1.2 Why Programming? Why Java?

What do Android phones, Minecraft, and Netflix have in common? They're all programmed in Java! Many of the apps you use in an Android phone or tablet are written in Java. If you've used App Inventor before, those apps are translated to Java before they are run on a phone or tablet. 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.

1.2.1 First Java Program

Every program in Java is written as a class. Java is an object-oriented language and we'll learn more about classes and objects in Unit 2. Inside the class, there can be a main method that starts the program. When you ask the Java run-time to run a class, it will always start execution in the main method. Here is the template for a simple Java program with a main method:


public class MyClass
{
    public static void main(String[] args)
    {
        // Put your code here!
    }
}

Note: In Java every open curly brace { must have a matched close curly brace }. These are used to start and end class definitions and method definitions.

1.2.2 Print Methods

Java has two different methods to print output to the screen:

System.out.println(value) : prints the value followed by a new line (ln)
System.out.print(value) : prints the value without advancing to the next line

System.out.println("Hi there!"); prints out the characters between the first " and the second " followed by a new line. The "Hi there!" is called a string literal, and it can have zero to many characters enclosed in starting and ending double quotes.

1.2.3 Syntax Errors and Debugging

Computers don't actually speak Java so we have to compile (translate) Java source files that we write into class files which is code that a computer can understand and run. Syntax errors are reported to you by the compiler if your Java code is not correctly written. Examples of syntax errors are a semicolon ; missing or if the code has a open curly brace { or open quote ", but no close curly brace } or close quote ".

Informally, a syntax error is called a bug, and the process of removing errors is called debugging. An early computer science pioneer Grace Hopper documented a real bug, a moth that flew into a computer in 1947!

1.2.4 Reading Error Messages

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.

1.2.5 Comments

In Java and many text-based coding languages, // is used to mark the beginning of a comment. For multi-line comments, use /* to start the comment and */ to end the comment. The compiler will skip over comments. However, it is a good idea to use comments to make notes to yourself and other programmers working with you.


/* MyClass.java
   Programmer: My Name
   Date:
*/

int max = 10; // this keeps track of the max score

1.2.6 Debugging Challenge

Working in pairs, debug the following code. Can you find all the bugs and get the code to run?

1.2.7 Summary

A basic Java program starts with public class NameOfClass { }
Most Java classes have a main method: public static void main(String[] args) { }
System.out.print() and System.out.println() display information
String literals are enclosed in double quotes (" ")
Java statements end in ; (semicolon)
{ } are used to enclose blocks of code
// and /* */ are used for comments

1.2.8 AP Practice

Test your understanding with these AP-style questions:

1.2.1.1 First Java Program

1.2.2.1 Print Commands

1.2.6.1 Debugging Challenge

AP Practice Questions

1. What is printed as a result of executing the following code segment?

System.out.print("Java is ");
System.out.println("fun ");
System.out.print("and cool!");

Java is fun and cool!
Java isfun
and cool!
Java is
fun
and cool!
Java is fun
and cool!

Additional Resources

For more detailed information and interactive exercises, visit the CSAwesome Java Introduction.