Bug 4240 Can't edit class path entry (1GKEPIF)
There is no way to edit an existing entry for a library on the Java Build Path project properties page (other than editing the .classpath file directly). NOTES: EG (9/22/2001 11:42:11 AM) this is particularly annoying for variables. we should have an Edit button when a variable is selected.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/wizards/buildpaths/NewContainerDialog.java
if (!pathValidation.isOK()) {
			fContainerFieldStatus.setError(NewWizardMessages.getFormattedString("NewContainerDialog.error.invalidpath", pathValidation.getMessage())); 
			return;
		}
		IFolder folder= fCurrProject.getFolder(pathStr);
		if (isFolderExisting(folder)) {
			fContainerFieldStatus.setError(NewWizardMessages.getString("NewContainerDialog.error.pathexists")); 
			return;
		}
		fContainerFieldStatus.setOK();
		fFolder= folder;
	}
	
	private boolean isFolderExisting(IFolder folder) {
		for (int i= 0; i < fExistingFolders.length; i++) {
			if (folder.equals(fExistingFolders[i])) {
				return true;
			}
		}
		return false;
	}
		
	
		
	public IFolder getFolder() {
		return fFolder;
	}
		
	
}