Lecture 2: Parts of a program

Objectives

• Understand basic Java syntax
• Explain the purpose of escape sequences
• Demonstrate identification and fixing of compilation errors

Navigating through the terminal

Last class, we introduced the terminal, which is a program you will use this semester to compile and run your Java programs.

Class Participation Activity 1 [2 min]

Open the terminal program on your computer however you normally open programs (using the Start menu on Windows, Finder on Mac, etc.)

Note: your computer may refer to this program as the command line or command line prompt instead of the terminal

Class Participation Activity 2 [5 min]

Let’s use terminals and navigate to a certain directory.

1. Use whatever you normally use to create a folder called CS1111 somewhere you’ll remember on your computer. Don’t put a space in the folder name.
2. Type pwd in your open terminal and hit enter. This will show you the path of the current directory you are on in your terminal. If the pwd command isn’t recognized, you are running a version that already shows the full path by your cursor, so you can skip this step.
3. Now type ls on Mac (or dir on Windows) and hit enter. This will show you all the sub-folders from the current directory.
4. In VSCode (or NotePad on Windows or TextEdit on Mac if you haven’t had your lab yet), open a new file and copy-and-paste the Hello World example from the previous class into a file. Then, save the file as Hello.java, in the folder you created in step 1. Make sure the filename is spelled and cased exactly like we show here.
5. Using the cd command in the Terminal (which stands for “change directory”), type cd FolderName where FolderName is the full path of where your CS1111 folder lives. For example, on my machine the full path of my folder, which I chose to save under Documents, is C:\Users\Dr_Kinga\Documents\CS1111. You can figure out what the correct path is by navigating through Finder (on Mac) or Windows Explorer (on Windows). In Windows Explorer, you can click on the path at the top of the window and it will convert it into the required format. On Mac, you may need to manually navigate through the folders, typing them as you go. You can also directly navigate the folders in the terminal itself by doing cd into a new directory, then typing ls/dir to see what its subfolders are, and repeat until you arrive in the CS1111 folder.
6. Check that you are in the directory were you saved Hello.java; you should be able to see the file in the current directory when you do ls/dir.

Take a screenshot of the terminal window that shows Hello.java in the current folder (using ls/dir) and submit to BB by 1pm under the link week 1 participation B.

Now that you can navigate to your Java in the files under the current directory you are in on the Terminal, you are ready to run your code (after you install Java)!

Files vs programs

Keep in mind to always verify that your file has the same name as the class it contains.

Your code is in a file called Hello.java

You are using a program, the Terminal, to access all the files on your computer.

You will use another program, javac, to compile your file. Then, you’ll use a different program, java to execute your compiled file (which will be called Hello.class, as you will see next class).

Now the confusing part: technically, all programs are also files stored somewhere on your system. For example, if you navigate to certain folders (such as Applications or Programs) you can find some of your programs there. The operating system, which you have direct access to in your Terminal program, allows you to run other programs (such as javac and java) from the Terminal.

Java syntax basics

public class Hello {

    public static void main(String[] args) {
        // generate some simple output
        System.out.println("Hello, World!");
        System.out.println("How are you?");
    }
}

Take a look at the program above – it’s Hello World, but we added another print statement.

Class Participation Activity 3 [1 min]

Click on this link to see the code above in the visualizer, and run it to see both print statements printed out.

Let’s examine the different elements in the file above; most of it won’t make sense now, but it will all make sense by the end of the semester!

public class Hello {
   ...
}

The first and last lines declare a class called Hello. These light blue words, like Hello, String, args, and System are called identifiers that, at some point, a human picked, to represent a variable or type.

The curly braces are used to group code into scope blocks (or blocks for now); the class itself is the only thing in this file.

The bolded blue words like public and class are reserved words that have special meaning in Java (so they can’t be used by humans as identifiers because that would confuse the compiler). For now, don’t worry about what they mean.

Let’s look at the next line:

public class Hello {

    public static void main(String[] args) {
        ...
    }
}

Here we see the reserved words public again, as well as static and void. Unfortunately, different text editors choose different colors for syntax highlighting, even for all reserved words, so you’ll have to learn what are identifers (you choose their names) vs reserved words (Java already has a meaning for them) the hard way :-)

You’ll also notice another identifier, main, which is the name of a function (technically a method) that contains the entire program we want to run. This main method is very special in Java – when you run a Java program, the java interpreter assumes and looks for a main method in whatever class name you passed to the program on the terminal. So when you do

javac Hello.java

java Hello

that second command is telling Java to open up this class we see above (called Hello) and find the main method in it and start executing code. Notice that the main method is followed by parentheses () that contain the arguments to the method.

The last new bit of syntax are the brackets [] which are used to refer to lists (called arrays). Here, they are used for command line arguments (but you can ignore them for now).

What next? Now that we know that whatever is inside main will get run, let’s look at those three lines – this is what we really care about at this point in the semester:

public class Hello {

    public static void main(String[] args) {
        // generate some simple output
        System.out.println("Hello, World!");
        System.out.println("How are you?");
    }
}

The first line is a comment in Java, which is just human text the compiler will ignore. You can use // at the start of a line to generate a comment, or use a comment block with /* */ as you will see soon.

And the last two lines are our two print statements. System, out, and println are all identifiers. System is a class in Java that represents the operating system; we use the dot operator . to access one of its components, out, which refers to a place we can write output to: the terminal. Then, another method called println is called using the dot operator: it will actually print something to the terminal. You can also see, once again, that both println methods are using the parentheses to specify arguments to this method (like main did above).

Next, you can see two strings in green: "Hello, World!" and "How are you?". The double quotes " around them make them strings; this is a type in Java (technically String – the same String you saw in the second line of this example). We’ll introduce them later, but for now you can see how a string can be passed as an argument to the println method, and it gets printed out :-)

Note that these two statements that actually do something (in this case, print) are terminated with a semicolon ; in Java.

Finally, everything in Java is case sensitive. Everything. Really, everything. When things aren’t compiling, check your cases :-)

Strings and escape sequences

In Java we use double quotes to indicate a string. For example,

System.out.println("System");

Here System is an identifier that refers to a class in Java that gives you access to the operating system, while "System" is just a string. The line above would print the string System to the terminal.

Sometimes, we want to put double quotes in strings. This is a problem, since the double quote is used as the delimiter:

System.out.println("She said "How are you?", and smiled.");

You can see the code above looks broken.

Class Participation Activity 4 [1 min]

Copy the broken code above into your main method in the visualizer; what happens?

You could get around this by replacing the double quote with a single quote:

System.out.println("She said 'How are you?', and smiled.");

But sometimes we don’t want to have to do that. To allow us to store double quotes (and other special characters) inside of string, we use the backslash \ as an escape character that tell the computer to ignore the upcoming character. For example, we could solve our issue above with:

System.out.println("She said \"How are you?\", and smiled.");

Class Participation Activity 4 [1 min]

Add in the \ in both places in the visualizer you have open, and verify the compilation errors go away and you can print out the string with the double quotes inside of it to the terminal.

There are many other escape sequences available in Java, where the \ is combined with the character after it to represent something unsual:

• \t insert a tab at this point
• \n insert a new line at this point
• \": insert a (literal) double quotes at this point (we just saw this)
• \\: insert a (literal) backslash at this point

Class Participation Activity 5 [3 min]

Modify the code here, using escape sequences, to make it print the following on the Visualizer:

She said
"Hello!"
to me.

Make sure you do this using only a single print statement! Your answer must use \n to receive full credit. When you’re finished, upload the visualizer link to Blackboard under “week 2 participation A”. Make sure you submit this by 1pm today to recieve credit.

Syntax vs semantics

We’ve covered a lot of syntax today, but the vast majority of this class will not be about Java syntax. This is not a course on Java; it is a course on problem decomposition and algorithm design: we will only introduce and worry about the minimal syntax we need this semester to write our basic programs. Keep in mind the difference below:

Syntax: The set of rules that define well formed sentences in a given language. In other words: Is the spelling correct, given the language (Java)? Example: the sentence He!lp the d,og ate the pizza has incorrect syntax (even if, as a human, you might be able to pick up on the meaning).

Semantics: This looks at the meaning of the sentences rather than at how they are written. In other words, does the code actually do what we want it to do?. Example: the sentence Help, the pizza ate the dog! has correct syntax but it does not reflect what the person attempted to say.

Next class

We’ll discuss variables and numeric types; this means we’ll be able to do basic math.

Prep for next time

Study for the 3-minute open-note BB quiz we’ll have at 11:10am the next class on the upcoming Lecture 03 notes.