Defining classes

• Fields are variables that hold the data stored in objects, also known as instance variables.

• Constructor. The default constructor goes away even if you write your own constructor that takes parameters, so you must define both explicitly.

• this keyword. A method invocation implicitly passes an object as a parameter called this.

• static keyword. A static field is a single variable that shared by a whole class of objects; its value does not vary from object to object, also known as class variables.

  • call it by class name
  • eg. System.in

int kids = Human.numberOfHumans / 4;

• A static method doesn't implicitly pass an object as a parameter.

  • It lets a method run without any instance of the class.
  • Cannot use non-static (instance) variables / methods.
  • The main() method is always static, because when we run a program, we are not passing an object in.

• Initializing a static variable

  • Static variables in a class are initialized before any object of that class can be created.
  • Static variables in a class are initialized before any static method of the class runs.

Lifetimes of Variables

• A class variable (static field) lasts as long as the program runs.

HFJ Reading

• A class describes what an object knows and what an object does.
• A method uses parameters. A caller passes arguments.
• Java is pass-by-value.
• Instance variable vs. local variable
  • Local variables do NOT get a default value.