Computer Science 121 Final Exam * F13 Review

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1 Computer Science 121 Final Exam * F13 Review Be sure to show all your work. Remember: exam is closed book. Cellphones, calculators, ear buds of any sort, slide rules, notes, talking, etc., are not permitted. Keep all of your scratch work on the exam sheets. Feel free to use the opposite blank side for your answer. WRITE LEGIBLY! IF WE CAN T READ IT, YOU WON T GET CREDIT 1. (25) Write a complete, working, one-class application that generates 1,000,000 random numbers r, 0.0 r < 1.0, and counts each number according to where it falls within the ten blocks 0.0 r <.1.1 r <.2.2 r <.3.9 r < 1.0 A 10 element array must be part of your counting process. Element 0 should correspond to numbers between 0.0 and 0.1, element 1 should correspond to numbers between 0.1 and 0.2, and so forth. After generating and tallying these numbers, your program should print out the count of occurrences of each, in a column. (Note: since random numbers are uniformly distributed between 0 and 1, the 10 numbers you print out should each be close to 100,000.) Hint: you can classify the random numbers in your program by the first digit to the right of the decimal point. For example, if one generated random number, say r, is , then the 4 determines how the number should be accounted for in the array. You can extract that digit of r using this expression: (int)(r*10). Public class Nums{ p.s.v.m(..){ int[] nums = new int[10]; for(int j = 0; j < ; j++){ int k = (int)(10*math.random()); nums[k]++; for(int j : nums) SOP(j); 2. (25) Consider the two classes below: public class MysteryDriver{ public static void main(string[] args){ DisplayWindow d = new DisplayWindow(); Mystery p = new Mystery(); d.addpanel(p); d.showframe(); import java.awt.*; import javax.swing.*;import java.awt.event.*; 1

2 public class Mystery extends JPanel implements ActionListener{ JButton circle = new JButton("Circle"); JButton square = new JButton("Square"); private int size =100; boolean drawcircle = true; public Mystery() { setpreferredsize(new Dimension(500,700)); this.add(circle); this.add(square); circle.addactionlistener(this); square.addactionlistener(this); public void paintcomponent(graphics g){ super.paintcomponent(g); g.setcolor(color.red); if(drawcircle) g.filloval(20,100,size,size); else g.fillrect(20,100,size,size); public void actionperformed(actionevent e){ if(e.getsource() == circle) drawcircle = true; else if (e.getsource() == square) drawcircle = false; repaint(); a. The GUI application above has two buttons, circle and square. Suppose you start up the program, and then click these buttons in succession as follows: Square-Circle-Square. Make 3 small sketches that show what the window looks like after each click. You get alt circles, squares b. What object is the listener for the square button? Listener is p c. Consider the expression e.getsource() in the actionperformed method. The getsource() method belongs to what class? In ActionEvent class d. This application draws geometric figures of size 100. Alter the code so that each time either button is pressed, the geometric figure drawn grows by 10 pixels in all dimensions (so a rectangle would be 10 pixels wider, 10 taller). Again, just rewrite/add-to the relevant code. just before repaint() add: size += 10; 3. (25) Consider the three classes below: import java.util.*; public class CountTester{ 2

3 public static void main (String[] args){ try{ Scanner s = new Scanner(System.in); System.out.println("Enter name of a text file"); String str = s.next(); Counter c = new Counter(str); c.readlines(); System.out.println(c.reportCt()); catch(exception e){system.out.println(e); import java.io.*; public class Counter extends Echo{ private int ct = 0; public Counter(String f) throws IOException{ super(f); public void processline(string s){ for(int j = 0; j < s.length()-1; j++) if((s.charat(j) == 'b')&&(s.charat(j+1) == 't')) ct++; public int reportct(){return ct; import java.util.scanner; import java.io.*; public class Echo{ String filename; Scanner scan; public Echo(String f) throws IOException{ filename = f; scan = new Scanner(new FileReader(fileName)); public void readlines(){ while(scan.hasnext()){ processline(scan.nextline()); scan.close(); public void processline(string line){ System.out.println(line); a) In a sentence or two at most, describe the role of the import statement in the driver class CountTester (Be brief here). Imports Scanner class b) What does the program do on this text file (what is printed when you run CountTester)? 3

4 No doubt: I have no debts To speak of - Except a debt to you! Prints 3 there are 3 occurences of bt letter pair b) How many times is processline called when processing the text file above? 3 c) In a sentence or two at most, explain the relationship between IOException and Exception. IOException is a subclass of Exception d) The readlines method in Echo uses a method called hasnext(). What class does this method belong to? Scanner f) Rewrite the Counter class so that it does something completely different: it should report the number of lines in the text file with name f that have more than 50 characters (of any sort spaces, letters, dashes - anything). (You should only change the Counter class, and you don t need to rewrite the parts of the Counter class that don t change). Just change process line: void processline(string line){ if line.length() > 50) ct++; (25)Consider the Crop class, given below: public class Crop{ private String name; private double acres; // acres planted in crop private double worth; // crop value per acre public Crop(String name, double a, double w){ this.name = name; acres = a; worth = w; public String getname(){return name; public double getacres(){return acres; public double getworth(){return worth; public void setworth(double val){worth = val; public String tostring(){ return(name + "acres " + acres + "value "+ worth); a) Suppose you are writing a driver class, and want to create a Crop object in the driver called wheatcrop, with name "wheat", on acres of land, and a worth of $70.00 per acre. Write a statement that will do this. Crop wheatcrop = new Crop( wheat,100.0,70.0); 4

5 b) Add a method to the Crop class called cropworth, which returns the total value of the calling Crop object. For example, the wheatcrop object in a), above, should have total value (return value of method) equal to 100.0*70.00 = $ double cropworth(){return acres*worth; c) Now write a static method called totalworth, which is passed an array of Crop objects, and which returns the sum of the total values of all of the Crops in the array. Use your answer to part b) as part of your answer here. double totalworth(crop[] crops){ double val = 0.0; for(crop c : crops) val += c.cropworth(); return val; d) Now write a static method called allsmall, which is passed an array of Crop objects, and which returns true if all of the Crops in the array are less than 10 acres; otherwise return false. boolean allsmall(crop[] crops){ for(crop c : crops) if (c.acres < 10) return false; return true; 5