Class: COP 2800 Java Programming
Homework #3 Model Solution

 1) How do you simplify the max method in the following, using the conditional operator?
    1:     /** Return the max of two numbers */
    2:     public static int max ( int num1, int num2 ) {
    3:         int result;
    4:
    5:         if ( num1 > num2 )
    6:             result = num1;
    7:         else
    8:             result = num2;
    9:         return result;
   10:     }

public static int max ( int num1, int num2 ) {
  return num1 > num2 ? num1 : num2;
}

 2) Write method headers (just the declaration, not the bodies) for the
    following methods:
     a. Compute a sales commission, given the sales amount and the commission
        rate.
     b. Display the calendar for a month, given the month and year.
     c. Compute a square root of a number.
     d. Test whether a number is even, and returning true if it is.
     e. Display a message a specified number of times.
     f. Compute the monthly payment, given the loan amount, number of
        years, and annual interest rate.
     g. Find the corresponding uppercase letter, given a lowercase letter.

a. public static int sales_commission ( int salesAmount, int commissionRate )
b. public static void displayCalendar ( int month, int year )
c. public static double sqrt ( double number )
d. public static boolean isNumEven ( int num )
e. public static void displayMessage ( String message, int numTimes )
f. public static double monthlyPayment ( double loanAmount, int numYears,
     double annualIntrestRate )
g. public static int toUppercase ( int letter )  // Use int for Unicode
Use int instead of char will accommodate Unicode.

 3) Identify and correct the errors in the following program.
     1. public class Test {
     2.   public static method1(int n, m) {
     3.     n += m;
     4.     method2 (3. 4);
     5.   }
     6.
     7.   public static int method2(int n) {
     8.     if ( n > 0 ) return 1;
     9.     else if (n == 0) return 0;
    10.     else if (n < 0) return -1;
    11.   }
    12. }

Without comments, you can see there is a problem but not what was
intended, so you must guess (and document in a comment) how to fix.  Here's
one reasonable way:

public class Test {
  public static void method1 ( int n, int m ) {  // missing "void" and "int".
    n += m;  // Does nothing; could be deleted.
    method2( 3.4 );  // space removed; method2 changed to match (double).
  }

  public static int method2 ( double n ) {  // change int to double.
    if ( Double.isNaN( n ) )
      throw new ArithmeticException();   // I bet Liang forgot this check. :-)
    if ( n > 0.0 ) return 1;  // for clarity, changed 0 to 0.0.
    if (n == 0.0 ) return 0;  // eliminated needless else, but not an error.
    else return -1;           // eliminated needless if statement.
  }
}

 4) What is pass-by-value? Show the results of the following two programs.

  a. 1: public class Test {
     2:     public static void main ( String [] args ) {
     3:         int max = 0;
     4:         max(1, 2, max);
     5:         System.out.println(max);
     6:     }
     7:
     8:     public static void max ( int value1, int value2, int max ) {
     9:         if ( value1 > value2 )
    10:             max = value1;
    11:         else
    12:             max = value2;
    13:     }
    14: }

  b. 1: public class Test {
     2:     public static void main ( String [] args ) {
     3:         int i = 1;
     4:         while ( i <= 6 ) {
     5:             method1( i, 2 );
     6:             i++;
     7:         }
     8:     }
     9:
    10:     public static void method1 ( int i, int num ) {
    11:         for ( int j = 1; j <= i; j++ ) {
    12:             System.out.print( num + " " );
    13:             num *= 2;
    14:         }
    15:         System.out.println();
    16:     }
    17: }

"Pass-by-value" is when the "actual" arguments are passed as if by
assignment to the "formal" parameters, when calling a method.

Output of program (a):
(It is legal, but confusing, to use the same for a variable and a method.)
0

Output of program (b):
2 
2 4 
2 4 8 
2 4 8 16 
2 4 8 16 32 
2 4 8 16 32 64 

 5) What is wrong with the following class?
    1: public class Test {
    2:     public static void method ( int x ) {
    3:     }
    4:     public static int method ( int y ) {
    5:         return y;
    6:     }
    7: }

Overloaded methods must have different signatures; here, both
have the same signature of "method(int)".

 6) a. Write an expression that obtains a random integer between 34 and 55, inclusive.
    b. Write an expression that obtains a random integer between 0 and 999, inclusive.
    c. Write an expression that obtains a random number between 5.5 (inclusive)
    and 55.5 (exclusive).
    d. Write an expression that obtains a random lowercase (English) letter.

a) 34 + (int) (Math.random() * (55 - 34 + 1)
b) 0 + (int) (Math.random() * (999 - 0 + 1)
c) 5.5 + Math.random() * (55.5 - 5.5 + 1)
d) 'a' + (int) (Math.random() * ('z' - 'a' + 1)

 7) How many times is the factorial method in the following invoked,
    for the call factorial(6)? 
    1: import java.util.Scanner;
    2: 
    3: public class ComputeFactorial {
    4:    public static void main ( String [] args ) {
    5:       Scanner input = new Scanner( System.in );
    6:       System.out.print( "Enter a nonnegative integer: " );
    7:       int n = input.nextInt();
    8: 
    9:       // Display factorial
   10:       System.out.println( "Factorial of " + n + " is " + factorial(n) );
   11:    }
   12: 
   13:    /** Return the factorial for the specified number */
   14:    public static long factorial ( int n ) {
   15:       if ( n == 0 ) // Base case
   16:          return 1;
   17:       else
   18:          return n * factorial( n - 1 ); // Recursive call
   19:    }
   20: }

The method is invoked 7 times with input "6":
factorial(6),
factorial(6) = 6 * factorial(5),
factorial(6) = 6 * 5 * factorial(4),
factorial(6) = 6 * 5 * 4 * factorial(3),
factorial(6) = 6 * 5 * 4 * 3 * factorial(2),
factorial(6) = 6 * 5 * 4 * 3 * 2 * factorial(1),
factorial(6) = 6 * 5 * 4 * 3 * 2 * 1 * factorial(0)

(You can add "static int count;" between lines 4 and 5, and
add "++count;" between lines 14 and 15, and print out count
between (the original) lines 10 and 11.)


 8) Suppose that the class F is defined as shown below.  Let f be an
    instance of F.  Which of the following statements are syntactically correct?

    (a)
    public class F {
      int i;
      static String s;
      void iMethod() {
      }
      static void sMethod() {
      }
    }

a. System.out.println(f.i);   // CORRECT
b. System.out.println(f.s);   // CORRECT
c. f.iMethod();               // CORRECT
d. f.sMethod();               // CORRECT
e. System.out.println(F.i);   // WRONG
f. System.out.println(F.s);   // CORRECT
g. F.iMethod();               // WRONG
h. F.sMethod();               // CORRECT


 9) Add the static keyword in the place of ?, if appropriate, in the code below.
     1: public class Test {
     2:    private int count;
     3:    public ? void main ( String [] args ) {
     4:       ...
     5:    }
     6:    public ? int getCount () {
     7:       return count;
     8:    }
     9:    public ? int factorial ( int n ) {
    10:       int result = 1;
    11:       for ( int i = 1; i <= n; i++ )
    12:          result *= i;
    13:       return result;
    14:    }
    15: }

ANSWER - Line 3 must be static, line 6 must not be static, and line 9 should be
         static but is correct either way.

10) Can each of the following statements be compiled?
    a. Integer i = new Integer("23");                     ANSWER - yes
    b. Integer i = new Integer(23);                       ANSWER - yes
    c. Integer i = Integer.valueOf("23");                 ANSWER - yes
    d. Integer i = Integer.parseInt("23", 8);             ANSWER - yes
    e. Double d = new Double();                           ANSWER - NO
    f. Double d = Double.valueOf("23.45");                ANSWER - YES
    g. int i = (Integer.valueOf("23")).intValue();        ANSWER - YES
    h. double d = (Double.valueOf("23.4")).doubleValue(); ANSWER - YES
    i. int i = (Double.valueOf("23.4")).intValue();       ANSWER - YES
    j. String s = (Double.valueOf("23.4")).toString();    ANSWER - YES

11) a. How do you convert an integer into a string?
    b. How do you convert a numeric string into an integer?
    c. How do you convert a double number into a string?
    d. How do you convert a numeric string into a double value?

ANSWER - 
    a.  Integer.toString( someInt );
    b. int i = Integer.parseInt( "123" );
    c. Double.toString( someDouble );
    d. double d = Double.parseDouble( "123.69" );

end