CSIS 10A Test 2 Name:_SOLUTIONS

Transcription

1 CSIS 10A Test 2 Name:_SOLUTIONS Multiple Choice: Each question is worth 2 points. Circle the letter of the best answer for the following questions. 1. A special return type that indicates a method does not return a value: a. boolean c. flag b. void d. none of the above 2. The type of a variable that can contain only the two values true and false: a. boolean c. conditional b. void d. logical 3. An operator that compares two values and produces a boolean that indicates the relationship between the operands. Examples include <, >,!= and ==: a. arithmetic operator c. logical operator b. conditional operator d. inverse operator 4. The word means to divide a large complex program into components (like methods) and isolate the components from each other (for example, by using local variables). a. overload c. generalize b. iterate d. encapsulate 5. A variable that is declared inside a method and that exists only within that method. a. local variable c. instance variable b. argument d. counter 6. To iterate through all the elements of a set performing a similar operation on each. a. increment c. encapsulate b. overload d. traverse 7. String objects are called because once created, we cannot change their state. a. inviolable c. robust b. immutable d. constant 8. The type of a variable that can contain a single symbol, for example, the letter x a. String c. char b. int d. void 9. The condition when two or more variables refer to the same object is known as a. illegal c. ambiguity b. aliasing d. chaining 10. The process of finding objects that have no references and reclaiming their storage space. a. garbage collection c. exception handling b. incremental development d. encapsulation Short Answer: Each question is worth 10 points. Fill in the blanks with the best answer.

2 11. What is printed by the following program? Output public static int multadd(int a, int b, int c){ return a*b+c; result1 is 80 result2 is 160 public static void main(string [] args){ int n = 5, d = 10; System.out.println("result1 is " + multadd(n,d,30)); System.out.println("result2 is " + multadd(30,n,d)); 12. What is the printed by the following loops? a) int x = 10, y = 30; do{ System.out.print(x); x = x + 5; while (x < y); b) int x = 1, y = 8; while ( x < y ){ System.out.print(x); if ( x > 4){ System.out.print(", "); x++; Output: Output: 12345, 6, 7, 13. What is the output of the following code? String name = "Alan Turing"; int index = name.length()-6; while (index < name.length()) { char letter = name.charat(index); System.out.println(letter); index++; T u r i n g Output 14. What is the output of the following program? Output public static void main(string[] args){ x = 5 int x = 5; p = (4, 8) Point p = new Point(3, 4); upgrade(x, p); System.out.println("x = " + x); System.out.print("p = (" + p.x + ", " + p.y + ")"); public static void upgrade(int val, Point pt){ val = val + pt.x; pt.x = pt.y; pt.y = val; Q: Why do the changes made in upgrade stick for p but not for x? A: See explanation at the end of this document.

3 15. The following program should print "A-S-T-R-O-L-O-G-I-C-". Fill in the blanks. public static String selectdash(string s) { String s2 = ""; while ( k < 5 ) { s2 = s2 + s.charat(k) + "-"; k++; return s2; // b // c // a public static void main(string [] args) { System.out.print(selectDash("ASTRONAUTS")); System.out.print(selectDash("LOGICALLY")); 16. You have a green lottery ticket, with ints a, b, and c on it. If the numbers are all different from each other, the result is 0. If all of the numbers are the same, the result is 20. If two of the numbers are the same, the result is 10. greenticket(1, 2, 3) 0 greenticket(2, 2, 2) 20 greenticket(1, 1, 2) 10 greenticket(3, 2, 2) 10 greenticket(1, 5, 1) 10 public static int greenticket(int a, int b, int c) { if (a == b && b == c){ return 20; else if (a!= b && b!= c && a!= c) return 0; else return 10;

4 17. (20 points) Fill in the definition for paint below with a loop that draws a line of plus symbols on the screen, 40 pixels wide and tall, with center points indicated: // assume we have imported all required libraries; public class PlusSigns extends Canvas { public static void main(string[] args) { // assume code inserted to set up canvas public void paint(graphics g) { Point p = new Point(50, 100); while (k < 8) { g.drawline(p.x-20, p.y, p.x + 20, p.y); g.drawline(p.x, p.y-20, p.x p.y+20); p.translate(50,0); k++; ALTERNATIVE (from Joseph Sommers) public void paint(graphics g) { Rectangle horiz = new Rectangle(30, 98, 40, 4); Rectangle vert = new Rectangle(48, 80, 4, 40); while (k < 8) { g.fillrect(horiz.x, horiz.y, horiz.width, horiz.height); g.fillrect(vert.x, vert.y, vert.width, vert.height); horiz.translate(50,0); vert.translate(50,0); k++

5 Problem 14 Discussion on references in Java Q: Why do the changes made in upgrade stick for p but not for x? A: Good question! The reason that x does not change is that the value of x is passed to the method. Even though the method changes x, the original value of x is unchanged in main. With objects, Java represents them differently. Object variables like Point p hold the address of (or "reference" to) where an object is stored in memory. One way to think of references is that they are kind of like hypertext links. For example, if I wanted to you instructions on how to install BueJ, I could just type the instructions entirely in the body of a very long and send it out. However if I send the out to 25 people, there are now 25 copies of my instructions clogging everyone's in-box, and if I realize I made a mistake after sending it out...too bad. The error has already been sent and I can't change it. I can send out another copy, but that gets messy fast. Another way to you instructions on how to install BlueJ is to send you a hypertext link, such as A hypertext link is like the address of where information is stored on the web. Sending a link instead of the actual instructions has a number of advantages. First, the is a LOT smaller so memory usage is greatly reduced. And second, there is only one copy of my instructions out there at the address I sent. If I realize I made a mistake after sending it out, I can go and correct it, possibly before anyone pulls up the link. A hypertext link is a great example of why object "references" in Java are so powerful. In the example you bring up, the variable p contains a reference (you can think of it as a hypertext link) to the Point object created in main. When we pass p to the the method upgrade, the method gets the hypertext link to the original object, the same one we created in main. So if upgrade changes that object, when we return to main, we see that the change has persisted. If you get this, you will eliminate a lot of confusion about working with objects in Java!