PIC20A sample final ans

Question 1
(a)

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class ChooseColor extends JFrame implements ActionListener {
    private JLabel label = new JLabel();
    private JPanel panel = new JPanel(new GridLayout(1,0));
    private JButton[] buttons= new JButton[3];
    
    private Container contentPane = getContentPane(); 
   
    public ChooseColor() {
        buttons[0] = new JButton("Red");
        buttons[0].setBackground(Color.red);
        buttons[1] = new JButton("Green");
        buttons[1].setBackground(Color.green);
        buttons[2] = new JButton("Blue");
        buttons[2].setBackground(Color.blue);
        label.setOpaque(true);
        contentPane.add(label);
        for(int i = 0; i < 3; i++) {
            panel.add(buttons[i]);
            buttons[i].addActionListener(this);
        }
        
        contentPane.add(panel, BorderLayout.SOUTH);
    }
    
    public void actionPerformed(ActionEvent e) {
        if(e.getActionCommand().equals("Red")) {
            label.setBackground(Color.red);
        } else if(e.getActionCommand().equals("Green")) {
            label.setBackground(Color.green);
        } else {
            label.setBackground(Color.blue);
        }
    }
    
     public static void main(String[] args) {
        JFrame f = new ChooseColor();
        f.setSize(250,150);
        f.setVisible(true);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

(b)

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class ChooseColor extends JFrame {
    private JLabel label = new JLabel();
    private JPanel panel = new JPanel(new GridLayout(1,0));
    private JButton[] buttons= new JButton[3];
    
    private Container contentPane = getContentPane(); 
   
    public ChooseColor() {
        buttons[0] = new JButton("Red");
        buttons[0].setBackground(Color.red);
        buttons[0].addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                label.setBackground(Color.red);
            }
        });
        
        buttons[1] = new JButton("Green");
        buttons[1].setBackground(Color.green);
        buttons[1].addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                label.setBackground(Color.green);
            }
        });
        
        buttons[2] = new JButton("Blue");
        buttons[2].setBackground(Color.blue);
        buttons[2].addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                label.setBackground(Color.blue);
            }
        });
        
        label.setOpaque(true);
        contentPane.add(label);
        for(int i = 0; i < 3; i++) {
            panel.add(buttons[i]);
        }
        
        contentPane.add(panel, BorderLayout.SOUTH);
    }
    
    
     public static void main(String[] args) {
        JFrame f = new ChooseColor();
        f.setSize(250,150);
        f.setVisible(true);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}

Question 2

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class Click extends JFrame implements ActionListener {
    JButton[] buttons = new JButton[3];
    Container contentPane = getContentPane();
    
    public Click() {
	contentPane.setLayout(new GridLayout(0,1));
	for(int i = 0; i < buttons.length; i++) {
	    buttons[i] = new JButton("0");
	    buttons[i].addActionListener(this);
	    contentPane.add(buttons[i]);
	}
	
    }
    
    public void actionPerformed(ActionEvent e) {
	// we can use e.getActionCommand
	// try to show you different methods of doing it
	JButton button = (JButton)e.getSource();
	int num = Integer.parseInt(button.getText());
	button.setText(Integer.toString(num+1));
    }
    
   public static void main(String[] args) {
        JFrame f = new Click();
	f.pack();
	f.setVisible(true);
	f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
	
}

Question 3
(a) Read my notes
(b)(i),(iii),(iv) can be compiled. We don't have to "take care" runtime excetpion.

Question 4

ArithmeticException
Finally
2

Explanation: 3/0 throws the exception first. The codes after it won't be exceuted.
Question 5

public class Date {
     int day;
     int month;
     int year;
     Date(int day, int month, int year) {
          this.day = day;
          this.month = month;
          this.year = year;
      }
      
      public boolean equals(Object obj) {
          if(obj instanceof Date) {
              return ((Date)obj).day == day &&
                          ((Date)obj).month == month &&
                          ((Date)obj).year == year;
        } else {
            return false;
        }
      }
        
        public Object clone() {
            return new Date(day, month, year);
        }
        
        public String toString() {
            return month + "/" + day + "/" + year;
        }
}

Question 6
(a) : refer to my notes
(b):

Integer Sum(Integer num1, Integer num2) {
    int s = num1.intValue() + num2.intValue();
    return new Integer(s);
}

(c): read my handout.
Question 7
(a)

    public static long getSize(File f) {
	if(f.isFile()) { // if it is a file
	    return f.length();
	} else { // if it is a folder
	    long length = 0;
	    File[] files = f.listFiles(); // list files
	    for(int i = 0; i < files.length; i++) {
		length+=getSize(files[i]); // recursive function
	    }
	    return length;
	}
    }

(b)

    public static void count(File f) {
	int fileNum = 0;
	int folderNum = 0;
	File[] files = f.listFiles();
	for(int i = 0; i < files.length; i++) {
	    if(files[i].isFile()) { // it is a file
	        fileNum++;
	    } else {
	        folderNum++;
	    }
	}
	System.out.println("File: " + fileNum);
	System.out.println("Folder: " + folderNum);
    }

(c)

new File("dir/dir1").mkdirs();
new File("dir/dir2").mkdirs();
new File("dir/dir3").mkdirs();

Question 8

import java.io.*;

public class Count {
       public static int countChar(File file, char ch) throws IOException {
              FileInputStream in = new FileInputStream(file);
	      int c;
	      int count = 0;
	      while((c=in.read()) != -1) {
		  if(c == ch) {
		      count++;
		  }
	      }	      
	      in.close();
	      return count;      	      
       }
       
       public static void main(String[] args) {
            PrintStream out = null;
	    File f = new File("in.txt");
	    try {
		out = new PrintStream(new FileOutputStream("count.txt"));
		for(char i = 'a'; i < 'z'+1; i++) {
		    out.println(i + ": " + countChar(f, i));
		}
	    } catch (IOException ex) {
		if(out != null) {
		    out.println("IOException");
		    out.close();
		}
	    }
       }
}

Question 9 (a)

    public static void increaseByOne(Vector v) {
	for(int i = 0; i < v.size(); i++) {
	    Integer x = (Integer)v.get(i);// casting
	    int y = x.intValue();// get int value
	    v.set(i, new Integer(y+1));
	}
    }

(b)

    public static Vector remove(Vector v, char ch) {
	Vector result = new Vector();
	for(int i = 0; i < v.size(); i++) {
	    String s = (String)v.get(i); // casting
	    if(s.indexOf(ch) == -1) { // s has no ch
		result.add(s); // add
	    }
	}
	return result;
    }

(c)
Read java doc
Question 10
(a)
Correct selection is: D
The code will compile and run; the cast in line 6 is required, because changing an Animal to a Dog is going "down" the tree.
(b)
Correct selection is: E
The cast in line 7 is required. Answer D is a preposterous statement expressed in a tone of authority.