Some basics about java

1. Real-world objects contain state and behavior.
2. A software object's state is stored in fields.
3. A software object's behavior is exposed through methods.
4. Hiding internal data from the outside world, and accessing it only through publicly exposed methods is known as data encapsulation.
5. A blueprint for a software object is called a class.
6. Common behavior can be defined in a superclass and inherited into a subclass using the extends keyword.
7. A collection of methods with no implementation is called an interface.
8. A namespace that organizes classes and interfaces by functionality is called a package.
9. The term API stands for Application Programming Interface.

What is method and instance variable ?

Things an object knows about itself are called instance variable.
While Things an object can do are called methods.

Why does everything have to be In a class ?

java is an object-oriented(00) language. It's not Iike the old days when you had steam driven
compliers and wrote one monolithic source file with a pile of procedures