BHSawesome AP CSA

Java has two different methods to print output to the screen which we saw in passing in the Introduction.

The value between the parentheses can be any Java expression which just means some bit of code that produces a value. One simple kind of expression is a literal value which is a way of representing a value directly—or literally—in our code. For instance 42 is a literal expression representing the int value 42. When we print a value with print or println, Java converts the value to its textual representation and then emits that to the screen. So we could write System.out.print(42) to print the text 42. Another trivial kind of expression is a variable, whose value is whatever value was last assigned to it.

In order to print out a value Java has to convert it to text. In Java, text is represented with a data type called a String. Why are they called "strings"? Who knows? Java didn’t make it up; computer scientists have been using “string” to refer to sequences of items since at least the 1940s. Think “string of pearls” or “string of ponies” but in this case a “string of characters”, where “character” is what computer scientists call letters, numbers, punctuation, spaces, etc.

If we wanted to store any text, from a person’s name to the contents of a novel, in a Java program, we would represent it as a String. As another data type, like int and double, we can declare String variables. We’ll learn a lot more about the String data type in Section 7.1 Manipulating strings but for now we just need to know we can write string literals by enclosing the text we want in quotation marks: "".

For instance "Hi there!" is a string literal whose value is the text: Hi there!. The quotation marks are not part of the value; they are the syntax for writing the string literal. Thus the code System.out.println("Hi there!"); prints the text Hi there! on the screen, without the quotation marks, and then advances to the next line.

Another handy String feature for printing output is the string concatenation operator, written with +, the same as arithmetic addition. When we write a + expression where one of the operands is a String, it produces a new String by smooshing together the String with the String representation of the other value. For instance if name and age were variables containing someone’s name and age we could write:

System.out.println("Name: " + name + "; Age: " + age);

to get output like:

Name: Sally Sue; Age: 17

Note that there’s an important difference between a variable in a string concatenation expression and the name of the variable in quotes "". The latter is just a String whose text happens to be the same as the name of the variable while the former will produce the String representation of the variable’s value. A common mistake is to write something like System.out.println("x") when we meant to write System.out.println(x) On the other hand, a really good way to write that would be: System.out.println("x: " + x) which would emit the value of x labeled with the name of the variable.

Also if we want to make readable output with spaces between the concatenated pieces variables, we need to the spaces inside the quotes of some String literal. Without spaces, we will get output like HiJose instead of Hi Jose.

What if we wanted to print out a double quote " character? Since " has a special meaning within Java’s string literals, it might seem there’d be no way to include it within a string. However, Java provides a way to write string literals that contain double quotes using a mechanism called escaping. To “escape” the normal syntax of a string literal, where a quotation mark would indicate the end of the string, we put a backslash (\) in front of the quotation mark which tells Java to include the following quotation mark in the string value rather than treating it as the end of the literal.

This is called a backslash escape sequence. Since backslash is used this way, that also makes it a special character, so if we want to include a backslash in a string literal, we need to escape it with another backslash! There are a handful of other backslash escape sequences. The only other one the College Board wants you to know about is \n which will be translated to a newline character, which will break the text across lines.

The flip side of emitting output to the user is getting input from the user. Just as there are lots of ways for computers to generate output, modern programs can accept all kinds of input: UI gestures such as the user clicking on a button, audio input from the computer’s microphone, visual input from a camera, etc. But as with output the simplest to deal with is textual input. In Chapter 12 text-files we’ll look at how to read input from files saved on the computer. But sometimes it’s useful to be able to get input typed by the user as a program runs.

The AP curriculum doesn’t require you to know any particular way of dealing with user input but does mention the Scanner class that comes with Java as one way. And knowing how to use Scanner will let us write programs whose behavior can change depending on their input.

The code below uses the Scanner class to read a name the user types in and then says hello. It will have different results depending on what the user types. Try it by typing in your name in the box below the code before you click “Run”. Try again with a friend’s name. The code works for any name.

Although you will not be tested in the AP CSA exam on using the Java input from the keyboard, learning how to do input in Java is very useful and fun. For more information on using the Scanner class, go to https://www.w3schools.com/java/java_user_input.asp, and for the newer Console class, https://howtodoinjava.com/java-examples/console-input- output/. We are limited with the one way communication with the Java server in this Runestone ebook, but in most IDEs, the input/output would be more interactive. You can try this Scanner input example in JuiceMind (click on Create Starter Code after login with a Google account) or Scanner input example in Replit using the Scanner class and Console input example using the Console class. We will also learn how to use Scanner with input files in a later unit.

Have you ever played MAD LIBS? In this game, you first choose a bunch of words following clues like give me a color or a plural noun, without looking at the story, and then those words are filled into the story to make it sound very wacky! Fill in the variables below with silly words, and then run to see the wacky story.

Then, working in pairs, come up with another silly story that uses at least 5 new String variables. When you’re done, try another team’s mad libs code. For more advanced programming, you could create this program in a Java IDE that can do input using the Scanner class to read in input into the variables.