» CS1: Lecture 6

CS1: Lecture 6 – String

September 13, 2019 by Chris Johnson. Filed under cs1, fall 2019, lectures.

Dear students,

We have been looking at the computer’s role as a calculator, but we will deviate from this theme today. Numbers are not the only thing that computers are good at crunching. They are also really good at crunching communication. They can send out our words (and propaganda) as fast as electricity travels. So, today we’ll talk about text. We introduce the String class, which stands for a String of characters.

Char

Last time we built a catalog of seven different types: boolean for true/false values; byte, short, int, and long for whole numbers; and float and double for real-ish numbers. There’s an eighth type for holding two-byte Unicode characters: char.

We can declare and assign a char variable just like the others, except we must surround char literals in single-quotation marks:

char c = '/';

Only one character is allowed between the quotation marks. If we want more, we’ll have to use a different type, which we’ll see in a moment.

Primitive vs. Objects

First, we should make it clear that there are two types of citizens in Java: primitives and objects. Primitives are the dregs of society, they are simple numbers or letters or trues or falses. Their type names are all lowercase. Objects are bigwigs, having their type names capitalized. We create them in slightly different ways:

Type myObject = new Type(...);
type myPrimitive = ...;

We also act upon them in different ways. Primitives are bullied about with operators like + and %. Objects are deferentially asked to do things through their methods:

prim1 + prim2 + prim3           // operators
object.pleaseDeleteAllMyFiles() // method call

So, we can make this analogy:

operators : primitives :: methods : objects

The data type of an object is its class. We’ve formally met one object class so far: Scanner. Now it’s time to meet String.

String

Based on our discussion above, we create a String like this:

String name = new String("Frank Lee");

The thing is, we use Strings so often, the Java folks have allowed us to say it more compactly:

String name = "Frank Lee";

The String class provides a handful of useful methods for dealing with text:

length()

charAt(int i)
substring(int from)
substring(int from, int to)

indexOf(char c)
lastIndexOf(char c)

equals(Object that)
startsWith(String prefix)
endsWith(String prefix)
isEmpty()
contains(String that)

Additionally, Scanner provides two methods for retrieving text: next, which grabs the next “word” of the input between whitespace boundaries; and nextLine, which grabs the next line of input.

Exercises

We’ll discuss how Strings are laid out in memory, how we can index into them, extract substrings and so on, through a few example problems:

You ask the user for a credit card number. Print it back to the console with all but the last four digits as asterisks.
Students and teachers get a discount at your website. How can you tell if someone’s a student or teacher?
You ask 125 people for their UWEC username—and username only. All of them append @uwec.edu anyway. How can you clean up one such input?
You are writing a MUD and want to respond only to the commands north, south, east, and west. But the players are entering in all sorts of perversions of this like NORTH and east (with trailing spaces). How can you “canonicalize” their input to what you expect?
Display at most 10 characters of a customer’s first name.
Ask the user for a mod problem (100 % 3) and compute its value.

Static vs. Instance

Notice that calling methods on Strings is different than calling methods of the Math class:

char first = name.charAt(0);
double score = Math.floor(points);

In the case of length, we call this method on an instance of the String class. There may be many instances of Strings in our program, and length will give back a different result for each one. With floor, there’s no instance of Math. We call it directly on the class.

This difference is what static refers to. Math.floor is a static method. String.charAt is not.

If something is static, like static electricity, it is not moving. It is in stasis. The Math class is not moving behind the scenes. Strings, however, are a moving target, as each String will have different contents.

Casting

We’ve got four integer types and two floating point types. Sometimes we need to move data from one type to another. Whether that’s legal or not depends on the direction you are going in this ladder:

String
double
float
long
int
short
byte

If you are going from a lower type to a higher type, then you can freely transition. This is called an upcast. For instance, let’s store a byte in an int:

byte b = 100;
int i = b;

But if you go the other direction in a downcast, you face the potential loss of information. The higher types can store a broader range of numbers that might not fit in the smaller type. The Java compiler requires that we sign off on this potential information loss with a cast:

int i = 10000;
byte b = (byte) i;

Promotion

Sometimes casting happens automatically. Consider this code:

int radius = 2;
int area = Math.PI * radius * radius;

This doesn’t compile. Math.PI is a double, and as soon as you involve a double in an operation, we’re going to get a double result. There’s no explicit cast, but there’s an implicit one. This is called promotion.

TODO

Here’s your TODO list of things to complete before next class:

CS 145, be sure to complete lab 1 before lab tomorrow. We will mark checkpoints only the first 10 minutes of tomorrow’s lab.
CS 145, lab 2 is posted. You are invited to complete it early.
Read about The Git Sandwich and Updating SpecCheckers to ensure a healthy Git experience.

See you next class!

Sincerely,

P.S. It’s time for a haiku!

Words and computers
They’re natural companions
Like lightning and drought

P.P.S. Here’s the code we wrote together in class…