Java Comments

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Java Comments

Comments can be used to explain Java code, and to make it more readable. It can also be used to prevent execution when testing alternative code.

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Single-line Comments

Single-line comments start with two forward slashes (//).

Any text between // and the end of the line is ignored by Java (will not be executed).

This example uses a single-line comment before a line of code:

Example

// This is a comment
System.out.println("Hello World");

Try it Yourself »

This example uses a single-line comment at the end of a line of code:

Example

System.out.println("Hello World"); // This is a comment

Try it Yourself »

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Java Multi-line Comments

Multi-line comments start with /* and ends with */.

Any text between /* and */ will be ignored by Java.

This example uses a multi-line comment (a comment block) to explain the code:

Example

/* The code below will print the words Hello World
to the screen, and it is amazing */
System.out.println("Hello World");

Try it Yourself »

Single or multi-line comments?

It is up to you which you want to use. Normally, we use // for short comments, and /* */ for longer.

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Test Yourself With Exercises

Exercise:

Insert the missing part to create two types of comments.

 This is a single-line comment
 This is a multi-line comment 

Start the Exercise

 

Java Syntax

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Java Syntax

In the previous chapter, we created a Java file called Main.java, and we used the following code to print "Hello World" to the screen:

Main.java

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Try it Yourself »

Example explained

Every line of code that runs in Java must be inside a class. In our example, we named the class Main. A class should always start with an uppercase first letter.

Note: Java is case-sensitive: "MyClass" and "myclass" has different meaning.

The name of the java file must match the class name. When saving the file, save it using the class name and add ".java" to the end of the filename. To run the example above on your computer, make sure that Java is properly installed: Go to the Get Started Chapter for how to install Java. The output should be:

Hello World

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The main Method

The main() method is required and you will see it in every Java program:

public static void main(String[] args)

Any code inside the main() method will be executed. You don't have to understand the keywords before and after main. You will get to know them bit by bit while reading this tutorial.

For now, just remember that every Java program has a class name which must match the filename, and that every program must contain the main() method.

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System.out.println()

Inside the main() method, we can use the println() method to print a line of text to the screen:

public static void main(String[] args) {
  System.out.println("Hello World");
}

Try it Yourself »

Note: The curly braces {} marks the beginning and the end of a block of code.

Note: Each code statement must end with a semicolon.

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Test Yourself With Exercises

Exercise:

Insert the missing part of the code below to output "Hello World".

public class MyClass {
  public static void main(String[] args) {
    ..("Hello World");
  }
}

Some PCs might have Java already installed.

To check if you have Java installed on a Windows PC, search in the start bar for Java or type the following in Command Prompt (cmd.exe):

C:\Users\Your Name>java -version

If Java is installed, you will see something like this (depending on version):

java version "11.0.1" 2018-10-16 LTS
Java(TM) SE Runtime Environment 18.9 (build 11.0.1+13-LTS)
Java HotSpot(TM) 64-Bit Server VM 18.9 (build 11.0.1+13-LTS, mixed mode)

If you do not have Java installed on your computer, you can download it for free at oracle.com.

Note: In this tutorial, we will write Java code in a text editor. However, it is possible to write Java in an Integrated Development Environment, such as IntelliJ IDEA, Netbeans or Eclipse, which are particularly useful when managing larger collections of Java files.

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Setup for Windows

To install Java on Windows:

1. Go to "System Properties" (Can be found on Control Panel > System and Security > System > Advanced System Settings)
2. Click on the "Environment variables" button under the "Advanced" tab
3. Then, select the "Path" variable in System variables and click on the "Edit" button
4. Click on the "New" button and add the path where Java is installed, followed by \bin. By default, Java is installed in C:\Program Files\Java\jdk-11.0.1 (If nothing else was specified when you installed it). In that case, You will have to add a new path with: C:\Program Files\Java\jdk-11.0.1\bin
   Then, click "OK", and save the settings
5. At last, open Command Prompt (cmd.exe) and type java -version to see if Java is running on your machine

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Java Quickstart

In Java, every application begins with a class name, and that class must match the filename.

Let's create our first Java file, called Main.java, which can be done in any text editor (like Notepad).

The file should contain a "Hello World" message, which is written with the following code:

Main.java

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}

Try it Yourself »

Don't worry if you don't understand the code above - we will discuss it in detail in later chapters. For now, focus on how to run the code above.

Save the code in Notepad as "Main.java". Open Command Prompt (cmd.exe), navigate to the directory where you saved your file, and type "javac Main.java":

C:\Users\Your Name>javac Main.java

This will compile your code. If there are no errors in the code, the command prompt will take you to the next line. Now, type "java Main" to run the file:

C:\Users\Your Name>java Main

The output should read:

Hello World

Try it Yourself »

 

JAVA

What is Java?

Java is a popular programming language, created in 1995.

It is owned by Oracle, and more than 3 billion devices run Java.

It is used for:

• Mobile applications (specially Android apps)
• Desktop applications
• Web applications
• Web servers and application servers
• Games
• Database connection
• And much, much more!

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Why Use Java?

• Java works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc.)
• It is one of the most popular programming language in the world
• It is easy to learn and simple to use
• It is open-source and free
• It is secure, fast and powerful
• It has a huge community support (tens of millions of developers)
• Java is an object oriented language which gives a clear structure to programs and allows code to be reused, lowering development costs
• As Java is close to C++ and C#, it makes it easy for programmers to switch to Java or vice versa

 Example

public class Main {
  public static void main(String[] args) {
    System.out.println("Hello World");
  }
}
Output 

 

 

The 2nd day session started under the leadership of minarva sister.  After that all the students presented their own presentation.

Below is a set of commands made by us accordingly.