ‘unstaged’.

You can use the git status command to check the status of the working directory.

git status

# On branch master
#
# Initial commit
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   a.txt
nothing added to commit but untracked files present (use "git add" to track)

3. Stage the changes to commit: Although Git has detected the file in the working directory, it will not do anything with the file unless you tell it to. Suppose you want to commit the current changes to the file. First, you should stage the file.

Select the fruits.txt and click on the Stage Selected button.

fruits.txt should appear in the Staged files panel now.

You can use the stage or the add command (they are synonyms, add is the more popular choice) to stage files.

git add fruits.txt
git status

# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#       new file:   fruits.txt
#

4. Commit the staged version of fruits.txt.

Click the Commit button, enter a commit message e.g. add fruits.txt into the text box, and click Commit.

Use the commit command to commit. The -m switch is used to specify the commit message.

git commit -m "add fruits.txt"

You can use the log command to see the commit history.

git log

commit 8fd30a6910efb28bb258cd01be93e481caeab846
Author: … < … @... >
Date:   Wed Jul 5 16:06:28 2017 +0800

  Add fruits.txt

Note the existence of something called the master branch. Git allows you to have multiple branches (i.e. it is a way to evolve the content in parallel) and Git auto-creates a branch named master on which the commits go on by default.

5. Do a few more commits.

1. Make some changes to fruits.txt (e.g. add some text and delete some text). Stage the changes, and commit the changes using the same steps you followed before. You should end up with something like this.

   

2. Next, add two more files colors.txt and shapes.txt to the same working directory. Add a third commit to record the current state of the working directory.

   

6. See the revision graph: Note how commits form a path-like structure aka the revision tree/graph. In the revision graph, each commit is shown as linked to its 'parent' commit (i.e., the commit before it).

Often, there are files inside the Git working folder that you don't want to revision-control e.g., temporary log files. Follow the steps below to learn how to configure Git to ignore such files.

1. Add a file into your repo's working folder that you supposedly don't want to revision-control e.g., a file named temp.txt. Observe how Git has detected the new file.

2. Tell Git to ignore that file:

temp.txt

temp.txt

Remote repositories are repos that are hosted on remote computers and allow remote access. They are especially useful for sharing the revision history of a codebase among team members of a multi-person project. They can also serve as a remote backup of your codebase.

It is possible to set up your own remote repo on a server, but the easier option is to use a remote repo hosting service such as GitHub or BitBucket.

When you clone from a repo, the original repo is commonly referred to as the upstream repo. A repo can have multiple upstream repos. For example, let's say a repo repo1 was cloned as repo2 which was then cloned as repo3. In this case, repo1 and repo2 are upstream repos of repo3.

Cloning, pushing, and pulling can be done between two local repos too, although it is more common for them to involve a remote repo.