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Java Quiz
CMSC 433
Programming Language Technologies and Paradigms
Fall 2003
September 2, 2003
This quiz (7 pages, both sides of the page) is designed to test your knowledge of basic, but tricky Java features. Don’t get worried about how you do on the quiz—this is for fun, and to get you thinking about Java again. However, if you do well, the quiz score can benefit your final grade. We will use the results of the quiz to understand how well everyone understands Java in the class. Thursday’s lecture will cover material that appears on this quiz.
Question Points Score
Total 100
- (Objects, 15 points)
class MyInt { int x; public MyInt(int x) { this.x = x; } } class Foo { void inc(int x) { x = x+1; } void inc(MyInt o) { o.x = o.x+1; } }
Given the above class definitions, what is the output of the following code sequence:
MyInt o = new MyInt(5); int x = 5; Foo f = new Foo();
f.inc(x); f.inc(o); System.out.println(x); System.out.println(o.x);
Answer:
5 6
true true false true true false true false false true true true true true
- (Subtyping, 30 points)
Foo
FooIfc
BarIfc
Bar
BifIfc
Bif Legend
extends
implements
Interface
Class
Given the above class and interface relationships, what will be the output of the following code sequence? Recall that o instanceof C returns true when the actual run-time type of o is a subtype of C. If any of the instanceof checks fail to compile, indicate which ones.
Foo f = new Foo(); System.out.println(f instanceof Foo); System.out.println(f instanceof FooIfc);
Bar b = new Bar(); System.out.println(b instanceof FooIfc); System.out.println(b instanceof BarIfc); System.out.println(b instanceof Object);
Object o = b; System.out.println(o instanceof Foo); System.out.println(o instanceof Bar);
FooIfc f2 = new Bar(); System.out.println(f2 instanceof Foo); System.out.println(f2 instanceof FooIfc);
Bif b2 = new Bif(); System.out.println(b2 instanceof FooIfc); System.out.println(b2 instanceof Bar); System.out.println(b2 instanceof BarIfc); System.out.println(b2 instanceof BifIfc);
Answer:
The statement System.out.println(b2 instanceof Bar) does not compile; if removed the output would be: true true true true true true true true true
- (Exceptions, 25 points)
class AppException extends Exception { } class AppFault extends AppException { } class AppError extends Error { }
(a) Which of the following methods will cause errors during compilation: class Foo { void f1() { throw new Exception(); } void f2() { throw new AppException(); } void f3() { throw new AppError(); } void f4() throws AppException { throw new Exception(); } void f5() throws AppException { throw new AppFault(); } }
Answer: Methods f1, f2, and f4 fail: they do not declare thrown exceptions.
(b) What will be the output of executing the main method of the Foo class defined next: class Foo { void f1() throws Exception { throw new AppFault(); } void f2() { throw new AppError(); }
public static void main(String args[]) { Foo f = new Foo(); try { f.f1() } catch (AppFault e) { System.out.println("Got AppFault"); } catch (Exception e) { System.out.println("Exception"); } finally { System.out.println("Done"); }
try { f.f2() } catch (Exception e) { System.out.println("Exception"); } finally { System.out.println("Done"); } } }
Answer: Got AppFault Done Done program then exits due to an uncaught AppError
