CSC 240 Computer Science III Spring 2018 Midterm Exam. Name

Transcription

1 CSC 240 Computer Science III Spring 2018 Midterm Exam Name Page Points Score Total P age

2 1. Tracing programs (1 point each value): For each snippet of Java code on the left, write down the value of each wanted variable after the code is finished executing. (Hint: none of these contains an error.) Body of main method Method definition Values (of variables) at end of main in LEFT COLUMN Mystery m1 = new Mystery(); Mystery m2 = new Mystery(2); Mystery m3 = new Mystery(2,7); public class Mystery { private int n; public Mystery ( ) { n = 0; public Mystery ( int x ) { n = x; public Mystery ( int x, int y) { n = Math.max(x, y); m1 = 0 m2 = 2 m3 = 7 Mystery m = new Mystery(); int x=2; int a, b; a = m.getn1(x); b = m.getn2(x); Mystery m = new Mystery(); int x=2; String a, b; m.setn1(x); a = m.tostring(); m.setn2(x); b = m.tostring(); public class Mystery { private int n = 1; public int getn1(int n) { n = this.n; return n; public int getn2(int n) { this.n = n; return n; public class Mystery { private int n = 1; public void setn1(int n) { n = this.n; public void setn2(int n) { this.n = n; public String tostring ( ) { return n= +n; m = {2 a = 1 b = 2 m = {2 a = 1 b = 2 2 P age

3 2. Method development. Complete the class MyArray with a private array attribute private String [] head and support the given class ArrayTest without throwing any exception. 1 public class ArrayTest{ 2 public static void main(string args[]){ 3 MyArray a = new MyArray(); 4 System.out.println(a); 5 6 String [] t = null; 7 MyArray b = new MyArray(t); 8 System.out.println(b); 9 10 t = new String [0]; 11 MyArray c = new MyArray(t); 12 System.out.println(c); t = new String [1]; 15 t[0] ="a"; 16 MyArray d = new MyArray(t); 17 System.out.println(d); t = new String [2]; 21 t[0] ="aa"; 22 t[1] ="zz"; 23 MyArray e = new MyArray(t); 24 System.out.println(e); MyArray g = new MyArray(t); 28 System.out.println(g); System.out.println("testing a-----"); 31 System.out.println(a.set(0, "b")); 32 System.out.println(a.set(-1, "b")); 33 System.out.println(a.get(0)); 34 System.out.println(a.get(-1)); System.out.println("testing b-----"); 37 System.out.println(b.set(0, "b")); 38 System.out.println(b.set(-1, "b")); 39 System.out.println(b.get(0)); 40 System.out.println(b.get(-1)); System.out.println("testing c-----"); 43 System.out.println(c.set(0, "b")); 44 System.out.println(c.set(-1, "b")); 45 System.out.println(c.get(0)); 46 System.out.println(c.get(-1)); System.out.println("testing d-----"); 49 System.out.println(d.set(1, "b")); 50 System.out.println(d.set(0, "b")); 51 System.out.println(d.set(-1, "b")); 52 System.out.println(d.set(1, "b")); 53 System.out.println(d.get(1)); 3 P age

4 54 System.out.println(d.get(0)); 55 System.out.println(d.get(-1)); set that accepts one Integer and one String as the argument. The first indicates the position of the array head and the second one indicates the target value for the update. The method will return a Boolean value whether such a set operation can be successful or not. get that accepts one Integer as the argument. The number value indicates the target position in the array head. The method will return a String value from target position, otherwise, null will be returned. The rest of methods including necessary constructor and accessors to achieve the desired execution as the follows. 4 P age

5 1 public class MyArray { 2 private String [] head; 3 // = 5 pts 4 public MyArray(){ 5 head = null; 6 7 // = 12 pts 8 public MyArray(String [] str){ 9 if (str==null) { 10 head = null; // or head = str; 11 return; head = new String [str.length]; // deep copy is needed. 14 for(int i = 0; i<str.length; i++) 15 head[i] = str[i]; // = 17 pts 18 public String tostring(){ 19 String ret = "["; 20 if (head == null head.length<1) {return ret+"null]"; 21 ret += head[0]; 22 for(int i = 1; i<head.length; i++) 23 ret += ","+head[i]; 24 return ret+"]"; // = 10 pts 28 public boolean set(int p, String n){ 29 if(head == null head.length ==0) 30 return false; 31 if (p <0 p >= head.length) 32 return false; 33 head[p] = n; 34 return true; // = 9 pts 37 public String get(int p) { 38 if(head == null head.length ==0) 39 return null; 40 if (p <0 p >= head.length) 41 return null; 42 return head[p]; 43 44

6 3. Complete program. Below is a partially filled-out Time class, which stores the hour and minute in private attributes. The constructor is also given. public class Time{ private int hour =0; private int minute =0; public Time (int h, int m){ if (h<0 h>23) return; if(m<0 m>59) return; hour = h; minute = m; // additional (instance) methods will go here. Implement an accessor compareto that accepts another Time object as an argument. It returns an integer with the following value: -1 if the time of this object is earlier than that of the parameter 0 if the two times are the same 1 if the time of this object is later than that of the parameter You should also develop necessary accessors of class Time in order to support such a method compareto. 12 // 10 pts 13 public int gethour(){ 14 return hour; public int getminute(){ 17 return minute; // 11 pts 21 public int compareto(time r){ 22 int t1 = hour * 60 + minute; 23 int t2 = r.gethour() * 60 + r.getminute(); 24 if (t1<t2) return -1; 25 else if(t1>t2) return 1; 26 else return 0; // 6 pts 29 public String tostring(){ 30 return "("+hour+", "+minute+")"; 31 6 P age

7 Use the above compareto method to complete the class called TimeCheck, in order to printout the earliest time in the array Time [] t. See the desired result: public class TimeCheck{ public static void main(string args[]){ Time [] t = {new Time(3, 15), new Time(16, 0), new Time(4, 29), new Time(11, 59), new Time(23, 11), new Time(2, 59), new Time (4, 9) ; // Your development will go here. Time min = t[0]; for(int i = 1; i<t.length; i++){ if(t[i].compareto(min)<0) min = t[i]; System.out.println("The earliest time is: "+min); // end of class TimeCheck 7 P age