Lecture 8, solutions

Exercise 1.

According to the API, sort throws ClassCastException - if the list contains elements that are not mutually comparable (for example, strings and integers), and UnsupportedOperationException - if the specified list's list-iterator does not support the set.

1. ClassCastException is an unchecked exception, and so is UnsupportedOperationException
2. According to the lecture slides, unchecked exceptions represent situations in which the client breaks a pre-condition. The documentation explicitly states the precondition that they must be mutually comparable, so that fit with ClassCastException being unchecked. The documentation says that the list must be modifiable, and if it is not, an UnsupportedOperationException is thrown, which also fits with the principle of unchecked exceptions representing a broken pre-condition.
3. No, the two errors should be "NotMutallyComparable" instead of ClassCastException, and "ListNotModifiable" instead of UnsupportedOperationException.
4. It throws the unchecked exception NullPointerException if the list reference is null. That precondition is not stated in the documentation. It might throw others, we have not found those others.
5. No, one should instead write programs so that the preconditions are satisfied when you call the method.

Exercise 2.1

Exercise 2.2

Exercise 3.1

public class MyDate implements Date{
	static final int[] daysInMonth={31,28,31,30,31,30,31,31,30,31,30,31,};
	public MyDate(int y, int m, int d) throws DateException{ // Constructor might throw exception
		setDate(y, m, d);
	}

	public void setDate(int y, int m, int d) throws DateException{
		year = y;
		if (m<1 || m>12) 
			throw new DateException("wrong month");
		month = m;
		if (d<1 || d> daysInMonth[m-1])
			throw new DateException("wrong day");
		day = d;
	}

	public int getYear(){return year;}
	public int getMonth(){return month;}
	public int getDay(){return day;}

	public String toString(){
		return day +"/"+month+"/"+year;
	}
}

Exercise 3.2.1

public static void main(String args[]) throws Date.DateException {
	System.out.println("The date is: " + editDate(new MyDate(2004,6,15)) );
}

Exercise 3.2.2

	private void storeDate(){
		int y,m,d;
		try{
			try{y = Integer.parseInt(yearField.getText());}
			catch(NumberFormatException nfe){
				messages.setText("Year is not a number");
				return;
			}
			try{m = Integer.parseInt(monthField.getText());}
			catch(NumberFormatException nfe){
				messages.setText("Month is not a number");
				return;
			}
			try{d = Integer.parseInt(dayField.getText());}
			catch(NumberFormatException nfe){
				messages.setText("Day is not a number");
				return;
			}
			date.setDate(y,m,d);
			this.hide();
		}catch(Throwable uups){
			messages.setText("Something is not right!?");
		}
	}

Exercise 3.2.3

The only thing to change in MyDate is the setDate method.

	public void setDate(int y, int m, int d) throws DateException{
		year = y;
		if (m<1 || m>12) throw
			new DateException("month");
		month = m;
		if (d<1 || d> daysInMonth[m-1])
			throw new DateException("day");
		day = d;
	}

The storeDate method is changed to:

private void storeDate(){
	int y,m,d;
	try{
		try{y = Integer.parseInt(yearField.getText());}
		catch(NumberFormatException nfe){
			messages.setText("Year is not a number");
			return;
		}
		try{m = Integer.parseInt(monthField.getText());}
		catch(NumberFormatException nfe){
			messages.setText("month is not a number");
			return;
		}
		try{d = Integer.parseInt(dayField.getText());}
		catch(NumberFormatException nfe){
			messages.setText("day is not a number");
			return;
		}
		date.setDate(y,m,d);
		this.hide();
	}catch(MyDate.MyDateException e){
		if ( e.field.equals("month") )
			messages.setText("Wrong month number");
		else
			messages.setText("Wrong day number");
	}catch(Throwable uups){
		messages.setText("Something is not right!?");
	}
}