Downloading and Installing Java
Downloading and Installing Eclipse
Writing a Java Program in Eclipse
Eclipse is an Integrated Development Environment (IDE) widely used by Java programmers. It helps with all the logistical and administrative tasks that go into developing software, therefore leaving you, the programmer, free to focus on the design of your program's logic, i.e. the data structures and algorithms. For instance, it compiles as you type--if there is a synatx error in the line you just typed, a red X shows up next to that line, along with an error message if you mouse over the X. So you can immediately fix the error before you move on. If you type an entire program and there are no red X's anywhere, your program is already compiled and you can run it.
I find that my programming time is much more effective and productive when I use Eclipse. Most of all, it is a lot more fun to program in Eclipse than to use jEdit or any other smart editor. Because Eclipse is widely used, there's a lot of third-party support in the form of "plugins". As you program more and more, you will start using these plugins for your work...
Before you install Eclipse, you want to install Java on your computer. (This also allows you to write Java programs outside of Eclipse if you choose to do so.)
Go to the Java
downloads page, and click the Java Platform (JDK) 8u71/8u72
download box (left box). This will bring up a page, Java SE
Development Kit 8 Downloads, with a list of downloadables for
various platforms. Pick the Windowsx86 download, or, if you have a
64-bit machine, the Windows x64 download. Launching the exe file
after you have downloaded it will install Java in your folder of
choice. (The default installation will put the JDK under c:\Program
Files.)
If you already have an
earlier update version of Java, as long as it's Java 1.5 or later,
that's fine, you don't NEED to install the latest version.
Macs comes with Java pre-installed. The latest Macs have Java 1.6 on them. On any Mac, you can find the latest version of Java in the directory
/System/Library/Frameworks/JavaVM.framework/Versions/
If you are looking for version 1.6 (on the latest Macs), the installation is under
/System/Library/Frameworks/JavaVM.framework/Versions/1.6
If you don't have Java 1.6 on your Mac, no need to worry. Java 1.5 will do. (But any version before 1.5 WILL NOT do.)
You can download and install Eclipse on your Windows, Mac, or Linux machine. Go to http://www.eclipse.org/downloads/ and download the Eclipse Installer for your Windows, Mac, or Linux machine, then follow the instructions to install Eclipse. (This is the latest release of Eclipse, code named Mars. Make sure you select the correct version - 32-bit or 64-bit - for your machine.)
At installation time, when asked, choose the Eclipse IDE for Java Developers package.
There are plenty of tutorials which are available from right inside the Eclipse program to help you write and run a Java program, but some of the basic tasks you will do for your programs are described in this document.
Views and Perspectives
To get started with Eclipse, a quick introduction to views and perspectives would be useful.
In Eclipse, go to Help -> Help Contents -> Workbench User Guide -> Getting Started -> Basic Tutorial -> Editors and Views -> Views and read the entire section to learn about views.
Go to Help -> Help Contents -> Workbench User Guide -> Getting Started -> Basic Tutorial -> Perspectives. Read the main Perspectives section, and the New Perspectives subsection.
Workspaces
When you start Eclipse, it will ask you for a workspace, which is a folder on your machine under which all your work will be stored. You may want to create a separate workspace folder for each class. See The Eclipse Workspace section below for more details.
To write a Java program in Eclipse you will need to do the following:
Point Eclipse to Java 1.8
Set up a new project
Develop one or more classes in the project
Go to Windows -> Preferences (on the Mac, it's Eclipse -> Preferences)
In the Preferences dialog that pops up, expand the Java item in the left frame
Click on the Compiler item (do not expand it)
In the Compiler frame on the right, check if the Compiler compliance level is set to 1.8. If not, select 1.8 from the drop down list. (If your Eclipse version doesn't support 1.8, then use 1.7 or 1.6 or 1.5.)
In the left frame, click on the Installed JREs item (do not expand it)
In the Installed JREs
frame on the right, if you see a listing for jre8, which is
checked, you are done. Otherwise, click on the Add... button
to the right of the list. Then select Standard VM, then
Next, then Directory... to select the home directory
of JRE 1.8 (it should be directly under the folder where you
installed the JDK 1.8), then Finish.
On
the Mac, it should be in
/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home/
In the list of installed JREs, you should now see the JRE 6 you picked above. Make sure the checkbox next to it is checked, then click OK.
For more details, refer to Help -> Help Contents ->
Java development user Guide -> Getting Started -> Basic
Tutorial -> Preparing Eclipse, and read the section Verifying JRE
installation and classpath variables.
When you want to build a Java application, you need to set up a new Java project. Different applications will be in different projects.
Suppose you want to build a new Java application called Maze. Here's how you would set up a project for this application:
If you are not already in the Java perspective for some reason (the Java perspective is the default), put yourself in it by doing Window -> Open Perspective -> Other -> Java (default)
In the package explorer view area (white space) right click, then New -> Java Project (or you can do File -> New -> Java Project)
In the New Java Project dialog that pops up:
Enter a name for the project (Maze, in this example)
In the Contents section, select Create new project in workspace
In the JRE section, select the radio button for Use default JRE (Currently 'jre6')
In the Project layout section, select the radio button for Create separate folders for sources and class files
In the Working sets section, leave the box unchecked
Click Finish
In the package explorer view, you should see a folder with
the project name (Maze, in this example).
Directly under it, you should see a folder called src - this
folder will contain all the Java classes you write for this
project.
Once you have a project set up (e.g. Maze), you can start building your Java classes.
Every class you build must be in some package. Eclipse will allow you to build classes that are not in any named package, but you will see a warning that this is not good. So, as a matter of good programming practice, always create a package (or packages) in which you can build your classes.
That being said, introductory programming courses may steer clear of packages for simplicity's sake, in which case all program files will go directy under the project's src folder.
For the sample Maze project, assume that you want to build a class called Amazing in a package called route. (Packages are conventionally named in lower case, classes alway have a capitalized first letter.) Here are the steps to start building your class:
Right click on src, then New -> Package (or, click on the New Package wizard icon - yellow square - in the task bar)
In the Java Package
dialog that pops up, enter the name of the package (e.g. route),
then click Finish
In the package explorer
view, you should see a folder under src with the name of the
package (e.g. route)
Right click on the package folder (e.g. route), then New -> Class (or, select the package folder in the Package Explorer, then click on the New Class wizard - green circle with a C - in the task bar)
In the Java Class dialog that pops up, enter the
name of the class (e.g. Amazing), leave
the rest of the fields as they are, and click Finish
In
the package explorer view, you should see the java file for the
class under the package folder. Also, Eclipse will switch focus to
the new class file in the editor. You should see a small starting
template of the code in the editor, including a package line at the
top, and the class outline below. You are all set to build your
class.
Every time you run Eclipse, it stores all your work in a workspace on your computer. This is a folder you can specify when you start up Eclipse. Say you specify the folder c:\eclipse\workspace as the workspace.
The projects, packages and classes you build are arranged in a hierarchy under the workspace folder. The hierarchy for the example discussed above would look like this:
c:\eclipse\workspace
|
Maze
_____________|____________
| |
src bin
| |
route route
| |
Amazing.java Amazing.class
Say you add another package called maze.util,
and under it a class called BasicAlgos. Then
the workspace would look like this:
c:\eclipse\workspace
|
Maze
_____________|____________
| |
src bin
_________|_______ _______|______
| | | |
route maze route maze
| | | |
Amazing.java util Amazing.class util
| |
BasicAlgos.java BasicAlgos.class
Notice how the package name maze.util gets converted into a hierarchy of folder maze with a folder util under it: for every dot in the package name, a new level is created in the workspace folder hierarchy.
Say Amazing is an application class that has a main method.
The very first time you run an application you need to do it the long way. In the package explorer view, right click on the file that has the main method (e.g. Amazing.java), then select menu item Run As, and click on Java Application. Eclipse will open a console view (below the editor, by default) for terminal-based input and output, and run the program.
Subsequently, you can run the program by pulling down the green arrow drop down list in the task bar at the top, and selecting your application by name (e.g. Amazing)
If you are going to ask for input files in the program, these files must be placed directly below the project folder. So, for example, if the Amazing program takes input from a file called amazingin.txt, then the file is placed like this, directly under the Maxe folder:
c:\eclipse\workspace
|
Maze
_______________________|______________
| | |
amazingin.txt src bin
| |
If you run a program that takes arguments, you need to set up the arguments for in the program properties.
Say that Amazing needs an argument which is the name of an input file from which some data will be read. (So if you were to run this program outside Eclipse, on the command line, you may type > java Amazing infile.dat)
This is how you would set up the argument in Eclipse:
In the package explorer view, right click on the file that has the main method (e.g. Amazing.java), then select menu item Run As, and click on Java Application. The program will run, but will give you an error since no argument was passed (assuming that the program is indeed accessing the argument). This step is just to "prime" Eclipse so that it sets up a run configuration for this program - see the next steps.
Now repeat the above by selecting Run As, but this time click on Run Configurations...
In the Run Configurations dialog that pops up, you should see an entry with the name of the main class (e.g. Amazing) in the left frame under Java Application - click on this if it is not already selected.
In the right frame, you should see the configuration set up for the program (e.g. Amazing), showing the settings under the Main tab. Click on the Arguments tab.
In the text area titled Program arguments immediately below the tab bar, enter the name(s) of the argument(s), one per line (e.g. input file infile.dat), and click the Apply button (toward the bottom).
IMPORTANT: If one or more of your program arguments are file names, you need to place the files directly under the project folder in Eclipse. So, if your file is stored on somewhere on your computer, you should drag and drop it into the project folder. (In the example above, you would need to drag and drop the file infile.dat into the folder Maze.)
You can now run the program using the green arrow shortcut as described in the previous section.
If you want to zip an entire project so it can be imported in another Eclipse install in its entirety, here's what you do:
Go to the project in the package explorer view, and right click on the project folder, then select Export...
In the Select dialog that pops up, expand the General item in the list of choices, then select Archive File and click Next
In the Archive file dialog that pops up, in the left text area you will see the project name checked (if you expand this, you will see bin and src checked as well), and in the right text area you will see .classpath and .project checked. In the "To archive file" text field below, choose the zip file you want for your output, then click Finish
Go to the project in the package explorer view, and right click on the src folder, and select Export...
In the Select dialog that pops up, expand the General item in the list of choices, then select Archive File and click Next
In the Archive file dialog that pops up, in the left text area you will see the project name with bin and src items, with the src checkbox selected. Do the following:
Expand the src folder - this will show all the package folders
Uncheck the src folder
Check all the package folders
In the "To archive file" text field, type the complete path name of the zip file, or click on Browse... to select the directory you want the zip file to be stored in, and type the name of the zip file.
Check the "Create only selected directories" radio button.
Click Finish
The zip file should have all the source files from all the packages in your project, placed in the respective directories.
Go to the File menu and select Import....
In the Select dialog that pops up, expand the General item, and choose Existing Projects into Workspace, then click Next
In the Import Projects dialog that pops up, click the radio button for Select archive file and select Browse.. to find and select the project .zip file on your computer
Back in the Import Projects dialog click Finish and you will see the project created in your workspace.
Suppose you want to add a jar file called ext.jar to your project.
Right click on the project name, and select Properties.
In the dialog that opens up, click on Java Build Path in the left frame.
In the right frame, click on the Libraries tab.
Click on the Add External Jars... button on the right.
Browse to the ext.jar file and click OK.
You will see ext.jar appear in the JARs and class folders list on the build path text area. Click OK.
You will also see ext.jar under Referenced Libraries in your project in the Java package explorer perspective.