The problem you are facing is due to lower version of java being specified in your facet settings.
Goto project properties -> Select Project Facets -> Check the Java version, if it is other than java 5.0 then click on Add/Remove Project Facets.
A new pop-up will open select "Facets Project" from the configurations dropdown.
Now you will see a Project Facet named "Java". select the checkbox and change the version to 5.0 and click finish.
Clean build your workspace.
Hope the above solution works for you. All the best.