Lecture Notes 02: Parts of a Program

Objectives

• Demonstrate knowledge of filename relationship to program
• Demonstrate identification of many key syntactic and stylistic elements.
• Demonstrate understanding of case-sensitivity.
• Differentiate between comments and statements.
• Explain how to identify a String within an instruction.
• Explain the purpose of escape sequences.
• Demonstrate familiarity with the structure and usage of methods.
• Demonstrate how to read simple programs.
• Demonstrate identification and fixing of compilation errors.

Navigating through the Terminal

Now that everyone has Java installed on their computer, let's run our first program through the Terminal. Although it's been a lot simpler to run this on the Java Visualizer, by the end of the semester (and in your future classes) you won't find that website very useful because it can only run a few lines of code in a single file.

Files vs Programs

Keep in mind:
Rule 1: 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 in a minute).

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.

Multiple Print Instructions

You can have as many instructions as you like inside the main method. Here, we have two print statements:

public class Hello2 {

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

Additional Structural Elements

Reserved Words:
We'll Highlight in red some words in HelloWorld which we call reserved words

public class MyHello {

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

Identifiers:
We'll Highlight in green some words in HelloWorld which we call identifiers

public class MyHello {

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

Braces:
We'll Highlight in blue the different types of blocks used in Java.

public class FutureExample {

    public static void main(String[] args) {
        ArrayList<Integer> mylist = new ArrayList<Integer> ();
        System.out.println("Hello, Past!");
    }
}

Code Formatting: White spaces and Indentation

Some code formatting elements are optional but recommended. The code shown below compiles but is very hard to read and understand.

public
  class Hello2 {public static void main(String
    args [] ) {// generate some simple output
        System.out.
    println("Hello, World!");
System.out.println("How are you?");
    }}
      

|-- indent Level 1
    |-- indent Level 2
        |-- indent Level 3

public class Hello2 {

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

We will download and use the Java CheckStyle command line tool throughout this semester to perform automated style checking. Although there are many common and slightly different opinions on what is good coding style, we're going to pick just one of these during the semester, for the sake of learning to conform to a coding standard and being able to use this tool. More details in lab this week!

Your homework assignments will grade your solutions based on how many test cases they pass (which you will have access to), and then will provide you with instructions how to call the command line style checker on your code to earn those points.

Strings and escape sequences

In Java, we use strings to represent text that is human-readable, rather than code. For example, "Hello, World!" is a string. We use double quotes to indicate what the start and end of a string should be to Java, as we saw above.

Some things inside a String denote literal text and others try to indicate formatting. Since Strings are surrounded by double quotes like in:
"How are you?"
, it would be hard to try to write double quotes as part of a text. like in
" She said "How are you?", and smiled ".
In that example, the compiler doesn't know you mean for the two internal quotes to be taken literally.

That's why, we use a special symbol: \ to tell the compiler that there starts an escape sequence, which is a sequence of characters that the compiler needs to interpret in a different way.
In Java, these are the most common escape sequences:

• \t: insert a tab at this point
• \n: insert a new line at this point
• \': insert a (literal) single quote at this point
• \": insert a (literal) double quotes at this point
• \\: insert a (literal) backslash at this point

  She said
  "Hello!"
  to me.

Only use a single print statement! Your answer must use \n to receive full credit. When you're finished, upload a screenshot to Blackboard under "week 2 participation B" (each person uploads their own screenshot). If you didn't finish this today in class, make sure to get help with it in your next lab, and submit it by the end of that session.

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