I mentioned that our next programming assignment will involve String manipulation which is really a CSE142 topic, but I didn't intend the String processing to be the hard part of the assignment. So I said that I wanted to solve a short problem to remind people how basic String manipulation works.
I said that I wanted to write a method called dashes that would take a String as input and that would return a new String with dashes inserted between letters. So given the input "hello", the method would return "h-e-l-l-o".
Someone said that we could build up a temporary String that the method could return. So the basic structure will look like this:
public static String dashes(String s) {
String result = ??
// fill up string
return result;
}
Someone said that we could loop over the characters of the string. Remember
that the string has a method called length that tells you the number of
characters in the string and a method called charAt that gets you the
individual characters of the string:
public static String dashes(String s) {
String result = ??
for (int i = 0; i < s.length(); i++) {
// do something with s.charAt(i)
}
return result;
}
Someone suggested that each time through the loop we want to add to the current
result a dash and the next character, so I modifed the loop to be:
for (int i = 0; i < s.length(); i++) {
result = result + "-" + s.charAt(i);
}
The problem with this is that it would add a leading dash that we don't want.
And putting the dash after the call on charAt would put an extra dash
afterwards. This is a classic fencepost problem that we can solve by
processing the first character before the loop.So we decided to initialize the string to the first character of the word:
String result = s.charAt(0);
Unfortunately, this isn't going to work. You can't assign a string the value
of a character. Someone suggested casting, but that doesn't work either. The
usual way to do this in Java is to append the character to an empty string,
because you can always concatenate a value to a string:
String result = "" + s.charAt(0);
But because we processed the first character before the loop, we had to change
the loop bounds to start at 1 instead of 0. So our final version became:
public static String dashes(String s) {
String result = "" + s.charAt(0);
for (int i = 1; i < s.length(); i++) {
result = result + "-" + s.charAt(i);
}
return result;
}
I mentioned one last detail about this method. It fails in one case. Someone
said that it doesn't properly handle an empty string, which is correct. We
have assumed that there is a character at position 0. That won't work for an
empty string. We could add a special case for that, but I said that in this
case I would probably add a precondition that makes this clear:
// pre: s is not an empty string
It's important to consider these cases and document any preconditions, but
sometimes assumptions like this are reasonable. For example, in the hangman
programming assignment, you will never be asked to deal with an empty string
because the program itself guarantees that all words are at least of length
1.I explored several issues having to do with classes by developing a class called Angle that could be used to keep track of the angles that are used to specify latitudes and longitudes. For example, Seatac Airport is at a latitude of 47 degrees 39 minutes North and a longitude of 122 degrees 30 minutes West. This will be a simple class for demonstration purposes only.
We are only going to keep track of degrees and minutes, so we began with a class that has fields for each and a constructor:
public class Angle {
private int degrees;
private int minutes;
public Angle(int degrees, int minutes) {
this.degrees = degrees;
this.minutes = minutes;
}
}
We started with some client code that constructed an array of Angle objects and
that printed it out:
import java.util.*;
public class Fri {
public static void main(String[] args) {
Angle a1 = new Angle(23, 26);
Angle a2 = new Angle(15, 48);
List<Angle> list = new ArrayList<Angle>();
list.add(a1);
list.add(a2);
System.out.println(list);
}
}
When we ran this program, it produced output like the following:
[Angle@42719c, Angle@30c221]
The text being generated in each case includes the name of the class (Angle)
followed by an at sign followed by a hexadecimal (base 16) address. This is
what the built-in toString method produces. To get better output, we decided
to add a toString method to the class.Ideally we would show something like 23 degrees and 26 minutes using the standard symbols used for degrees and minutes:
23° 26′But those characters aren't easy to work with in Java, so instead we'll settle for "d" for degrees and "m" for minutes:
23d 26mWe wrote the following toString method to produce that output:
public String toString() {
return degrees + "d " + minutes + "m";
}
When we ran the client code again, it produced the following output:
[23d 26m, 15d 48m]
Then I discussed how to include a method that would add two of these angles
together. In a sense, what I'd like to be able to say in the client code is
something like this:
Angle a1 = new Angle(23, 26);
Angle a2 = new Angle(15, 48);
Angle a3 = a1 + a2;
We can't write it that way because the "+" operator is used only for numbers
and String concatenation. Some programming languages like C++ allow you to do
something called "operator overloading," but Java doesn't allow that. So we
have to settle for having a method called "add" that we can use to ask one of
these angles to add another one to it:
Angle a1 = new Angle(23, 26);
Angle a2 = new Angle(15, 48);
Angle a3 = a1.add(a2);
I pointed out that this is a common convention in the Java class libraries.
For example, there is a class called BigInteger for storing very large integers
that has a method called add that is similar.We wrote this as the first version of our add method:
public Angle add(Angle other) {
int d = degrees + other.degrees;
int m = minutes + other.minutes;
return new Angle(d, m);
}
I pointed out that we're referring to other.degrees and other.minutes, which
are private fields. This works because the understanding of the word private
is that it is "private to the class." This is not at all the way that we as
humans understand the meaning of private (if something is private to me, then
it shouldn't be available to other humans). But in Java, one Angle object can
access private elements of another Angle object because they are both of the
same class.I modified the client code to add this angle to the list as well:
Angle a1 = new Angle(23, 26);
Angle a2 = new Angle(15, 48);
Angle a3 = a1.add(a2);
List<Angle> list = new ArrayList<Angle>();
list.add(a1);
list.add(a2);
list.add(a3);
System.out.println(list);
When we ran it, we got the following output:
[23d 26m, 15d 48m, 38d 74m]
Clearly the program has added the two angles to get a third, but the third
angle is listed as having 74 minutes, which isn't right. The first thing we
did was to discuss the constructor. We added the following precondition:
// pre: minutes <= 59 and minutes >= 0 and degrees >= 0
public Angle(int degrees, int minutes) {
this.degrees = degrees;
this.minutes = minutes;
}
Then we added code to throw an exception when the precondition was not
satisfied:
// pre: minutes <= 59 and minutes >= 0 and degrees >= 0
// (throws IllegalArgumentException if not true)
public Angle(int degrees, int minutes) {
if (minutes < 0 || minutes > 59 || degrees < 0)
throw new IllegalArgumentException();
this.degrees = degrees;
this.minutes = minutes;
}
But we still needed to handle the case where the minutes become larger than 59.
We were able to fix this with a simple if statement:
public Angle add(Angle other) {
int d = degrees + other.degrees;
int m = minutes + other.minutes;
if (m >= 60) {
m -= 60;
d++;
}
return new Angle(d, m);
}
It also would have been possible to use integer division and the mod operator
to figure this out.When we ran the code again, we got this output, indicating that it had correctly added the two angles together:
[23d 26m, 15d 48m, 39d 14m]
Then I said that I wanted to explore how to modify the class so that we can put
a collection of Angle objects into sorted order. For example, I added the
following client code to add some randomly generated Angle objects to our list
and to put them into sorted order:
Random r = new Random();
for (int i = 0; i < 5; i++)
list.add(new Angle(r.nextInt(90), r.nextInt(60)));
System.out.println(list);
Collections.sort(list);
System.out.println(list);
Unfortunately, this code did not compile. That's because we haven't told Java
how to compare Angle objects to put them into order. For example, we know that
an angle of 45 degrees and 15 minutes is more than an angle of 30 degrees and
55 minutes, but how is Java supposed to figure that out?If you want to use utilities like Arrays.sort and Collections.sort, you have to indicate to Java how to compare values to figure out their ordering. There is an interface in Java known as the Comparable interface that captures this concept.
Not every class implements Comparable because not all data can be put into sorted order in an intuitive way. For example, the Point class does not implement the Comparable interface because it's not clear what would make one two-dimensional point "less than" another two-dimensional point. But many of the standard classes we have seen like String and Integer implement the Comparable interface. Classes that implement the Comparable interface can be used for many built-in operations like sorting and searching. Some people pronounce it as "come-pair-a-bull" and some people pronounce it as "comp-ra-bull". Either pronunciation is okay. The interface contains a single method called compareTo.
public interface Comparable<T> {
public int compareTo(T other);
}
So what does compareTo return? The rule in Java is that:
public class Angle implements Comparable<Angle> {
...
}
We completed the compareTo method fairly quickly. I pointed out that in
general the degrees field tells you which Angle is larger, so we can almost get
by with writing the method this way:
public int compareTo(Angle other) {
return degrees - other.degrees;
}
Consider the case where an Angle object has a value for degrees that is larger
than the other object's value of degrees. Then the expression above returns a
positive integer, which indicates that the object is greater. If an Angle
object has a lower value for degrees than the other one, then this expression
returns a negative value, which indicates that it is less than the other Angle
object.The only case where this fails is when the degree values are equal. In that case, we should use the minutes fields in a similar manner to break the tie. So the right way to code this is as follows:
public int compareTo(Angle other) {
if (degrees == other.degrees)
return minutes - other.minutes;
else
return degrees - other.degrees;
}
When we ran the client code with this new version of the code, we got output
like the following:
[23d 26m, 15d 48m, 39d 14m]
[23d 26m, 15d 48m, 39d 14m, 13d 57m, 57d 59m, 3d 21m, 30d 42m, 10d 19m]
[3d 21m, 10d 19m, 13d 57m, 15d 48m, 23d 26m, 30d 42m, 39d 14m, 57d 59m]
We went through this somewhat quickly, but I said that we'd have several more
examples in section.