Java Fundamentals: A fast-paced and pragmatic introduction to one of the world's most popular programming languages

Gazihan Alankus

Isola

Miles Obare

Basheer Ahamed Fazal

Rogério Theodoro de Brito +1 more

€24.99

1 (1 Ratings)

Paperback Mar 2019 408 pages 1st Edition

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Java Fundamentals

Chapter 1. Introduction to Java

Note

Learning Objectives

By the end of this lesson, you'll be able to:

Describe the working of the Java ecosystem
Write simple Java programs
Read input from the users
Utilize classes in the java.util package

The Java Ecosystem

The three most basic parts of the Java ecosystem are the Java Virtual Machine (JVM), the Java Runtime Environment (JRE), and the Java Development Kit (JDK), which are stock parts that are supplied by Java implementations.

Figure 1.1: A representation of the Java ecosystem

Every Java program runs under the control of a JVM. Every time you run a Java program, an instance of JVM is created. It provides security and isolation for the Java program that is running. It prevents the running of the code from clashing with other programs within the system. It works like a non-strict sandbox, making it safe to serve resources, even in hostile environments such as the internet, but allowing interoperability with the computer on which it runs. In simpler terms, JVM acts as a computer inside a computer, which is meant specifically for running Java programs.

Note

It is common for servers to have many JVMs in execution simultaneously.

Up in the hierarchy of stock Java technologies is the JRE. The JRE is a collection of programs that contains the JVM and also many libraries/class files that are needed for the execution of programs on the JVM (via the java command). It includes all the base Java classes (the runtime) as well as the libraries for interaction with the host system (such as font management, communication with the graphical system, the ability to play sounds, and plugins for the execution of Java applets in the browser) and utilities (such as the Nashorn JavaScript interpreter and the keytool cryptographic manipulation tool). As stated before, the JRE includes the JVM.

At the top layer of stock Java technologies is the JDK. The JDK contains all the programs that are needed to develop Java programs, and it's most important part is the Java Compiler (javac). The JDK also includes many auxiliary tools such as a Java disassembler (javap), a utility to create packages of Java applications (jar), system to generate documentation from source code (javadoc), among many other utilities. The JDK is a superset of the JRE, meaning that if you have the JDK, then you also have the JRE (and the JVM).

But those three parts are not the entirety of Java. The ecosystem of Java includes a very large participation of the community, which is one of the reasons for the popularity of the platform.

Note

Research into the most popular Java libraries that are used by the top Java projects on GitHub (according to research that has been repeated in 2016 and 2017) showed that JUnit, Mockito, Google's Guava, logging libraries (log4j, sl4j), and all of Apache Commons (Commons IO, Commons Lang, Commons Math, and so on), marked their presence, together with libraries to connect to databases, libraries for data analysis and machine learning, distributed computing, and almost anything else that you can imagine. In other words, for almost any use that you want to write programs to, there are high chances of an existing library of tools to help you with your task.

Besides the numerous libraries that extend the functionality of the stock distributions of Java, there is a myriad of tools to automate builds (for example, Apache Ant, Apache Maven, and Gradle), automate tests, distribution and continuous integration/delivery programs (for example, Jenkins and Apache Continuum), and much, much more.

Our First Java Application

As we briefly hinted before, programs in Java are written in source code (which are plain text, human-readable files) that is processed by a compiler (in the case of Java, javac) to produce the Java bytecode in class files. The class files containing Java bytecode are, then, fed to a program called java, which contains the Java interpreter/JVM that executes the program that we wrote:

Figure 1.2: The process of compilation in Java

Syntax of a Simple Java Program

Like all programming languages, the source code in Java must follow particular syntaxes. Only then, will the program compile and provide accurate results. Since Java is an object-oriented programming language, everything in Java is enclosed within classes. A simple Java program looks similar to this:

public class Test { //line 1
    public static void main(String[] args) { //line 2
        System.out.println("Test"); //line 3
    } //line 4
} //line 5

Every java program file should have the same name as that of the class that contains main (). It is the entry point into the Java program.

Therefore the preceding program will compile and run without any errors only when these instructions are stored in a file called Test.java.

Another key feature of Java is that it is case-sensitive. This implies that System.out.Println will throw an error as it is not capitalized correctly. The correct instruction should be System.out.println.

main() should always be declared as shown in the sample. This is because, if main() is not a public method, it will not be accessed by the compiler, and the java program will not run. The reason main() is static is because we do not call it using any object, like you would for all other regular methods in Java.

Note

We will discuss these the public and static keywords later in this book, in greater depth.

Comments are used to provide some additional information. The Java compiler ignores these comments.

Single line comments are denoted by // and multiline comments are denoted by /* */.

Exercise 1: A Simple Hello World Program

Right-click the src folder and select New | Class.
Enter HelloWorld as the class name, and then click OK.

Enter the following code within the class:

public class HelloWorld{    
public static void main(String[] args) {  // line 2
        System.out.println("Hello, world!");  // line 3
    }
}

Run the program by clicking on Run | Run 'Main'.
The output of the program should be as follows:
```
Hello World!
```

Exercise 2: A Simple Program for Performing Simple Mathematic Operations

Right-click the src folder and select New | Class.
Enter ArithmeticOperations as the class name, and then click OK.

Replace the code inside this folder with the following code:

public class ArithmeticOperations {
    public static void main(String[] args) {
            System.out.println(4 + 5);
            System.out.println(4 * 5);
            System.out.println(4 / 5);
            System.out.println(9 / 2);
    }
}

Run the main program.
The output should be as follows:
```
9
20
0
4
```
In Java, when you divide an integer (such as 4) by another integer (such as 5), the result is always an integer (unless you instruct the program otherwise). In the preceding case, do not be alarmed to see that 4 / 5 gives 0 as a result, since that's the quotient of 4 when divided by 5 (you can get the remainder of the division by using a % instead of the division bar).
To get the result of 0.8, you would have to instruct the division to be a floating-point division instead of an integer division. You can do that with the following line:
```
System.out.println(4.0 / 5);
```
Yes, this does mean, like most programming languages, there is more than one type of number in Java.

Exercise 3: Displaying Non-ASCII Characters

Right-click the src folder and select New | Class.
Enter ArithmeticOperations as the class name, and then click OK.

Replace the code in this folder with the following code:

public class HelloNonASCIIWorld {
    public static void main(String[] args) {
            System.out.println("Non-ASCII characters: ☺");
            System.out.println("∀x ∈ ℝ: ⌈x⌉ = −⌊−x⌋");
            System.out.println("π ≅ " + 3.1415926535); // + is used to concatenate 
    }
}

Run the main program.

The output for the program should be as follows:

Non-ASCII characters: ☺
∀x ∈ ℝ: ⌈x⌉ = −⌊−x⌋
π ≅ 3.1415926535

Activity 1: Printing the Results of Simple Arithmetic Operations

To write a java program that prints the sum and the product of any two values, perform the following steps:

Create a new class.
Within main(), print a sentence describing the operation on the values you will be performing along with the result.
Run the main program. Your output should be similar to the following:
```
The sum of 3 + 4 is 7
The product of 3 + 4 is 12
```
Note
The solution for this activity can be found on page 304.

Getting Input from the User

We previously studied a program that created output. Now, we are, going to study a complementary program: a program that gets input from the user so that the program can work based on what the user gives the program:

import java.io.IOException; // line 1

public class ReadInput { // line 2
    public static void main(String[] args) throws IOException { // line 3
        System.out.println("Enter your first byte");
        int inByte = System.in.read(); // line 4
        System.out.println("The first byte that you typed: " + (char) inByte); // line 5
        System.out.printf("%s: %c.%n", "The first byte that you typed", inByte); // line 6
    } // line 7
} // line 8

Now, we must dissect the structure of our new program, the one with the public class ReadInput. You might notice that it has more lines and that it is apparently more complex, but fret not: every single detail will be revealed (in all its full, glorious depth) when the time is right. But, for now, a simpler explanation will do, since we don't want to lose our focus on the principal, which is taking input from the user.

First, on line 1, we use the import keyword, which we have not seen yet. All Java code is organized in a hierarchical fashion, with many packages (we will discuss packages in more detail later, including how to make your own).

Here, hierarchy means "organized like a tree", similar to a family tree. In line 1 of the program, the word import simply means that we will use methods or classes that are organized in the java.io.Exception package.

On line 2, we, as before, create a new public class called ReadInput, without any surprises. As expected, the source code of this program will have to be inside a source file called ReadInput.java.

On line 3, we start the definition of our main method, but, this time, add a few words after the closing parentheses. The new words are throws IOException. Why is this needed?

The short explanation is: "Because, otherwise, the program will not compile." A longer version of the explanation is "Because when we read the input from the user, there may be an error and the Java language forces us to tell the compiler about some errors that our program may encounter during execution."

Also, line 3 is the line that's responsible for the need of the import in line 1: the IOException is a special class that is under the java.io.Exception hierarchy.

Line 5 is where the real action begins: we define a variable called inByte (short for "byte that will be input"), which will contain the results of the System.in.read method.

The System.in.read method, when executed, will take the first byte (and only one) from the standard input (usually, the keyboard, as we already discussed) and give it back as the answer to those who executed it (in this case, we, in line 5). We store this result in the inByte variable and continue the execution of the program.

With line 6, we print (to the standard output) a message saying what byte we read, using the standard way of calling the System.out.println method.

Notice that, for the sake of printing the byte (and not the internal number that represents the character for the computer), we had to use a construct of the following form:

An open parenthesis
The word char
A closing parenthesis

We use this before the variable named inByte. This construct is called a type cast and will be explained in much more detail in the lessons that follow.

On line 7, we use a different way to print the same message to the standard output. This is meant to show you how many tasks may be accomplished in more than one way and that there is "no single correct" way. Here, we use the System.out.println function.

The remaining lines simply close the braces of the main method definition and that of the ReadInput class.

Some of the main format strings for System.out.printf are listed in the following table:

Table 1.1: Format strings and their meaning

There are many other formatting strings and many variables, and you can find the full specification on Oracle's website.

We will see some other common (modified) formatted strings, such as %.2f (which instructs the function to print a floating-point number with exactly two decimal digits after the decimal point, such as 2.57 or -123.45) and %03d (which instructs the function to print an integer with at least three places possibly left filled with 0s, such as 001 or 123 or 27204).

Exercise 4: Reading Values from the User and Performing Operations

To read two numbers from the user and print their product, perform the following steps:

Right-click the src folder and select New | Class.
Enter ProductOfNos as the class name, and then click OK.
Import the java.io.IOException package:
```
import java.io.IOException;
```

Enter the following code within the main() to read integers:

public class ProductOfNos{
public static void main(String[] args){
System.out.println("Enter the first number");
int var1 = Integer.parseInt(System.console().readLine());
System.out.println("Enter the Second number");
int var2 = Integer.parseInt(System.console().readLine());

Enter the following code to display the product of the two variables:

System.out.printf("The product of the two numbers is %d", (var1 * var2));
}
}

Run the program. You should see an output similar to this:

Enter the first number
10
Enter the Second number
20
The product of the two numbers is 200

Well done, this is your first Java program.

Packages

Packages are namespaces in Java that can be used to avoid name collisions when you have more than one class with the same name.

For example, we might have more than one class named Student being developed by Sam and another class with the same name being developed by David. We need a way to differentiate between the two classes if we need to use them in our code. We use packages to put the two classes in two different namespaces.

For example, we might have the two classes in two packages:

sam.Student
david.Student

The two packages look as follows in File Explorer:

Figure 1.3: Screenshot of the sam.Student and david.Student packages in File Explorer

All the classes that are fundamental to the Java language belong to the java.lang package. All the classes that contain utility classes in Java, such as collections, classes for localization, and time utilities, belong to the java.util package.

As a programmer, you can create and use your own packages.

Rules to Follow When Using Packages

Here are a few rules to be considered while using packages:

Packages are written in lowercase
To avoid name conflicts, the package name should be the reverse domain of the company. For example, if the company domain is example.com, then the package name should be com.example. So, if we have a Student class in that package, the class can be accessed with com.example.Student.
Package names should correspond to folder names. For the preceding example, the folder structure would be as follows:
Figure 1.4: Screenshot of the folder structure in File Explorer

To use a class from a package in your code, you need to import the class at the top of your Java file. For example, to use the Student class, you would import it as follows:

import com.example.Student;
public class MyClass {

}

Scanner is a useful class in the java.util package. It is an easy way of inputting types, such as int or strings. As we saw in an earlier exercise, the packages use nextInt() to input an integer with the following syntax:

sc = new Scanner(System.in);
int x =  sc.nextIn()

Activity 2: Reading Values from the User and Performing Operations Using the Scanner Class

To read two numbers from the user and print their sum, perform the following steps:

Create a new class and enter ReadScanner as the class name
Import the java.util.Scanner package
In the main() use System.out.print to ask the user to enter two numbers of variables a and b.
Use System.out.println to output the sum of the two numbers.

Run the main program.

The output should be similar to this:

Enter a number: 12
Enter 2nd number: 23
The sum is 35.

Note

The solution for this activity can be found on page 304.

Activity 3: Calculating the Percent Increase or Decrease of Financial Instruments

Users expect to see the daily percentage of increase or decrease of financial instruments such as stocks and foreign currency. We will ask the user for the stock symbol, the value of the stock on day 1, the value of the same stock on day 2, calculate the percent change and print it in a nicely formatted way. To achieve this, perform the following steps:

Create a new class and enter StockChangeCalculator as the class name
Import the java.util.Scanner package:
In the main() use System.out.print to ask the user for the symbol of the stock, followed by the day1 and day2 values of the stock.
Calculate the percentChange value.
Use System.out.println to output the symbol and the percent change with two decimal digits.

Run the main program.

The output should be similar to:

Enter the stock symbol: AAPL
Enter AAPL's day 1 value: 100
Enter AAPL's day 2 value: 91.5
AAPL has changed -8.50% in one day.

Note

The solution for this activity can be found on page 305.

Key benefits

Get introduced to Java, its features, and its ecosystem

Understand how Java uses object-oriented programming

Become an expert Java exception handler

Description

Since its inception, Java has stormed the programming world. Its features and functionalities provide developers with the tools needed to write robust cross-platform applications. Java Fundamentals introduces you to these tools and functionalities that will enable you to create Java programs. The book begins with an introduction to the language, its philosophy, and evolution over time, until the latest release. You'll learn how the javac/java tools work and what Java packages are - the way a Java program is usually organized. Once you are comfortable with this, you'll be introduced to advanced concepts of the language, such as control flow keywords. You'll explore object-oriented programming and the part it plays in making Java what it is. In the concluding chapters, you'll get to grips with classes, typecasting, and interfaces, and understand the use of data structures, arrays, strings, handling exceptions, and creating generics. By the end of this book, you will have learned to write programs, automate tasks, and follow advanced courses on algorithms and data structures or explore more advanced Java courses.

What you will learn

Create and run Java programs

Use data types, data structures, and control flow in your code

Implement best practices while creating objects

Work with constructors and inheritance

Understand advanced data structures to organize and store data

Employ generics for stronger check-types during compilation

Learn to handle exceptions in your code

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