2.1. Characters¶

So far in this course, we’ve worked storing and manipulating various kinds of data. We’ve used ints and doubles to hold numbers and booleans to store true and false values. Characters are a new type that can store single letters, punctuation, symbols, spaces, tabs, etc.

To create a character variable we’d declare and instantiate a variable like this:

char letterA = 'a';

Here use the Java type char to declare a variable named letterA that can hold a single character, and initialize the variable to a lowercase “a”. Note that we use single quotes (‘) around a literal character value we want to treat as part of our program text. Some programmers pronounce the type char the same as the word “care”, the first syllable of the word “character”. Others pronounce it the same as the word “char”, as in “char-broiled”, because that is how it is spelled. You’ll hear both pronunciations, and either is acceptable (and there are even more).

Character literals, like ‘a’, appear in single quotes. Unlike string literals, which appear in double quotes, character literals can only contain a single character.

Characters work like the other data types we have seen so far. You can compare them using relational operators:

if (letter == 'a')
{

}

The increment and decrement operators work with characters. So the following code would work:

char letter = 'a';
letter = letter + 1;  // now holds the value 'b'

Java uses Unicode to represent characters, so strings can store text in other alphabets like Cyrillic and Greek, and non-alphabetic languages like Chinese. You can read more about it.

In Unicode, each character is represented by a “code unit”, which you can think of (and sometimes treat) as an integer.

2.2. Strings¶

Phrases that appear in quotation marks are called strings, because they contain a sequence of “characters” strung together. Characters can be letters, numbers, punctuation marks, symbols, spaces, tabs, etc. Strings are designed to store text and can be considered a string of characters. Strings also provide methods for manipulating character data. While individual characters in Java are primitive data values, String is actually the name of a class, and Strings are actually objects.

Lets say we wanted to create a String variable. We could create one like this:

String food = "banana";

This example will create a String variable called food that refers to the word “banana”.

We can also create an empty string (which contains no characters at all) by doing this:

String food = "";

This example will initialize the variable with an object representing an empty string. The empty string has the literal value “”–-that is, a pair of double quotes that contain no characters.

2.3. Accessing Single Characters in Strings¶

Strings provide a method named charAt() that extracts a single character. It returns a char (as opposed to a string of multiple characters).

String fruit = "banana";
char letter = fruit.charAt(0); // stores just the letter b

The argument 0 means that we want the letter at position 0. Like with Pixels and Pictures from last week, the first letter in a string is stored at position (or index) 0, not 1. In Java, all sequences of items start at position 0.

2.4. Strings are Immutable¶

Strings provide methods, toUpperCase() and toLowerCase(), that convert all of the string’s letters to the specified upper- or lowercase form. These methods are often a source of confusion, because it sounds like they with modify the string. But neither these methods nor any others can change a string, because strings are immutable–meaning the value of the object cannot be changed. Effectively, all strings are read-only objects, and the methods on strings return new String objects as their result, without modifying the original.

When you invoke toUpperCase() on a string, you get a new String` object as a return value. For example:

String name = "Ada Lovelace";
String nameUpper = name.toUpperCase();

After these statements run, upperName refers to the string “ADA LOVELACE”. But name still refers to “Ada Lovelace”. The two variables refer to two completely independent objects.

One way to get around this (if we want) is to change what the name variable refers to. For example:

String name = "Ada Lovelace";
name = name.toUpperCase();

To be clear, we cannot change the string value by running toUpperCase(), and there are still two separate objects. But we can change what the variable refers to, assigning it to refer to the second, uppercase version of the original text. By doing this, we no longer have a way to refer to the original mixed-case String object, but if we no longer need it, we can assign the name to refer to the new object instead.

Another useful String method is replace(), which finds and replaces instances of one string within another.

String text = "Computer Science is fun!";
text = text.replace("Computer Science", "CS");

This example demonstrates a common way to work with String methods. It invokes text.replace(), which returns a reference to a new string, “CS is fun!”. Then it assigns the new string to variable text, updating the variable to refer to the new object.

This assignment is important; if you don’t save the return value, invoking text.replace() will appear to have no effect, since the variable text will continue to refer to the original (unchanged) object.

2.5. Viewing Strings¶

Jeroos and Pixels are a really handy way of teaching what objects are and how they can change by running methods. Unlike these objects, we don’t have a graphical way of immediately observing the content of String objects. Instead, if we want to see what value a variable represents, we have to write some code. We can use what are called print statements:

System.out.print("Hello World");

When this code is run in BlueJ, it produces textual output. BlueJ will pop open its terminal window, which shows the textual output produced by your code. You should see this window pop up to display the text “Hello World” without the quotation marks. Notably, you may not get the results you expect if you run:

System.out.print("Hello");
System.out.print("World");

This would display “HelloWorld” all on one line without any spaces. Often, it can be easier to read your output if different strings are on different lines. For that, we’d change our print statement slightly from print to println.

System.out.println("Hello");
System.out.println("World");

This would cause “Hello” and “World” to be printed out on separate lines.

System.out.println() appends a special character, called a newline, that marks the end of one line and moves following output to the beginning of the next line. If you don’t want a newline at the end, or if you want to combine many pieces of information on the same line, you can use print() instead of println().

2.6. Operators on strings¶

In general, you cannot perform mathematical operations on strings, even if the strings look like a number. The following expressions are illegal:

// None of these work!
"Hello" - 1;
"World" / 3;
"Hello" * "World";

Note, sometimes you will see strings that look like numbers. Any time you see quotation marks though, Java will treat the data inside as a string so the following code won’t work either:

// these won't work either
"123" - 1;
"562" / 4;
"99" * "2";

However, the + operator works with strings, but it might not do what you expect. For strings, the + operator performs concatenation, which means joining two strings end-to-end. So "Hello, " + "World!" yields the string "Hello, World!".

This works with variables too:

String firstName = "Katie";
String lastName = "Brian";
String space = " ";

// create a string that will store the value "Katie Brian"
String firstAndLast = firstName + space + lastName;

Since + is defined for both numbers and strings, Java performs automatic conversions you may not expect:

System.out.println(1 + 2 + "Hello");
// produces: 3Hello

System.out.println("Hello" + 1 + 2);
// produces: Hello12

Java executes these operations from left to right. In the first line, 1 + 2 is 3, and 3 + "Hello" is "3Hello". But in the second line, "Hello" + 1 is "Hello1", and "Hello1" + 2 is "Hello12".

Concatenation can come in handy in many situations. For example, if you have a long sequence of characters inside double quotes, be aware that a string literal cannot span across two lines in a program:

// Will not compile!
String long = "This is a really
long line.";

Instead, the double-quotes surrounding a string literal must both appear on the same line. However, you can still write long strings. Just use concatenation:

String long = "This is a really "
    + "long line.";

If you ever have a string that is too long, break it into pieces so you can write each piece on its own line, and combine them using +.

2.7. Frequently Used String Methods¶

Here are some common string methods you might find useful.

int len = str.length();

if (str1.equals(str2))
{
    ...
}

if (str1.startsWith(str2))
{
    ...
}

if (str1.endsWith(str2))
{
    ...
}

String sub = str.substring(3, 5);

String sub = str.substring(3);

int pos = str.indexOf('a');

String lower = str.toLowerCase();

String upper = str.toUpperCase();

2.8. Substrings¶

A substring is a subsequence of characters within a string–a complicated way of saying a part of the string. The substring() method returns a new string that copies letters from an existing string, starting at the given index.

String fruit = "banana";
String sub1 = fruit.substring(0); // returns "banana"
String sub2 = fruit.substring(2); // returns "nana"
String sub3 = fruit.substring(6); // returns ""

The first example returns a copy of the entire string, since it starts at position 0. The second example returns all but the first two characters (it starts at position 2, which is the third letter). As the last example shows, substring() returns the empty string if the argument is the length of the string.

There are other versions of substring() that have different parameters. If it’s invoked with two arguments, they are treated as a start and end index:

String fruit = "banana";
String part1 = fruit.substring(0, 3); // returns "ban"
String part2 = fruit.substring(2, 5); // returns "nan"

Notice that the character indicated by the end index is not included. The substring() method includes all characters beginning with the first position, and going up to but not including the last position.

Defining substring() this way simplifies some common operations. For example, to select a substring with length len, starting at index i, you could write:

String result = fruit.substring(i, i + len);

4.1. Traversing Strings¶

You can use a for loop to loop through the characters of a string. The following loop traverses the characters in fruit and displays them, one on each line:

for (int i = 0; i < fruit.length(); i++)
{
    char letter = fruit.charAt(i);
    System.out.println(letter);
}

Strings provide a method called length() that returns the number of characters in the string. Because it is a method, you have to invoke it with the empty argument list, ().

The condition is i < fruit.length(), which means that when i is equal to the length of the string, the condition is false and the loop terminates.

To find the last letter of a string, you might be tempted to try something like:

String fruit = "banana";
int length = fruit.length();
char last = fruit.charAt(length); // wrong!

This code compiles and runs, but invoking the charAt() method throws a StringIndexOutOfBoundsException. The problem is that there is no character at index 6 in “banana”. Since positions start at 0, the 6 letters are indexed from 0 to 5. To get the last character, you have to subtract 1 from length.

String fruit = "banana";
int length = fruit.length();
char last = fruit.charAt(length - 1); // correct!

This is an important thing to remember because when using for loops to traverse through a string this is a common mistake:

String fruit = "banana";
for (int i = 0; i <= fruit.length(); i++) // wrong!
{
    ...
}

We need to write the condition in the for loop as either i < fruit.length() or i <= fruit.length() - 1. It is much more common in Java to see loops written using <, and this kind of comparison fits more naturally with positions starting at 0.

Many string traversals involve reading one string and creating another. For example, to reverse a string, we simply add one character at a time:

public String reverse(String s)
{
    String result = "";
    for (int i = s.length() - 1; i >= 0; i--)
    {
        result = result + s.charAt(i);
    }
    return result;
 }

The initial value of result is "", which is the empty string. The loop traverses the letters of s in reverse order. Each time through the loop, it creates a new string that is one character longer and assigns it to result. When the loop exits, result contains the letters from s in reverse order. So the result of reverse("banana") is "ananab".

6.1. Generating Random Numbers¶

Sometimes you might find it helps to make random choices in some of your programs where you want your choices to more independent and varied. Java provides a built-in class called Random for generating random numbers, and our student class library includes a special version of this class that is helpful for beginners. To use this class, add the following import statement at the top of your file:

import student.util.Random;

The Random class provides a method called generator() to get an object that represents a random number generator. Here, we only need to deal with generating random integers, and the generator provides a method that is very useful for this purpose. You can use it like this:

Random generator = Random.generator();   // local variable to refer to the random number generator
int value = generator.nextInt(4);        // generate a random number from 0 - 3

The generator provides a method called nextInt() that generates a random integer. It takes a single parameter, which is an upper limit. When you provide this upper limit, the nextInt() method will generate a number from 0 (inclusive) up to (but not including) the upper limit.

So, for example, if you want to generate a number from 0-99, you would call nextInt(100).

Suppose that you would like to perform some action 15% of the time. You could do this:

Random generator = Random.generator();   // local variable to refer to the random number generator
int value = generator.nextInt(100);
if (value < 15)
{
    //code in here will happen 15% of the time
}

Here, the call to nextInt() will produce a number from 0-99 (that is 100 possible values), and the if statement will execute its true branch if the generated number is in the range 0-14 (which is 15 possible values, or 15% of the time).

6.2. Testing Random Behaviors¶

Random behaviors are great for chance-based events. But random behaviors also make software testing hard. When you add random behavior to your code and then want to test it, what will your test case do? Suppose you want your Actor to turn left in a specific situation half the time, and right the other half. If you write a test case where the Actor is in that situation, it might turn left … or it might not. How can you write tests for that?

The answer is simple: the Random class helps you. Consider the following code sequence, which generates three random numbers less than 100:

// using the same local variable "generator" from before
int x = generator.nextInt(100);
int y = generator.nextInt(100);
int z = generator.nextInt(100);

It would be difficult to write a test case that used this code, since you have no way of controlling what values end up in x, y, and z. For test cases, however, the Random class provides a special method called setNextInts() that lets you control what numbers are generated for testing purposes. You use it like this:

// In your test case, do this:
Random.setNextInts(40, 50, 60);

// In the code you are testing, this happens:
int x = generator.nextInt(100);
int y = generator.nextInt(100);
int z = generator.nextInt(100);

// You know x will get the value 40, while y is 50, and z is 60

So, when you are testing behaviors that are random, you can force the actions to be predictable just by saying in your test cases what sequence of values you want the random number generator to produce. Outside of test cases, the generator will produce a truly (pseudo-)random sequence, but inside your test cases, the numbers will be completely determined by you.

8.1. Overriding the toString() Method¶

For this example, lets consider a class representing an ATM that tracks its location and the amount of money the machine has.

public class ATM
{
    private String location;
    private double moneyInside;

    public ATM(String where, double amount)
    {
        this.location = where;
        this.moneyInside = amount;
    }
}

And let’s make an object of this class. This ATM will be at the bank and have $4000.00 (USD) inside of it.

ATM bankATM = new ATM("bank", 4000.00);
System.out.println(bankATM);
// output: ATM@149f5b42

Printing the bankATM automatically calls toString() on the ATM, but we see the default behavior for toString() in the output, which wouldn’t be very helpful for telling us anything about this machine.

Instead, let’s write a toString() method that will be more helpful for telling us about where this ATM is.

Since a toString() method is already built-in for every class, we need to override the method and replace its behavior with one of our own. To override a method, you simply define a method with the same signature in the subclass. If you call toString() with an instance of the subclass, its version of the method will be used. In this way, the subclass method overrides the superclass version.

First, let’s add the method signature we saw above to the ATM class.

public class ATM
{
    private String location;
    private double moneyInside;

    public ATM(String where, double amount)
    {
        this.location = where;
        this.moneyInside = amount;
    }

    public String toString()
    {

    }
}

Since we are overriding the toString() method objects inherently have, this method signature needs to look exactly like what is defined above. Next, we’ll create a String variable that will tell us where this ATM is.

public class ATM
{
    private String location;
    private double moneyInside;

    public ATM(String where, double amount)
    {
        this.location = where;
        this.moneyInside = amount;
    }

    public String toString()
    {
        String data = "This ATM is at the: " + this.location;
    }
}

Then, because the return type in the toString() method is a string, we need to return that string.

public class ATM
{
    private String location;
    private double moneyInside;

    public ATM(String where, double amount)
    {
        this.location = where;
        this.moneyInside = amount;
    }

    public String toString()
    {
        String data = "This ATM is at the: " + this.location;
        return data;
    }
}

Now let’s consider our bankATM again …

ATM bankATM = new ATM("bank", 4000.00);
System.out.println(bankATM);
// output: "This ATM is at the: bank"

Now that we’ve overridden the toString() method, we can see some useful information.

While this new method may not play an important role in the ATM class, it does provide a very brief, understandable description of the state of the object. This is the reason that the toString() method was included in the Object class.