Bug 7252 Can add a variable with no associated jar file to a build path
It is possible to add a variable to the build path for a project using add variable but this will not pick up the classes in that directory. The user although warned can still select it. STEPS 1) Define a classpath variable PATH_TEST that is a directory with classes in it 2) Open the properties dialog on a JavaProject 3) Select the Libraries Tag 4) Select Add Variable 5) Select PATH_TEST 6) You will get a message that Resolved path does not exist. However OK is enabled so you can select this and believe it is valid.The UI correctly shows the empty circle to indicate it is not found but it is easy to miss the cause of this.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/dialogs/MessageLine.java
public MessageLine(Composite parent) {
		this(parent, SWT.LEFT);
	}
	/**
	 * Creates a new message line as a child of the parent and with the given SWT stylebits.
	 * Error message will be shown in  <code>RED</code>.
	 */
	public MessageLine(Composite parent, int style) {
		super(parent, style);
		fDefaultColor= getForeground();
		fErrorRGB= fgDefaultErrorRGB;
	}
	/**
	 * Creates a new message line as a child of the parent and with the given SWT stylebits.
	 * Error message will be shown with in the given rgb color.
	 */
	public MessageLine(Composite parent, int style, RGB errorRGB) {
		super(parent, style);
		fDefaultColor= getForeground();
		fErrorRGB= errorRGB;
	}
	/**
	 * Clears the currently displayed error message and redisplayes
	 * the message which was active before the error message was set.
	 */
	public void clearErrorMessage() {
		setErrorMessage(null);
	}
	/**
	 * Clears the currently displayed message.