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.

Files vs Programs

From your this past week, you should remember:
Rule 1: Verify that your file has the same name as the class it contains.

Multiple Print Instructions

Activity 2: Follow along using a Text Editor and the Terminal.

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 braces used in Java.

public class FutureExample {

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

Cake<Chocolate>

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

Activity 3 [3 minutes]: Work individually to modify the code below in your Text Editor, compile it, and run it.

public class Escape {

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

  She said
  "Hello!"
  to me.
      

When you have finished, submit this code on Blackboard under the "Class participation 02_1" link on Blackboard.

Reading Programs

We can't emphasize this enough: reading programs is an all-important skill.

Start with the class:

• (3) When you read the class name, pause, and ask whether the file name matches.
• When you see the opening brace (4), immediately look for and identify the closing brace (5).
• In general, for every left bracket, your eye should linger on the matching right one, sort-of absorbing the space between.

Next, go to main:

• Check to see that it's properly formed (6 - 10).
• Check the method braces (11-12) and get a sense for how much "stuff" there is between.

Next, read the one statement inside:

• Read System.out.println as one entity (13).
• When you see the opening paren, let your eyes find the right one (14).
• Same thing for the double-quote (15).
• Don't forget the semicolon (16).

As we proceed, we will point out good reading habits.

Reading Exercises

Activity 4: The following programs might have Syntax errors that will cause the compiler to complain. Try to identify them by eye: