While:
Just as with our conditional (if / else) statements a while loop employs boolean test that must evaluate to true in order for the instructions enclosed in the curly brackets to be executed. The difference is that the instructions continue to be executed until the test condition becomes false.

            int x = 0;
            while (x < 10) 
            {
                println("x is less than 10.");
                x++;
            }

Consider doing a striped background. You could do the following:

	void setup()
	{
	  size(400,400);
	  background(128);
	  stroke(255,0,0);
	  strokeWeight(5);
	}
	
	void draw()
	{
	 line(0,0,0,height);
	 line(20,0,20,height);
	 line(40,0,40,height);
	 // and so on..
	 
	 // or you could use a variable:
	 int x = 60;
	 line(x,0,x,height);
	 x = x + 20;
	 line(x,0,x,height);
	}	

but that would get very tedious.

With a "while" loop we could do it much more quickly and easily:

	void setup()
	{
	  size(400,400);
	  background(128);
	  stroke(255,0,0);
	  strokeWeight(5);
	}

	void draw()
	{
	 int x = 0;
	 while (x <= width)
	 {
	   line(x,0,x,height);
	   x = x + 20;
	 }
	}

For Loop:
Since this is a common use of a loop (creating a variable, checking it's value and incrementing by a certain amount) there exists an even quicker way to do the above, all in one step.

          for (int x = 0; x <= width; x = x + 20)
          {
             line(x,0,x,height);
          }

You can read this as, create variable x and set it to 0, while x is less than or equal to the width, do the following. Increment x by 20 at the end.
Exactly what we are doing in the while loop but in one quick step.

Execute a block of code based on the result of an expresssion that utilizes relational or logical (boolean) operators

If Statement:

            int anint = 1;
            if (anint >= 0)
            {
                // Execute some code
            }

If Else Statement:

            int anint = 1;
            if (anint >= 0)
            {
                // Excute some code
            } 
            else
            {
                // Execute some other code
            }   

If, Else If, Else:

            int anint = 1;
            if (anint >= 0)
            {
                // Execute some code
            }
            else if (anint < -1)
            {
                // Execute some other code
            }
            else
            {
                // Execute this code if none of the other conditions are true
            }

You can use the boolean operations to use boolean logic on multiple expressions.

            int anint = 1;
            int anotherint = 2;
            if (anint > 0 && anotherint <= 2)
            {
                // Execute some code
            }        

Assignment:
=

Mathematical Operators:
+, -, /, * (addition, subtraction, division and multiplication)

Example:

        int anumber = 0;
        anumber = anumber + 1;
        println("anumber = " + anumber);
        anumber = anumber + anumber;
        println("anumber = " + anumber);
        anumber = anumber/2;
        println("anumber = " + anumber);
        anumber = 7 * 8;
        println("anumber = " + anumber);
        
        anumber++;  // Short-cut for adding one, called "increment"
        println("anumber = " + anumber);
        anumber--; // Short-cut for subtracting one, called "decrement"
        println("anumber = " + anumber);

Relational Operators:
>= (greater than or equal to)
<= (less than or equal to)
== (equality)
!= (inequality)

Logical Operators: boolean logic
|| (logical OR)
&& (logical AND)
! (logical NOT)

Truth tables:

mouseX and mouseY:
Define where the mouse (finger) is in relation to the window or screen.
Follow the cursor around:

        void setup()
        {
            size(500,500);
            fill(255,0,0);
            frameRate(10);
        }
        
        void draw()
        {
            rect(mouseX,mouseY,50,50);        
        }

width and height:
Contain the width and height of the sketch window. They are only set after size() is called (in the standard version, with Android it may be set already).

        void setup()
        {
            size(400,200);
            println("The width is: " + width);
            println("The height is: " + height);
        }

More information about the screen dimensions and other built-in variables available on Android can be found on the Processing Android Wiki Page

Variables are containers for data. Essentially a variable is something that can hold a value and that value can be changed (They can vary).

Data Types:
Variables in Processing (and other strictly typed languages) must be defined or declared with their type.

The different (primitive) types that Processing/Java support are as follows:
int – An integer. Whole numbers only.

int someint = 5;

boolean – True or False. Often the result of an expression.

boolean somebool = true;

float – A number with a decimal point. 1.5, 3.987 and so on.

float somefloat = 99.76;

byte – An 8 bit value. Ranges from -127 to 128 in value.

byte mybyte = -90;

char – A single character.

char mychar = 'A';

Declaration:
Variables must be declared with their type as shown (in the examples above as well as) here:

int myinteger;
float yourfloat;
boolean mybool;

Assignement:
Once they are declared, you are free to assign values to them and subsequently use them in operations:

myinteger = 5;
myinteger = 5 + 5;
myinteger = myinteger - 5;
yourfloat = 5.5;

Of course, as shown in the examples above, you can take a shortcut and do both declaration and assignement in one step:

int someint = 89; 

VARIABLE SCOPE

You may have noticed the integer is declared outside of the setup() method we wrote here. This is due to something called variable scope. The concept is that variables are only accessible to the block of code they are defined in.

For instance, if we declared a variable in the setup function, we would not be able to access that variable in the draw function.

Blocks of code are defined by “{” and “}”. Any code that is within the brackets is considered in the same block. Therefore, setup and draw have their own blocks of code.

A code block:
        {
            // Some code
            // Some more code
        }

If you declare a variable outside of a block of code, in the main processing code section it is a global variable. If you declare it inside of a block of code, it is a local variable. Local to that block.

A global and a local variable:

        int myint = 90;  // Global
        
        void setup()
        {
            int myotherint = 100; // Local to the setup function
        }