Things to know when you write your first program

One class per file

• Although it's technically possible to have multiple classes in a single file, for now please have one class per file. You may have problems otherwise.

Java is case-sensitive

• DrawSquare is not the same as Drawsquare. Upper and lowercase are different identifiers to Java.

File and class name must match

• If your class name doesn't match the file name exactly (but without the .java extension), your code won't compile. For example, DrawSquare.java should contain class DrawSquare.

Spaces are not allowed in class names and variable names.

• Additionally, you cannot begin a class or variable name with a numerical value. You can have numbers after the first letter, however. Some special characters such as underscores are allowed but not conventionally used. This isn't C programming.

Braces matter for compiling

• Make sure every opening brace has a matching closing brace { }

Indenting matters for readability

• The contents inside every pair of braces { } should be indented one tab.

"gpdraw.*" cannot be found

• If you get this error then you didn't correctly follow steps 4 and 5 on the Java Setups Lesson. It's also possible that you downloaded gpdraw.jar incorrectly. When downloading gpdraw.jar some web browsers try to change the filename to gpdraw.zip. Don't allow that. Change it back to gpdraw.jar.

Selective debugging

• If you have an error but don't know what line it's on (BlueJ doesn't always tell you the correct line number) then a very good debugging technique is to comment out all of the individual instruction lines (any line ending in a semicolon) and look at the overall structure. If you still have compiling problems then you're missing a brace somewhere. Otherwise, you can start uncommenting the instruction lines one by one until you find the problem.