Sample final exam questions

We can discuss these in class on Wednesday, but you might want to try to work them out yourself in advance.

The first two are from a take-home exam.  Questions for take-home exams are written with the assumption that people can use search engines and compiler tools, and without quite the time constraint of an in-class exam.

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1. Suppose class B extends (is a subclass of) class A, and we have a method M with the following Java signature:
B M(B o)
or, in Cool syntax
M(B o): B

The Java and Cool type systems do not allow passing an A object to M, but they do allow assigning the return value of M to a variable of type A. Why?

Be brief but clear and concrete. Don't tell me it is because it would be a violation of the type rules; I want to know why those type rules are in Java and Cool.

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2. Storage for a Java variable of type int is allocated in the stack (activation record), as is storage for other primitive values like booleans. However, while references to objects are stored in the stack, storage for the objects themselves (including for example Int objects) is allocated in the heap. Why the difference? Be clear, precise, and brief.

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The next question makes more sense if you are familiar with stack architectures and stack-oriented virtual machines (e.g., JVM), which we did not talk about this term. Nonetheless you may be able to figure it out.

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3. Some of you generated code for the stack-oriented Java virtual machine, and some of you generated code for a physical processor with registers. If you generated Java byte code, you had to calculate the maximum number of stack slots required for expression evaluation. It is relatively simple to translate stack-oriented code into register-oriented code, provided you have as many available registers as the maximum required stack depth. Briefly describe how, and illustrate with an example of translation from postfix or stack-oriented code to register  oriented code for the expression

a + b − (c + d).

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I'll try to make up a few new questions before Wednesday, since my stash of (suitable) old questions seems pretty small.  Ideally I would like to cover type checking, code generation, data flow analysis, and register allocation.  Realistically, in a 50-minute exam, I will not be able to cover everything, and it will difficult to craft questions that require some real understanding and yet can be answered quickly.  Probably there will be less of the "explain why" kind of question and more of the "generate code for this" kind of question, though there may be some of each.