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MORE CS111 PRACTICE PROBLEMS FOR EXAM 1
Problem 1: Booleans and Conditionals
a. (Circle True or False) true and false denote objects in Java. Briefly justify your answer.
b. Betty Bother has written the following method (in a rather unfortunate programming style) which does not compile properly: public boolean isColdAndHeadingNorth () { if (getColor().equals(Color.blue) && getHeading().equals(Direction.NORTH)) { return true; } else if (!getColor().equals(Color.blue) || !getHeading().equals(Direction.NORTH)) { return false; } } (1) Explain the compiler error and (2) rewrite Betty’s method in a much simpler style.
c. Which of the following program fragments are equivalent to: if (A && B) { return bluePatch; } else { return redPatch; }
- if (A && B) { return bluePatch; } return redPatch;
- if (!A) { return redPatch; } if (B) { return bluePatch; } return redPatch;
- if (!A || !B) { return redPatch; } return bluePatch;
d. Define a Buggle method named isBoxedIn() that has no parameters and returns true if a buggle is in a cell surrounded by walls on all four sides, and otherwise returns false. The final state of the buggle when isBoxedIn() returns should be the same as the state of the buggle when isBoxedIn() is invoked. You may not use recursion or iteration in your solution, but you may define auxiliary methods if you want
Problem 2: Invocation Trees
public class ExamPictureWorld extends PictureWorld {
public Picture meth1 (Picture a) { Picture b = beside(a, empty()); return overlay(meth2(b), b); }
public Picture meth2 (Picture c) { return clockwise90(above(c, empty(), 0.75)); } }
Figure 1: A sublcass of PictureWorld.
Consider the subclass of PictureWorld shown in Fig. 1. Supose that: EPW is an instance
of ExamPictureWorld, P0 is a Picture instance denoting the empty picture, P1 is a Picture instance denoting the rightmost picture below:
ExamPictureWorld EPW
Picture P0 Picture P
red blue
The dashed grid lines are not part of the pictures. They indicate coordinates within pictures. The colors names are not part of picture P1. They indicate the color of the two rectangles. Each of the two rectangles is a solid color without any separately colored border. On the next page, you are to draw an invocation tree that models the instance method invocation EPW .meth1( P1 ). In the area labeled Execution Land, you should draw an invocation tree that contains the following eight nodes, arranged appropriately into a tree. You should use each node exactly once.
above( , , ): clockwise90( ): empty(): meth2( ):
beside( , ): empty(): meth1( ): overlay( , ):
The empty circles in the nodes are skeletons for object references that you should fill in with one of the labels P0, P1, P2, P3, P4, or P5 to refer to the appropriate Picture instance in Object Land (see below). A circle enclosed by parentheses is a reference to an actual argument of the method invocation. A circle appearing after a colon is a reference to the result of the method invocation. The root of the invocation tree is the meth1() node, which has already been drawn for you, and whose actual argument has been filled in (you need to fill in its result). In the area labeled Object Land are the skeletons for the six Picture instances that are used during the execution. The pictures labeled P0 and P1 have already been drawn for you; you
should draw the pictures for P2 , P3 , P4 , and P5. In each picture, you should label red areas with the letter R and blue areas with the letter B. All other areas are presumed to be white. (Note: for simplicity, the receiver object EPW for each of the method invocations has been omitted. This instance has also been omitted from Object Land.)
Problem 3: Java Execution Model Consider the following two class definitions:
public class RelayRace extends BuggleWorld { public void run() { RelayRunner r1 = new RelayRunner(); RelayRunner r2 = new RelayRunner(); RelayRunner r3 = new RelayRunner();
r2.setColor(Color.green); r3.setColor(Color.blue); r1.firstLeg(3, r2,r3); } }
class RelayRunner extends Buggle { public void firstLeg(int length, RelayRunner next, RelayRunner last) { forward(length); next.setPosition(this.getPosition()); next.secondLeg(length, last); } public void secondLeg(int length, RelayRunner next) { forward(length); next.setPosition(this.getPosition()); next.thirdLeg(length); } public void thirdLeg(int length) { forward(length); } }
The Java Execution Model diagram below shows the state of the program after evaluating the first line of the run() method. Show the diagram after the completion of the run() method. Include in Object Land all instances of the RelayRunner and Point classes referenced by local or instance variables. Also include all execution frames opened during the execution of the run() method. You may abbreviate references to instances of the Direction and Color classes as ovals surrounding appropriate identifying information (as shown below).
.run()
RLAY Point x 1
y 1
P
RelayRunner position P
heading EAST
color red
brushDown true
RUN
Object Land
Execution Land
RelayRace
RLAY
RelayRunner r3 = new RelayRunner(); r2.setColor(Color.green);
this RLAY r1 RUN RelayRunner r1 = new RelayRunner();
r1.firstLeg(3, r2,r3);
r3.setColor(Color.blue);
RelayRunner r2 = new RelayRunner();
