Java eLearning Kit For Dummies (2014)

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Importing single classes

Before you use a class in the Java API, you normally import it. The import keyword adds a class or an entire package to your application so that you can use it. In the following exercise, you create a new application that lets you interact with Java in a new way. You can type something at the command line, and Java will use a special class to import that information into the application.

Files needed: SayHello.java

1. Open the editor or IDE that you’ve chosen to use for this book.

2. Type the following code into the editor screen.

// Import the required API classes.
import java.util.Scanner;
import java.lang.String;

class SayHello
{
public static void main(String args[])
{
// Create an instance of the Scanner class to
// use for keyboard access.
Scanner getName = new Scanner(System.in);

// Create a variable to hold the user's name.
// This variable is designed to hold text.
String userName;

// Ask the user's name and place this name in
// userName.
System.out.print("What is your name?");
userName = getName.nextLine();

// Display a message to the user with the user's
// name in view.
System.out.println("Hello" + userName + "!");
}
}

In this example, getName is a Scanner object. The Scanner object provides access to the keyboard so that you can store what the user types to a variable. A variable is a sort of box used to store information. To save what the user types, the application must grab the keystrokes from the keyboard and place them in the variable. When the application detects that the user has pressed Enter, it stops gathering keystrokes.

3. Save the code you’ve written to disk using the name of the class and a .java extension, which is SayHello.java in this example.

The name of the file must match the name of the class exactly, or the application won’t compile.

4. Open a command prompt to the directory for this example.

5. Type javac SayHello.java and press Enter.

The Java compiler creates a .class file for you.

6. Type java SayHello and press Enter.

The application asks you to type your name.

7. Type your name and press Enter.

You see a hello message that includes your name. Figure 1-9 shows a typical example of the output from this application.

Figure 1-9:

Save the file and close the IDE or editor.

Importing an entire package

When you find yourself using more than one class of a particular package, it’s often more convenient to import the entire package. In this case, you follow the package name with an asterisk (*). For example, suppose you want to use multiple types from the java.lang package, such asjava.lang.String and java.lang.Integer. In this case, you can import the entire package by using import java.lang.*. In the following exercise, you create a new application that uses multiple members of the java.lang package.

Files needed: SayHello.java

1. Open the editor or IDE that you’ve chosen to use for this book.

2. Type the following code into the editor screen.

// Import the required API classes.
import java.util.Scanner;

// Import the required API packages.
import java.lang.*;

class SayHello2
{
public static void main(String args[])
{
// Create an instance of the Scanner class to
// use for keyboard access.
Scanner getName = new Scanner(System.in);

// Create a variable to hold the user's name.
// This variable is designed to hold text.
String userName;

// Ask the user's name and place this name in
// userName.
System.out.print("What is your name?");
userName = getName.nextLine();

// Create a variable to hold the number of letters
// in the user's name.
Integer letterCount;

// Get the number of letters in the user's name.
letterCount = userName.length();

// Display a message to the user with the user's
// name in view.
System.out.println("Hello" + userName +
"your name has" + letterCount + "letters in it!");
}
}

This example extends the SayHello example to include a count of the number of letters in the user’s name. Imagine that! Java can count letters and tell you how many there are.

3. Save the code you’ve written to SayHello2.java.

4. Open a command prompt to the directory for this example.

5. Type javac SayHello2.java and press Enter.

The Java compiler creates a .class file for you.

6. Type java SayHello2 and press Enter.

The application asks you to type your name.

7. Type your name and press Enter.

You see a hello message that includes your name and the number of letters in your name. Figure 1-10 shows a typical example of the output from this application.

Figure 1-10:

Save the file and close the IDE or editor.

Obtaining Help for Java

Java could be a little scary for someone who’s just starting out. Don’t worry about memorizing everything (or anything for that matter) or making mistakes. You can find help for using the Java API, all the utilities supplied with the JDK, and the various utilities output error messages when something is incorrect in your code. No one is expecting you to remember anything or to code perfectly. The following sections tell you how to obtain help when you need it.

Getting local help

The advantage of using local help, rather than online help, is that local help is always available — even when your Internet connection isn’t working. A lot of people find that local help also provides faster access because you aren’t using a network connection. However, local help does have a disadvantage — it’s current as of the moment you download it. When an update appears to the Java help system, you won’t see it unless you download a new copy of help. Often, the help on your hard drive simply continues to get outdated because there isn’t any system in place to remind you to apply updates (or even tell you that they’re available).

Even with its limitations, having a local copy of help comes in handy. In the following exercise, you download and install local help for your Java installation.

Files needed: None

1. Open the page at https://jdk8.java.net in your browser.

You see the options for working with Java.

2. Click the JDK 8 Release link.

You see a new page with download options for the JDK.

3. Locate the Additional Resources section of this page. (Scroll down if necessary to find it.)

4. Click the Download icon next to the Java SE 8 Documentation entry.

You see a new page with a licensing agreement.

5. Click Accept.

The JDK documentation download link becomes active.

6. Click the download link.

Your browser downloads the documentation, which is in a .zip file.

7. Open the .zip file in a directory on your local hard drive.

Use a folder related to the Java installation on your machine. For example, on a Windows system, you may want to create a \Program Files\Java\docs folder and place the help file there.

8. Open the index.html file in your browser.

You see an introductory page with a conceptual diagram. You can choose the help topic you want to use. For example, if you want to read about the java.lang package, you click the lang and util link. Figure 1-11 shows the conceptual diagram, which is part of a much larger page.

Figure 1-11:

Getting local utility help

All the utilities supplied with the JDK provide their own help, so you don’t need to memorize their usage. To gain access to the help information, you type the name of the utility followed by –help. For example, to access help information about the java utility, you type java –help. Figure1-12 shows a typical utility help screen.

Figure 1-12:

Locating Java information online

The advantage of using online help is that the information is always up-to-date. You don’t have to ever worry about getting old information that could cause problems with your application. You’ll always access the online help starting athttp://download.java.net/jdk8/docs/api/index.html. As mentioned in the earlier “Introducing the Java API” section of the chapter, you begin by selecting a package, and then you select a class within a package to obtain help about that class.

Summing Up

Here are the key points you learned about in this chapter:

· Java is one of the most popular languages available because it works well on so many platforms.

· Standardization is a key reason why Java is able to work in so many different environments.

· The JCP provides input on how Java should continue to change as a language.

· You can use any text editor to write Java applications, but using an IDE provides some significant advantages in the form of color coding, help, and convenience.

· Installing either the JDK or the JRE will let you run Java applications on your machine.

· To obtain the best functionality, the version of Java supported by your JRE must match the version of Java supported by the JDK used to write the application.

· When manually creating applications for Java, especially on Windows systems, you must ensure that your system has a path configured to find the Java executable files.

· An IDE will normally locate and configure the Java executable file path automatically for you.

· Classes can contain both methods and other classes.

· The javac command compiles your application from the source code you provide.

· The java command runs an application in a command window, whereas the javaw command runs an application without a command window.

· Java supports both single-line and multiline comments, both of which document your application so you can remember what it does later.

· The Java API is so huge that you can’t memorize it all, but you can memorize the parts you use most often.

· When you use the Java API, you normally access a class hierarchy, such as java.lang.String.

· Import an entire package when you need to use more than one class from it.

· Local help has the advantages of being more reliable and faster than remote help.

· Remote help has the advantage of providing updates that could make it easier to write an application.

Try-it-yourself lab

For more practice with the features covered in this chapter, try the following exercise on your own:

1. Open the GetStarted application.

2. Change the text that the application outputs.

For example, you might change it to read, “This text has been updated!”

3. Compile the application.

4. Run the application.

You should see the new text you’ve provided on screen.

5. Repeat Steps 1 through 4 for any other strings you’d like to see the application output.

Know this tech talk

· Application Programming Interface (API): Software consisting of classes contained in packages that come with the JDK. This software lets you write Java application with greater ease and reduces the amount of code you need to write.

· argument: Information provided to a method so that the method has data to manipulate.

· byte code: The compiled form of a Java application. Byte code normally appears within a .class file.

· class: A method of combining code and data. Classes are the basis of OOP techniques. Using classes means that Java applications are self-contained and easier to work with.

· code block: Any code that appears within a starting curly bracket ({) and an ending curly bracket (}).

· compile: To turn source code into executable byte code. The compiled application resides in a .class file.

· cross-platform: Software that can run on more than one platform.

· Integrated Development Environment (IDE): A special kind of editor used by developers to write application code. An IDE normally provides features such as color coding and access to various forms of help. It also lets you provide special configurations to make compiling, running, and debugging applications significantly easier.

· Java Community Process (JCP): A group dedicated to making Java a better language. Members of this group help improve standardization, discuss language additions, and work on fixes for language errors (bugs).

· Java Development Kit (JDK): A special kind of SDK used specifically to write and test Java applications.

· Java Runtime Engine (JRE): The special piece of software used to run Java applications.

· object-oriented programming (OOP): A form of programming that makes development simpler by packaging data and code together in a way that makes it easier to access and manage. OOP relies on the use of objects that correspond to real world entities. As with real world objects, OOP relies on properties to describe the object, methods to interact with the object, and events to let the object communicate with the outside world.

· package: A container used to combine a number of classes into an easily accessible whole. A package wraps up the classes so that you can find them with greater ease.

· platform: A combination of hardware and operating software that creates a specific environment in which to run Java applications.

· Software Development Kit (SDK): A combination of tools and software used to provide an application development environment.

· variable: A kind of box used to hold information in an application.