Recap

A class, a main method, and a statement. Let us summarize these three elements:

A class: The class declaration in our example has three components:

1. Optional modifiers like public (the meaning of which you will learn later).
2. A name in noun form with the first letter capitalized, following Java naming conventions.
3. A class body surrounded by curly braces, {}.

At a minimum, a class must have a name and a set of curly braces for the JDK to compile it. However, to run it from the command line, it must also have a main method.

A main method: The main method must have:

1. The public and static keywords (in any order).
2. A void return type.
3. The method name main.
4. A parameter of type String array, represented by String[]. You can name this parameter whatever you want, although args is typically used.
5. A method body containing optional statements enclosed between curly braces, which forms the main method's code block.

A statement: A statement is a single line of executable code that ends with a semicolon (;). For example, the System.out.println statement allows us to write to the console, which is why we see "Hello World" printed.

What is a Java program (a.k.a. application)?

A typical Java desktop program is a collection of one or more classes. Typically, one of these classes contains a main method, which serves as the program's entry point. A Java source file can contain more than one class definition; however, the file name must match the name of the public class. Each class definition is compiled into a separate .class file containing Java bytecode. The name of the .class file matches the name of the class defined in the .java file. All .java files must first be compiled into .class files before they can be run.

What is a library?

A library is a collection of pre-written code that you can use in your own programs.

What is an API?

API stands for Application Programming Interface. It is a set of rules and specifications that software programs follow to communicate with each other.

Example: Think of a restaurant menu. The menu is the API. It lists the dishes you can order and what you need to provide (the name of the dish). You don't need to know how the kitchen (the system) prepares the food; you only need to follow the rules on the menu to get your meal.

In Java, the Math class provides an API for mathematical operations. For example, to find a square root, you use: Task: Calling a method from the Math library API.

Math.sqrt(16.0);

The API for Math.sqrt specifies that you must provide a double value, and it will return the result as a double. You don't need to know the complex algorithms used to calculate the square root; you just need to know how to interact with the API.

What is a framework?

A framework is a pre-written set of code that provides a structure for developing applications. It offers a foundation on which you can build your own software, saving you from writing everything from scratch. Frameworks often include libraries, APIs, and other tools to help you develop your application more efficiently.

What is the difference between a library and a framework?

The key difference between a library and a framework is "inversion of control."

When you use a library, you are in control; you call the library's methods when you need them. When you use a framework, the framework is in control; it calls your code when it needs to.

For example, when you use a math library, you call the library's functions to perform calculations. When you use a web framework, the framework calls your code to handle incoming requests.

In short, your code calls a library, but a framework calls your code. We will NOT be working with framworks in this course.