Error or Exception Message

Possible Cause(s)

Comments

You have omitted an argument in a keyword message.

missing [ ]

An earlier method expects an argument. E.g. Evaluating 2 + in the Workspace would produce this error

The method you used expected a block. i.e. [ ]

A flashing GC cursor .

Endless loopcaused by a method that references itself from within the method. .
E.g. If you have a method called, method1. Do NOT use code similar to, self method1, within the method, which will cause an endless loop, and if you are not careful a computer crash. Often the use of super instead of self cures this problem

Check that whileTrue: methods will iterate towards a false answer

Check that whileFalse: methods will iterate towards a true answer

REMEMBER Ctrl + q if this happens.

Then ensure that the method is NOT referenced from within the method.

The flashing GC cursor is a fairly common error, until you know how to avoid it. TMAs are written in such a way that the student is likely to fall into the "GC" trap.

To break out of loop. Hold down Ctrl then press q (or possibly Ctrl + Q)
Other ways of rectifying can result in Loss of your work.
Often occurs when self is used rather than super

Be aware of this from TMA 2 onwards

‘ My subclass should have overridden one of my messages ’

Chap 21 page 10. Chap 45 page 20

Missing . (Full stop) or a full stop where there should not be one

Look immediately before the error message.

Missing ( Left parentheses

Look before the error message.

Right parentheses is expected

Missing ) Right parentheses

Look before the error message. The error is often only detected when a full stop is reached

Unhandled exception: xxxx

xxxx is a GUI property name

Missing end "

Misspelling of name.

Look after the error message.

See Using GUI (Chap 39) Simplified steps

Unhandled exception: Message not understood #XXX

The receiver is not a suitable object to receive that message.

E.g.
'two' + 1

two' is a string object, which does NOT have the binary message + 1 in its protocol. The number object 2 understands the message + 1.

Missing [ ]

1. If you receive the System Error Unhandled exception notifier window. Figure 1. This one was carried out by executing LB 17 Practical 15.
2. Your expression or code has created an error.
3. The wording that follows, in this case Message not understood: is stating the problem. Note that this error is in the list above.
4. # Discount this bit
5. position: is the message that was not understood.
6. Click the debug button.
7. In the Debugger Book. Figure 2. Click the Messages tab
8. See Figure 3.
9. Enlarge the Debugger Book to see all or most of the code etc. in the bottom pane
10. look through the messages in the top pane. Click them in turn and look at the code in the bottom pane
11. Usually you are looking for code that YOU have written, i.e. the source of the problem. Usually the source in in the messages at the bottom of the list
12. Make a note of the code, the portion of the code that is highlighted, and where it is found:- Workspace or Name of Method.
13. The example in Figure 4 is an expression in the Workspace, the Text Editor Evaluation points to this
14. Close the Debugger Book and then you can correct your error.

LearningBook will not open

Note if a LearningBook will not open the source of the problem is usually in a Class initialize method

• Use the above steps to find the Class initialize method.
• When you find it, place double quotes before and after all your code in that method.
• Click the Accept button.
• Click restart.