eclipse is not linked to the java libraries or they are not installed
To setup Eclipse to use the JDK you must follow these steps.
1.Download the JDK
First you have to download the JDK from Suns site. (Make sure you download one of them that has the JDK)
Install it and it will save some files to your hard drive. On a Windows machine this could be in c:\program files\java\jdk(version number)
Go to the Eclipse Preferences -> Java -> Installed JREs
4.Add the JDK
Click Add JRE and you only need to located the Home Directory. Click Browse... and go to where the JDK is installed on your system. The other fields will be populated for you after you locate the home directory.
Click Okay. If you want that JDK to be the default then put a Check Mark next to it in the Installed JRE's list.
go to the eclipse menu options Window -> Preferences or you can right click on your project and select Build Path -> Add Libraries... from the context menue and select JRE System Library from the Add Library dialog box
Sometimes when I am using Eclipse on different computers, I will have different JRE's installed. For example, I use JRE7 on one and the other has JRE6 installed. When I go from one to the other, Eclipse throws a ton of errors. So make sure whatever JRE you used to originally make the program is the same version as the one you're using now.