CSC212 Lab 5 2014

--D. Thiebaut (talk) 16:12, 23 September 2014 (EDT)

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Problem 1: Generic Classes

from research.microsoft.com/en-us/um/people/akenn/generics/cupt.jpg

• Store the program below in a file called Pair.java

public class Pair<T, P> {
    private T first;
    private P second;

    public Pair( T f, P s ) {
        first = f;
        second = s;
    }

    public T getFirst() { return first; }
    public P getSecond() { return second; }
    public void setFirst( T f ) { first = f; }
    public void setSecond( P s ) { second = s; }
    public String toString( ) { return "(" + first + ", " + second + ")"; }
}

• Store the program below in a file called TestPair.java

class TestPair {
    static public void main( String[] args ) {
        Pair<Integer, Integer> p1 = null, p2 = null;

        p1 = new Pair<Integer, Integer>( 10, 3 );
        p2 = new Pair<Integer, Integer>( 5, -9 );

        System.out.println( "p1 = " + p1 );
        System.out.println( "p2 = " + p2 );
        p1.setFirst( p2.getSecond() );
        System.out.println( "p1 = " + p1 );
    }
}

Question 1

Compile and run your program:

javac TestPair.java Pair.java 
java TestPair

Verify that your program works.

Question 2

Modify the toString() method of Pair.java, so that it prints a pair this way: [10|3], rather than (10, 3). Verify that your edited program works.

Question 3

Change TestPair.java and replace Integer with Double, so that your program can hold pairs of real numbers. Update the initialization of p1 and p2, so that you store real numbers in the pairs (e.g. 3.14159). Verify that your edited program works. Notice that you do not have to modify Pair.java at all; that's the beauty of a generic class: it will work for any type of data you want it to contain.

Question 4

Change TestPair.java again, and this time modify it so that p1 and p2 will contain ( "Smith College", 1871 ), and ( "Hampshire College", 1965). Verify that your edited program works.

Question 5

Study the solution program for Exercise 2 (seen in class). Modify TestPair.java so that it now holds an array of 6 (String, Integer) pairs. Initialize the array with these university names and the year they were founded:

• U. Bologna, 1088
• U. Oxford, 1167
• U. Cambridge, 1209
• U. Salamanca, 1218
• U. Padua, 1222
• U. Naples, 1224

Make your program output the 6 pairs. Verify that your program works.

Problem #2: Create your Own Generic Class

• Create a new generic class that will hold a triplet (i.e. 3 items). Call it Triplet.java and use Pair.java as an example.
• Create a new class called TestTriplet.java that you will adapt from TestPair.java. Make your TestTriplet.java use 3-integer triplets. Verify that your edited program works.

Question 1

Modify TestTriplet.java so that it now holds two ( String, Double, Integer) triplets: p1, and p2. Verify that your edited program works.

Question 2

Modify TestTriplet.java so that now it holds ( String, Integer, Pair( Integer, Integer ) ) !!!
You shouldn't have to modify Triplet.java at all! Initialize p1 to ( "Smith College", 1871, ( 2600, 2014 ) ), and p2 to ( "Hampshire College", 1965, ( 1400, 2014 ) ). Verify that your edited program works.

Question 3

Modify TestTriplet.java and make it hold an array of 5 (Integer, Integer, Integer) triplets. Initialize the triplets as you wish. Verify that your edited program works.

Problem #3: Review Continue and Break Statements

• This tiny section is just to make sure you remember how break and continue work in loops. They work equally well in for-loops as in while-loops.
• Study the program below.
• Guess its output.
• Run the program and verify that your intuition was correct!

public class ContinueBreak {

	public static void main(String[] args) {

		for ( int i=0; i<10000; i++ ) {
			if ( i % 3 == 0 )
				continue;
			if ( i >= 50 )
				break;
			System.out.println( "i = " + i);
		}
	}

}

Problem #4: Exceptions

Simple Exceptions

• Create a new program called Bomb.java with the following code:

import java.util.Scanner;

public class Bomb {
	
	public static void main(String[] args) {
		Scanner input = new Scanner( System.in );
		
		while ( true ) {
			System.out.println( "Please enter 2 integers"
                                                    +" (-1 to stop): ");
			int n1 = input.nextInt();
                        if ( n1==-1 ) 
                              break;
			int n2 = input.nextInt();
			System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );
		}

	}

}

• Run your program and enter some numbers. Nothing terribly exciting will happen, until you enter 0 as the second number, of course. Try it.
• Boom!
• Note the name of the exception that appears on your screen: java.lang.SomeNameException.
• Edit your program so that you can now catch the exception and process it. Put the try/catch statement as close to the problematic code as possible (and replace xxxxxxxxxxxxxxx by the actual exception name).

	
		while ( true ) {
			System.out.println( "Please enter 2 integers (-1 to stop): ");
			int n1 = input.nextInt();
			int n2 = input.nextInt();
                        if ( n1==-1 || n2==-1 ) 
                              break;
			try {
				System.out.println( n1 + " / " + n2 + " is " + (n1/n2) );
			}
			catch (  xxxxxxxxxxxxxxx  e ) {
				System.out.println("Exception: " + e.getMessage() + 
									"\nReenter numbers." );
			}
		}

More Complex Exceptions

• The ArithmeticException exception above is known to the java compiler. However, other exceptions are associated with library functions, and the compiler may not recognize them until your indicate in your program that the code for the exception should be included in the program. We illustrate this situation in this section.

• Use the same program (Bomb.java) as above. Try inputting a string instead of an integer
• Note that an exception occurs. Write down its name.
• Add a try/catch statement around the input sections of your program. Make sure you declare your variables before the try/catch statement. (This is because if you do not, n1 and n2 would not "exist" outside the try/catch block.

                        int n1, n2;
			try {
				n1 = input.nextInt();
				if ( n1==-1 ) break;
				n2 = input.nextInt();
			} catch ( InputMismatchException e ) {
                                // add your code here...
                        }

• Compile your program. Observe that you have an error... The compiler is complaining that the InputMismatchException type is unknown. So we have to import it in our program:

import java.util.Scanner;
import java.util.InputMismatchException;

public class Bomb {
	
	public static void main(String[] args) {
		Scanner input = new Scanner( System.in );
		
		while ( true ) {
			System.out.println( "Please enter 2 integers (-1 to stop): ");
			int n1, n2;
			try {
				n1 = input.nextInt();
				if ( n1==-1 ) break;
				n2 = input.nextInt();
			} catch ( InputMismatchException e ) {
                                // add your code here...
			}
			
        //... some code missing...

• Make your program skip printing the section that prints n1 / n2 if the user inputs a string instead of an integer.

• Are you getting an infinite loop? You should be getting an infinite loop. The reason is that if the user enters a string and your program executes an input.nextInt() statement, nextInt() stops on the first character that is not a number, and never removes the string from the input buffer. So when the loop starts again, the next nextInt() to run gets stuck on the same string the user entered the first time.

Solution: Add an input.nextLine() statement in your catch statement to clear the input buffer.

• Try it. Verify that your program works.