📓1.1: Algorithms & Programming

Algorithms
Java Programs (Classes)
- Compiling and Running Java Programs
- Java Syntax Overview
  - Syntax Errors and Debugging
Summary

📖 This page is a condensed version of CSAwesome Topic 1.1

✴✴✴ NEW UNIT/SECTION! ✴✴✴
Create a blank Java program to take your class notes in for the next few lessons.
Click on the collapsed heading below for GitHub instructions ⤵

📓 NOTES PROGRAM SETUP INSTRUCTIONS

Go to the public template repository for our class: BWL-CS Java Template
Click the button above the list of files then select Create a new repository
Specify the repository name: CS2-Unit1PartA-Notes
For the description, write: Primitive data types, variables, printing output
Click

Now you have your own personal copy of this starter code that you can always access under the Your repositories section of GitHub! 📂
Now on your repository, click and select the Codespaces tab
Click Create Codespace on main and wait for the environment to load, then you’re ready to code!
📝 Take notes in this Codespace during class, writing code & comments along with the instructor.

🛑 When class ends, don’t forget to SAVE YOUR WORK! Codespaces are TEMPORARY editing environments, so you need to COMMIT changes properly in order to update the main repository for your program.

There are multiple steps to saving in GitHub Codespaces:

Navigate to the Source Control menu on the LEFT sidebar
Click the button on the LEFT menu
Type a brief commit message at the top of the file that opens, for example: updated Main.java
Click the small ✔️ checkmark in the TOP RIGHT corner
Click the button on the LEFT menu
Finally you can close your Codespace!

☕️ What do Android phones, Minecraft, and Netflix have in common? They’re all coded in Java! Many of the apps you use in an Android phone or tablet are written in Java. Netflix uses Java for some of its software too. Java is a programming language that is used worldwide to create software that we all use.

INTRODUCTORY ACTIVITY

Form pairs, then pick a role:
- 🗣️ Instructor: Will give simple verbal instructions on how to draw some secret thing, without revealing what the thing is.
- ✍️ Drawer: Will follow the instructions to create the drawing exactly as the instructor describes.
DRAWERS leave the room while INSTRUCTORS are told about the secret thing to draw.
When the partner returns, move your chairs so you are sitting back-to-back with your teammate.
If you are the DRAWER:
- Have a blank sheet of paper and a pencil or pen ready.
- You will not know what you are drawing.
- DO NOT ask any questions during the instructions!
If you are the INSTRUCTOR:
- Give directions step-by-step, as detailed as possible.
- You can only describe shapes, sizes, and positions.
- DO NOT use any specific words for objects, body parts, comparisons, etc!!!
Once the drawing is complete, hand it in, then move your chairs back to normal.
💬 As a group, go through each of the drawings and discuss:
- Was the drawing correct? Can you tell what it is?
- How precise did the instructions need to be?
- Did the order of the instructions matter?
- What problems came from vague language?

Algorithms

Algorithms define step-by-step processes in order to complete a specific task or solve a problem in a certain way. Algorithms are used in many areas of life, not just in computer science:

A recipe is an algorithm for cooking a meal.
A set of directions to a friend’s house is an algorithm for getting there.

Algorithms can be used to plan and design code by writing the steps down in English or another language or in a diagram or in pseudocode, which is writing simplified code on paper. It’s important to plan the algorithm step by step where each step can be implemented by a line of code. Sequencing defines an order for when steps in a process are completed. Steps in a process are completed one at a time.

Java Programs (Classes)

Every program in Java is written as a CLASS. Java is an object-oriented language, and classes and objects created from classes are the basic building blocks in object-oriented programming.

Inside the class, there can be a main method that starts the program.

A method is a block of code that performs a specific task.
In other programming languages, methods are called functions or procedures.
The main() method is the entry point for the program.

Template for a simple Java program with a main() method:

public class MyClass
{
    public static void main(String[] args)
    {
        System.out.println("Hi there!");
    }
}

Note: In Java every open curly brace { must have a matched close curly brace }.

Compiling and Running Java Programs

An Integrated Development Environment (IDE) is often used to write programs because it provides tools for a programmer to write, compile, and run code.

Computers don’t actually speak Java, so we have to compile (translate) our .java source files into .class files, which a computer can understand and run. A compiler checks your code for errors and translates it to executable code.

From the intro activity:

The 🗣️ INSTRUCTOR represents the source code - all the statements we write in a .java file.

The ✍️ DRAWER represents the execution of a program.

The way the drawer interprets the instructions is like the compiler.

Java Syntax Overview

Every programming language has its own syntax, which is like its “grammar” rules. Here are some of Java’s most important rules:

Single-line comment: // comment
Multi-line comment:
```
/* comment text */
```
Keywords are reserved words that have special meaning in Java. Keywords such as public, class, and void must be in lowercase, but class names such as System and String are capitalized.
Lines in a Java program that express a complete action must end with a semicolon (;). Such a line is called a statement.

You can think of the semicolon (;) in Java like a period (.) in English. The same way you use a period to end a sentence in English, you use a semicolon to end a statement in Java.

Note also that not every line of Java ends with a semicolon! If the line starts a block like an if statement, there is no semicolon before the opening curly brace { nor one after the closing }.

Syntax Errors and Debugging

Syntax errors are reported by the compiler if your Java code is not correctly written. Syntax errors cause a program’s execution to STOP ✋🛑, either before the program starts running or at a certain point during the run.

In contrast, a logic error would be an issue in a working algorithm or program that causes unexpected behavior.

Examples of common syntax errors:

Missing semicolon ; at the end of a statement
Missing closing curly brace } around a block
Missing closing quote " around a String

🐞 Informally, a syntax error is called a bug, and the process of removing errors is called debugging.

Grace Hopper's log showing a real bug, 1947

An early computer science pioneer Grace Hopper documented a real bug, a moth that flew into a computer in 1947!

Compile-Time Errors

Example error message:

MyClass.java:5: error: unclosed string literal
       System.out.println("Hi there!);
                          ^
1 error

The filename, line number, error type, and caret ^ help locate the issue.

Error messages aren’t always 100% accurate about where the error actually is; sometimes you actually need to change something a bit earlier in the program and sometimes a bit later. But the line number is the best place to start looking.

After the line number, you will find the actual error type. These can be kind of cryptic at first, but they almost always contain some useful 🔍 clues, like in the example above:

unclosed string literal - you may not know what a string literal is (yet) but “unclosed” suggests something was opened and then not closed.
The caret (^) under the line of code is positioned to point at exactly where in the line the Java compiler thinks the problem is. In this case it’s pointing at the quotation mark (”) before “Hi”…

Type out the code below exactly, run it, and look at the error message. Then fix it so it prints Hi there!.
```
  System.out.println("Hi there!);
```
Repeat for this line of code, making sure to look at the error message before fixing it:
```
  System.out.println("Hi there!";
```
This version has TWO errors. Can you fix them?
```
  system.out.println("Hi there!")
```

Run-Time Errors

Some errors cannot be detected by the compiler. These are called run-time errors. These errors occur while the program is running, after the code has been compiled.

They can be caused by a variety of things, such as dividing by zero or trying to read from a file that doesn’t exist or a logic error in the code.

⚠️ An exception is a type of run-time error that occurs as a result of an unexpected error that was not detected by the compiler. It interrupts the normal flow of the program’s execution.

Type these two lines into your program and run it. What happens? Why?

System.out.println("It makes no sense to divide a number by zero!");
System.out.println(3/0);

Summary

(AP 1.1.A.1) Algorithms define step-by-step processes to follow when completing a task or solving a problem. These algorithms can be represented using written language or diagrams.
(AP 1.1.A.2) Sequencing defines an order for when steps in a process are completed. Steps in a process are completed one at a time.
(AP 1.1.B.1) An Integrated Development Environment (IDE) is often used to write programs because it provides tools for a programmer to write, compile, and run code.

A basic Java program looks like the following:

   public class MyClass
   {
       public static void main(String[] args)
       {
           System.out.println("Hi there!");
       }
   }

A Java program starts with public class NameOfClass { }. If you are using your own files for your code, each class should be in a separate file that matches the class name inside it, for example NameOfClass.java.
Most Java classes have a main method that will be run automatically. It looks like this: public static void main(String[] args) { }.
The System.out.println() method displays information given inside the parentheses on the computer monitor.
Java statements end in ; (semicolon). { } are used to enclose blocks of code. // and /* */ are used for comments.
(AP 1.1.B.2) A compiler translates Java code into a class file that can be run on your computer and checks code for some errors. Errors detectable by the compiler need to be fixed before the program can be run.
(AP 1.1.C.1) A syntax error is a mistake in the program where the rules of the programming language are not followed. These errors are detected by the compiler. Some things to check for are ; at end of lines containing complete statements and matching { }, (), and "".
(AP 1.1.C.2) A logic error is a mistake in the algorithm or program that causes it to behave incorrectly or unexpectedly. These errors are detected by testing the program with specific data to see if it produces the expected outcome.
(AP 1.1.C.3) A run-time error is a mistake in the program that occurs during the execution of a program. Run-time errors typically cause the program to terminate abnormally.
(AP 1.1.C.4) An exception is a type of run-time error that occurs as a result of an unexpected error that was not detected by the compiler. It interrupts the normal flow of the program’s execution.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.