Getting Started

This trail provides everything you'll need to know about getting started with the Java programming language.

The Java Technology Phenomenon

About the Java Technology

both a programming language and a platform

The Programming Language

all source files written in plain text files ending in .java extension
these source files are then compiled by the javac compiler into files containing bytecode and ending in .class extension
- this bytecode is not native to any particular processor
- only native to the machine language of Java Virtual Machine (JVM)
Java launcher tool then runs the application inside an instance of the JVM

overview of the java software development process

JVM will be available on any operating system, same .class file is capable of being used
some JVMs, such as Java SE HotSpot at a Glance, performs additional steps at runtime will give the application a performance boost
- includes various tasks such as finding performance bottlenecks and recompiling (to native code) frequently used sections of code

The Java Platform

The Java platform has two components:

the Java Virtual Machine (JVM)
the Java Application Programming Interface (API)
- grouped into libraries of related classes and interfaces
- libraries are known as packages

What Can the Java Technology Do?

The general-purpose, high-level Java programming language is a powerful software platform. Every full implementation of the Java platform gives you the following features:

Development Tools: The development tools provide everything you'll need for compiling, running, monitoring, debugging, and documenting your applications. As a new developer, the main tools you'll be using are the javac compiler, the java launcher, and the javadoc documentation tool.
Application Programming Interface (API): The API provides the core functionality of the Java programming language. It offers a wide array of useful classes ready for use in your own applications. It spans everything from basic objects, to networking and security, to XML generation and database access, and more. The core API is very large; to get an overview of what it contains, consult the Java Platform Standard Edition 8 Documentation.
Deployment Technologies: The JDK software provides standard mechanisms such as the Java Web Start software and Java Plug-In software for deploying your applications to end users.
User Interface Toolkits: The JavaFX, Swing, and Java 2D toolkits make it possible to create sophisticated Graphical User Interfaces (GUIs).
Integration Libraries: Integration libraries such as the Java IDL API, JDBC API, Java Naming and Directory Interface (JNDI) API, Java RMI, and Java Remote Method Invocation over Internet Inter-ORB Protocol Technology (Java RMI-IIOP Technology) enable database access and manipulation of remote objects.

How will the Java Technology Change My Life?

Java will likely to make your programs better and requires less effort than other languages by helping to do the following:

Get started quickly: Although the Java programming language is a powerful object-oriented language, it's easy to learn, especially for programmers already familiar with C or C++.
Write less code: Comparisons of program metrics (class counts, method counts, and so on) suggest that a program written in the Java programming language can be four times smaller than the same program written in C++.
Write better code: The Java programming language encourages good coding practices, and automatic garbage collection helps you avoid memory leaks. Its object orientation, its JavaBeans™ component architecture, and its wide-ranging, easily extendible API let you reuse existing, tested code and introduce fewer bugs.
Develop programs more quickly: The Java programming language is simpler than C++, and as such, your development time could be up to twice as fast when writing in it. Your programs will also require fewer lines of code.
Avoid platform dependencies: You can keep your program portable by avoiding the use of libraries written in other languages.
Write once, run anywhere: Because applications written in the Java programming language are compiled into machine-independent bytecodes, they run consistently on any Java platform.
Distribute software more easily: With Java Web Start software, users will be able to launch your applications with a single click of the mouse. An automatic version check at startup ensures that users are always up to date with the latest version of your software. If an update is available, the Java Web Start software will automatically update their installation.

The "Hello World!" Application

In your favorite editor/IDE create a file called HelloWorldApp.java and write the following code inside that file:

package helloworldapp;

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to standard output.
 */
public class HelloWorldApp {
    public static void main(String[] args) {
        System.out.println("Hello World!"); // Display the string.
    }

}

Compile the above file into a .class file

javac HelloWorldApp.java

Run the "HelloWord" app

java -cp HelloWorldApp

java HelloWorldApp

You should see the following output:

Hello World!

A Closer Look at the "Hello World!" Application

The "Hello World" app consists of three main components:

Source code comments
HelloWorldApp class definition
main method

Source code comments

/**
 * The HelloWorldApp class implements an application that
 * simply prints "Hello World!" to standard output.
 */

comments are ignored by the compiler
there are three different kinds of comments in Java
1. multi-line: /* text */ - java compiler ignores everything between /* */
2. documentation: /** documentation */ - indicates a documentation comment
  - javadoc tool uses doc comments when preparing automatically generated documentation
3. single line: // text - compiler ignores everything from // to the end of the line

Class Definition

class HelloWorldApp {
    
}

keyword class begins the class definition for a class named HelloWorldApp
code for each class appears between the opening and closing curly braces ({})

The main method

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

every application must contain a main method whose signature is: public static void main(String[] args)
modifiers public and static can be written in either order (public static or static public), but the convention is to use public static as shown above
you can name the argument anything you want, but most programmers choose "args" or "argv"
main method is the entry point for your application and will subsequently invoke all the other methods required by your program
it accepts a single argument: an array of elements of type String
this array is the mechanism through which the runtime system passes information to your application
for example: java MyApp arg1 arg2
each string in the array is called a command-line argument
command-line arguments let users affect the operation of the application without recompiling it
the "Hello World!" application ignores its command-line arguments, but you should be aware of the fact that such arguments do exist

Finally the line: System.out.println("Hello World!");

ses the System class from the core library to print the "Hello World!" message to standard output

Common Problems (and Their Solutions)

Compiler Problems

Common Error Messages on Microsoft Windows Systems

javac is not recognized as an internal or external command, operable program or batch file

If you receive this error, Windows cannot find the compiler (javac).

Here's one way to tell Windows where to find javac. Suppose you installed the JDK in C:\jdk1.8.0. At the prompt you would type the following command and press Enter:

C:\jdk1.8.0\bin\javac HelloWorldApp.java

If you choose this option, you'll have to precede your javac and java commands with C:\jdk1.8.0\bin\ each time you compile or run a program. To avoid this extra typing, consult the section Updating the PATH variable in the JDK 8 installation instructions.

Class names, HelloWorldApp, are only accepted if annotation processing is explicitly requested

If you receive this error, you forgot to include the .java suffix when compiling the program. Remember, the command is javac HelloWorldApp.java not javac HelloWorldApp.

Common Error Messages on UNIX Systems

javac: Command not found

If you receive this error, UNIX cannot find the compiler, javac.

Here's one way to tell UNIX where to find javac. Suppose you installed the JDK in /usr/local/jdk1.8.0. At the prompt you would type the following command and press Return:

/usr/local/jdk1.8.0/javac HelloWorldApp.java

Class names, HelloWorldApp, are only accepted if annotation processing is explicitly requested

If you receive this error, you forgot to include the .java suffix when compiling the program. Remember, the command is javac HelloWorldApp.java not javac HelloWorldApp.

Syntax Errors (All Platforms)

If you mistype part of a program, the compiler may issue a syntax error. The message usually displays the type of the error, the line number where the error was detected, the code on that line, and the position of the error within the code. Here's an error caused by omitting a semicolon (;) at the end of a statement:

Testing.java:8: error: ';' expected
            count++
                   ^
1 error

If you see any compiler errors, then your program did not successfully compile, and the compiler did not create a .class file. Carefully verify the program, fix any errors that you detect, and try again.

Semantic Errors

In addition to verifying that your program is syntactically correct, the compiler checks for other basic correctness. For example, the compiler warns you each time you use a variable that has not been initialized:

Testing.java:8: error: variable count might not have been initialized
            count++;
            ^
Testing.java:9: error: variable count might not have been initialized
        System.out.println("Input has " + count + " chars.");
                                          ^
2 errors

Again, your program did not successfully compile, and the compiler did not create a .class file. Fix the error and try again.

Runtime Problems

Error Messages on Microsoft Windows Systems

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class.

One of the places java tries to find your bytecode file is your current directory. So, for example, if your bytecode file is in /home/jdoe/java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Return:

cd /home/jdoe/java

If you enter pwd at the prompt, you should see /home/jdoe/java. If you enter ls at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH environment variable. To see if this is necessary, try clobbering the classpath with the following command.

unset CLASSPATH

Now enter java HelloWorldApp again. If the program works now, you'll have to change your CLASSPATH variable in the same manner as the PATH variable above.

Error Messages on UNIX Systems

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp/class

A common mistake made by beginner programmers is to try and run the java launcher on the .class file that was created by the compiler. For example, you'll get this error if you try to run your program with java HelloWorldApp.class instead of java HelloWorldApp. Remember, the argument is the name of the class that you want to use, not the filename.

Exception in thread "main" java.lang.NoSuchMethodError: main

The Java VM requires that the class you execute with it have a main method at which to begin execution of your application. A Closer Look at the "Hello World!" Application discusses the main method in detail.

Applet or Java Web Start Application Is Blocked

If you are running an application through a browser and get security warnings that say the application is blocked, check the following items:

Verify that the attributes in the JAR file manifest are set correctly for the environment in which the application is running. The Permissions attribute is required. In a NetBeans project, you can open the manifest file from the Files tab of the NetBeans IDE by expanding the project folder and double-clicking manifest.mf.
Verify that the application is signed by a valid certificate and that the certificate is located in the Signer CA keystore.
If you are running a local applet, set up a web server to use for testing. You can also add your application to the exception site list, which is managed in the Security tab of the Java Control Panel.

Last modified: 10 March 2024