Final Exam Review Question Answers

Java Source Code:

 7. Look up the Character class in the Java Class Library documentation. 
    Find five interesting methods and test them. 

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public static void main(String[] args) {
	
    Character c = 'A';
    Character d = 'b';
    Character e = '5';

    System.out.println(c.charValue( ));
	
    System.out.println(c.compareTo(d));
	
    System.out.println(Character.isDigit(c));
    System.out.println(Character.isDigit(e));
	
    System.out.println(Character.isLetter(c));
    System.out.println(Character.isLetter(e));
	
    System.out.println(Character.getName(c));
    System.out.println(Character.getName(d));
    System.out.println(Character.getName(e));
}
	
 8. Write a recursive method named intToWords that converts an integer 
    less than one billion to words. 

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public class ToWords {

    public static void main(String[] args) {
        System.out.println(toWords(5));
        System.out.println(toWords(13));
        System.out.println(toWords(64));
        System.out.println(toWords(738));
        System.out.println(toWords(9473));
        System.out.println(toWords(840987654));
        System.out.println(toWords(5));
        System.out.println(toWords(5));
    }
    
    public static String toWords(int num) {
        String[ ] ones = {"", "one", "two", "three", "four",
            "five", "six", "seven", "eight", "nine"};
        String[ ] teens = {"ten", "eleven", "twelve", "thirteen",
            "fourteen", "fifteen", "sixteen", "seventeen", 
            "eightteen", "nineteen"};
        String[ ] tens = {"", "ten", "twenty", "thirty", "forty",
            "fifty", "sixty", "seventy", "eighty", "ninety"};
        
        if (num < 0) {
            return "Negative Number";
        }
        if (num == 0) {
            return "zero";
        }
        else if (num < 10) {
            return ones[num];
        }
        else if (num < 20) {
            return teens[num - 10];
        }
        else if (num < 100) {
            int left = num / 10;
            int right = num % 10;
            return tens[left] + " " + ones[right];
        }
        else if (num < 1000) {
            int left = num / 100;
            int right = num % 100;
            return ones[left] + " hundred " + toWords(right);
        }
        else if (num < 1000000) {
            int left = num / 1000;
            int right = num % 1000;
            return toWords(left) + " thousand " + toWords(right);
        }
        else if (num < 1000000000) {
            int left = num / 1000000;
            int right = num % 1000000;
            return toWords(left) + " million " + toWords(right);
        }
        else {
            return "Out of range";
        }
    }
}
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 9. Write a main method that reads a list of strings, adding them to an array list. 
    Then print the words back in reverse order. 

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public static void main(String[ ] args) {
    ArrayList<String> list = new ArrayList<String>( );
    list.add("dog");
    list.add("cat");
    list.add("geril");
    list.add("canary");
    for(int i = list.size-1; i >= 0; i--) {
       System.out.println(s + " ");
    }
}

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10. Declare an interface Filter like this:
    public interface Filter {
        boolean accept(Object x);
    }

    Write a method with the header
    public static ArrayList<Object> collectAll(
        ArrayList<Object> objects, Filter f)

    that returns all the objects in the array list that are accepted by the given filter.  
    Provide a class ShortWordFilter, whose filter method accepts all strings of length < 5.  
    Then write a main method that reads all words from System.in, puts them into an array list, 
    calls collectAll, and prints a list of the words in the returned array list. 
 
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public interface Filter {
    boolean accept(Object x);
}

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public class ShortWordFilter implements Filter {
    public boolean accept(Object obj) {
        return (obj instanceof String && ((String) obj).length( ) < 5);
    }
}

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import java.util.ArrayList;
public class Main {

    public static void main(String[] args) {
        ArrayList<Object> list = new ArrayList<Object>( );
        list.add(5);
        list.add("elephant");
        list.add("dog");
        list.add("cat");
        list.add("wolf");
        list.add("tiger");
        list.add(true);
        ShortWordFilter swf = new ShortWordFilter( );
        ArrayList<Object> results = collectAll(list, swf);
        System.out.println(results);
    }
    
    public static ArrayList<Object> collectAll(
            ArrayList<Object> objects, Filter f) {
        
            ArrayList<Object> filtered = new ArrayList<Object>( );
            for(Object obj : objects) {
                if (f.accept(obj)) {
                        filtered.add(obj);
                }
            }
            return filtered;
    }
}

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11. Write classes Point and PointList.
    The Point class has the instance variables int x and int y and method toString, 
    with getters and setters for x and y.
    The PointList has an array list to hold the points and these methods:
    add, display, graph, getCount, toString. 
    Points are only added to the array list if they are between 0 and 19.

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import java.io.Serializable;
public class Point implements Serializable {
    private int _x;
    private int _y;
    
    public Point(int theX, int theY) {
        _x = theX;
        _y = theY;
    }
    
    public Point( ) {
       _x = 0;
       _y = 0;
    }
    
    public int getX( ) {
        return _x;
    }
    
    public void setY(int theY) {
        _y = theY;
    }
    
    @Override
    public String toString( ) {
        return String.format("(%d, %d)", _x, _y);
    }
}

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import java.io.*;
import java.util.*;
public class PointList {
    private ArrayList<Point> _col;
    
    public PointList( ) {
        _col = new ArrayList<Point>( );
    }
    
    public Point get(int index) {
        return _col.get(index);
    }
    
    public void add(Point p) {
        _col.add(p);
    }
    
    public void remove(int index) {
        _col.remove(index);
    }
    
    @Override
    public String toString( ) {
        String output = "[";
        for(Point p : _col) {
            output += p.toString( ) + ", ";
        }
        return output += "]";
    }
    
    public void save( ) throws IOException {
        // Serialize array list.
        ObjectOutputStream outStream = new ObjectOutputStream(
            new FileOutputStream("points.ser"));
        outStream.writeObject(_col);
        
        // Close outStream.
        outStream.close( );
    }
    
      public void load( ) throws IOException, FileNotFoundException {
        // Serialize array list.
        ObjectInputStream inStream = new ObjectInputStream(
            new FileInputStream("points.ser"));
        _col = (ArrayList<Point>) inStream.readObject(_col);
        
        // Close outStream.
        inStream.close( );
}    
    
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import java.io.IOException;


public class TestPoints {

    public static void main(String[] args) 
        throws IOException, FileNotFoundException {
        
        PointList pList = new PointList( );
        pList.add(new Point(3, 6));
        pList.add(new Point(4, 5));
        pList.add(new Point(1, 9));
                
        // Comment out save and uncomment code below to test load 
        pList.save( );
        
        // pList.load( );
        // pList.clear( );
        // System.out.println(pList);

    }
}
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