Computer Science II - Test 1

Transcription

1 Computer Science II - Test 1 Question 1. (15 points) a) For each of the access modifier keywords, what does each of them signify? public - private - protected - b) For each of the following programming constructs, which access modifier do we normally use and why? class - instance variable - method - c) For each of the following programming constructs, under what circumstances would we use a different access modifier that in part (b)? class - instance variable - method - 1

2 Question 2. (15 points) For each pair of concepts, briefly explain what they have in common and how they differ from each other. Be clear and concise in your answers. If it helps your answer, you may refer to specific examples in the code at the end of the exam. a) class vs. object b) class variable vs. instance variable c) extends vs. implements d) final vs. static e) behavior vs. implementation 2

3 Question 3. (5 points) What is the concept of information hiding? How does Java help us to implement the concept? Question 4. (5 points) What do we mean when we talk about Event-Driven Programming? Briefly discuss how event-driven programming was used in the CannonGame. Question 5 (10 points) Complete the following Java program that reads integers from the command-line and prints their sum to System.out. Recall that Integer.parseInt( ) can be used to convert from a String to an integer. The program would be run as java CalculateSum and print The sum is 28 to System.out. public class CalculateSum { public static void main( String[] args ) { // end main // end CalculateSum 3

4 Question 6. (25 points) Attached is my MyMemoDatebase solution to homework #3. When loading from the file, it calls the insert( ) method for each Key>>Value pair. If the Key is already in the database, the Key>>Value pair read from the file is thrown away. Below you are to rewrite the load method so that any duplicate Keys causes the corresponding Key>>Value pairs to be printed to a file named duplicates.dat. In other words, the load method will work the same, but it will generate a file containing Key>>Value pairs for Keys that are already in the datebase. 4

5 Question 7. (25 points) Attached is the code discussed in class for the CannonBall and CannonWorld. For the following two modifications, you do not have to completely rewrite the classes. Instead, you may: 1. write additional code on the existing code, and/or 2. write additional code for the class below, indicate in a comment for each block of code whether it replaces existing code or is added to existing code, and where the new code would go in the current code. a) Modify the class CannonBall so that the ball is colored blue when the ball is moving upward, and colored red when the ball is descending. b) Modify the CannonWorld program so that it maintains both a count of the number of balls fired and the number of times the target was hit. Display both of these values in the message area, as well as the angle of the cannon. 5

6 MyMemoDatabase: import java.io.*; import java.util.stringtokenizer; public class MyMemoDatabase implements MemoDatabase { private int MAX_SIZE = 100; // max. # of array elements private int memocount = 0; // count of the number of // MemoAssociations in the database private MemoAssociation [] associations; // reference to the array // containing the database // constructs an empty database public MyMemoDatabase() { Initialize(); // end MyMemoDatabase constructor // constructs an empty database with a max. size that's specified by // 'arraysize' public MyMemoDatabase( int arraysize ) { MAX_SIZE = arraysize; Initialize(); // end MyMemoDatabase constructor // helper method to aid in construction of a MyMemoDatabase private void Initialize() { associations = new MemoAssociation[MAX_SIZE]; memocount = 0; System.out.println("constructing a MyMemoDatabase object"); // end MyMemoDatabase constructor // Uses the 'savetofile' as the name of a file to write all of the // MemoAssociations in associations in the form key>>value. public void save( String savetofile ) throws IOException { PrintWriter outputfile = new PrintWriter( new FileWriter(saveToFile )); System.out.println("Saving to <" + savetofile + ">"); for (int i = 0; i < memocount; i++) { outputfile.println(associations[i].key()+ ">>" + associations[i].value()); System.out.println(associations[i].key()+ ">>" + associations[i].value()); // end for outputfile.flush(); outputfile.close(); // end save 6

7 // Uses the 'loadfromfile' as the name of a file to read into the // "associations" array. The 'loadfromfile' is assumed to be lines in the // form key>>value. Only entries with new keys from the file are added to the // array. public void load( String loadfromfile ) throws IOException { BufferedReader inputfile = new BufferedReader( new FileReader(loadFromFile) ); String buffer = null; String delimiters = ">"; while( true ) { buffer = inputfile.readline(); if ( buffer == null ) break; buffer = buffer.tolowercase(); StringTokenizer words = new StringTokenizer( buffer, delimiters ); String key = words.nexttoken(); String value = words.nexttoken(); if (words.hasmoreelements() ) { System.out.println("Invalid entry when loading from file " + loadfromfile + " Entry: " + buffer); else { insert(new MemoAssociation(key, value)); // end if // end while(true)... // end load // if newentry's key is already in the database or the database is full, // then the database is unmodified and 'false' is returned; // otherwise the 'newentry' is added to the database and 'true' is // returned. public boolean insert( MemoAssociation newentry ) { if ( memocount < MAX_SIZE && // if enough room and locatekey(newentry.key()) == -1 ) { // key not found associations[memocount] = newentry; memocount++; return true; else { return false; // end if // end insert // If 'key' is in the database, the associated 'value' is returned; // otherwise 'null' is returned. public String find( String key ) { int keyindex = locatekey( key ); if ( keyindex == -1) { return null; else { return associations[keyindex].value(); // end if // end find 7

8 // If 'key' is in the database, it is deleted and returns 'true'; // otherwise it returns 'false' and the array in unmodified. public boolean remove( String key ) { int keyindex = locatekey( key ); if (keyindex == -1) { // key not found return false; else { // slide associations over to remove the deleted key for (int i = keyindex+1;i < memocount; i++) { associations[i-1] = associations[i]; // end for memocount--; return true; // end if // end remove // returns 'true' if key is in the array; otherwise it returns 'false' public boolean containskey( String key ) { return (locatekey(key)!= -1); // end containskey // (a helper function) returns the index of the key if it is in // the array; otherwise it returns -1 private int locatekey( String key ) { for (int i = 0; i < memocount; i++) { if (key.equals(associations[i].key())) { return i; // end if // end for return -1; // end containskey // end class MyMemoDatabase 8

9 CannonGameDriver: public class CannonGameDriver { public static void main ( String[] args ) { CannonWorld world = new CannonWorld( ); world.show(); CannonWorld: import java.awt.*; import java.awt.event.*; public class CannonWorld extends Frame { public static final int FrameWidth = 600; public static final int FrameHeight = 400; private int angle; private String message; private CannonBall cannonball; private Scrollbar slider; public CannonWorld() { setsize ( FrameWidth, FrameHeight ); settitle( "Cannon Game" ); angle = 45; message = "Angle: " + angle; cannonball = null; slider = new Scrollbar( Scrollbar.VERTICAL, angle, 5, 0, 90 ); slider.addadjustmentlistener( new ScrollBarListener() ); add( "East", slider ); Button fire = new Button( "fire" ); fire.addactionlistener( new FireButtonListener() ); add( "North", fire ); // end CannonWorld constructor public static int dy( int y ) { return FrameHeight - y; // end dy public void paint( Graphics g ) { drawcannon ( g ); drawtarget ( g ); drawcannonball( g ); writemessage ( g ); // end paint /* helper methods */ protected void drawcannon( Graphics g ) { int x = 20; int y = 15; double radianangle = angle * Math.PI / 180.0; int lv = (int) (30 * Math.sin(radianAngle)); int lh = (int) (30 * Math.cos(radianAngle)); int sv = (int) (10 * Math.sin(radianAngle + Math.PI/2)); int sh = (int) (10 * Math.cos(radianAngle + Math.PI/2)); g.setcolor(color.green); g.drawline(x, dy(y), x+lh, dy(y+lv)); 9

10 g.drawline(x+lh, dy(y+lv), x+lh+sh, dy(y+lv+sv)); g.drawline(x+lh+sh, dy(y+lv+sv), x+sh, dy(y+sv)); g.drawline(x+sh, dy(y+sv), x, dy(y)); g.drawoval(x-8, dy(y+10), 12, 12); // end drawcannon protected void drawtarget( Graphics g ) { g.setcolor(color.red); g.fillroundrect( FrameWidth-100, dy(12), 50, 10, 6, 6 ); // end drawtarget protected void drawcannonball( Graphics g ) { if ( cannonball!= null ) { cannonball.move(); cannonball.paint( g ); try { Thread.sleep( 40 ); catch (InterruptedException e) { System.exit( 0 ); if ( dy(cannonball.y()) > 0 ) repaint(); else { int targetx = FrameWidth - 100; if ( (cannonball.x() > targetx) && (cannonball.x() < (targetx + 50)) ) message = "You Hit It!"; else message = "Missed!"; cannonball = null; // end if // end if // end drawcannonball protected void writemessage( Graphics g ) { g.drawstring( message, FrameWidth/2, FrameHeight/2 ); // end writemessage /* inner classes */ private class FireButtonListener implements ActionListener { public void actionperformed( ActionEvent e ) { double radianangle = angle * Math.PI / 180.0; double sinangle = Math.sin( radianangle ); double cosangle = Math.cos( radianangle ); cannonball = new CannonBall ( 20 + (int) (30 * cosangle), dy(5+(int) (30 * sinangle)), 5, 12 * cosangle, -12 * sinangle ); repaint(); // end actionperformed // end inner class FireButtonListener private class ScrollBarListener implements AdjustmentListener { public void adjustmentvaluechanged (AdjustmentEvent e) { angle = slider.getvalue(); message = "Angle: " + angle; repaint(); // end adjustmentvaluechanged // end inner class ScrollBarListener // end class CannonWorld 10

11 CannonBall: public class CannonBall extends MovableBall { public CannonBall (int sx, int sy, int r, double dx, double dy) { super(sx, sy, r, dx, dy); public void move () { setmotion( xmotion(), ymotion() ); super.move(); MovableBall: public class MovableBall extends Ball { private double dx; private double dy; public MovableBall( int x, int y, int r, double dx, double dy ) { super( x, y, r ); this.dx = dx; this.dy = dy; public void move() { region().translate( (int) dx, (int) dy ); protected void setmotion( double ndx, double ndy ) { dx = ndx; dy = ndy; protected double xmotion() { return dx; protected double ymotion() { return dy; // end MovableBall 11

12 Ball: import java.awt.color; import java.awt.graphics; import java.awt.rectangle; public class Ball { private Rectangle location; private Color color; public Ball( int x, int y, int r ) { location = new Rectangle( x-r, y-r, 2*r, 2*r ); color = Color.blue; public void paint( Graphics g ) { g.setcolor( color ); g.filloval( location.x, location.y, location.width, location.height ); public void setcolor( Color newcolor ) { color = newcolor; public Color color() { return color; protected int radius() { return location.width / 2; protected int x() { return location.x + radius(); protected int y() { return location.y + radius(); protected Rectangle region() { return location; protected void moveto( int x, int y ) { region().setlocation( x, y ); // end class Ball 12