Read: 04 - OOP

Java OO Tutorial

What Is an Object?

• Real-world objects share two characteristics: state and behavior.

• Software objects are similar: they too consist of state and related behavior.

• An object stores its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some programming languages).

• Data Encapsulation a fundamental principle of object-oriented programming that about Hiding internal state and requiring all interaction to be performed through an object’s methods.

• Software objectsbenefits:
  1. Modularity: The source code for an object can be decompose into a set of modules so that can be written and maintained independently of the source code for other objects.

1. Information-hiding: the details of object’s internal implementation remain hidden from the outside world.

2. Code re-use: If an object already exists (in differnet program by different developer), you can use that object in your program.

3. Pluggability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your application and plug in a different object as its replacement.

Java Classes

Declaring Classes

class MyClass {
    // field, constructor, and 
    // method declarations
}

• The convention to write Class Name is Capital.

• Kinds of variables:
  1. Member variables in a class-> are called fields.
  2. Variables in a method or block of code -> are called local variables.
  3. Variables in method declarations -> are called parameters.
• You can use a construct called varargs to pass an arbitrary number of values to a method. To use varargs, you use (three dots, …), then a space, and the parameter name.

  public Polygon polygonFrom(Point... corners) {
    int numberOfSides = corners.length;
    double squareOfSide1, lengthOfSide1;
    squareOfSide1 = (corners[1].x - corners[0].x)
                     * (corners[1].x - corners[0].x) 
                     + (corners[1].y - corners[0].y)
                     * (corners[1].y - corners[0].y);
    lengthOfSide1 = Math.sqrt(squareOfSide1);
  
    // more method body code follows that creates and returns a 
    // polygon connecting the Points
  }
  

    1. **Pass By Value** -> when you pass Primitive arguments, such as an int or a double.
    2. **Pass By Reference** -> when you pass Reference data type parameters, such as objects, as this example: ``` public void moveCircle(Circle circle, int deltaX, int deltaY) {   // code to move origin of circle to x+deltaX, y+deltaY   circle.setX(circle.getX() + deltaX);   circle.setY(circle.getY() + deltaY);
  

  // code to assign a new reference to circle circle = new Circle(0, 0); } ```

• Using the this Keyword -> this is a reference to the current object.

• Controlling Access to Members of a Class:

• To create an object from a class -> use the new operator and a constructor.
• The new operator returns a reference to the object that was created.

Binary, Decimal and Hexadecimal Numbers

• The Decimal Number System OR Base 10, because it is based on the number 10, with these 10 symbols (from 0-9).

• Binary Numbers OR Base 2 (0&1).

• Hexadecimal Numbers 16Symbols from 0-F