Java Introduction

What is a programming language?

• A programming language is what allows us to talk to computers
• When we write and run a program, a computer will "read" our program and do what we ask it to
  • We write programs to perform tasks like controlling a robot, calculating things, , etc.

What makes Java different from other languages?

• There are many languages you might've heard of, like Python, C, etc.
• Many differences, but the biggest ones are in syntax and purpose
  • Syntax
    • Syntax for a programming language is like spelling in english
    • When writing a program, we have to "spell" our words correctly so the computer can properly understand
  • Purpose
    • Different languages are written for different reasons
      • HTML is written to render web-pages, MATLAB for engineering analysis, etc.
      • Java is an object-oriented language, like Python or C++
        • Java is used for a wide variety of purposes
        • E.g. Robots, Android Apps, and even MINECRAFT

The Building Blocks of Java

The most important building blocks of a program are variables, operators, classes, and functions.

Variables

• A variable (like in math) is used to represent a piece of data

  • Just as an equation uses a variable x to symbolize a number, a program uses a variable to symbolize a block of the computer's memory

  • We declare a variable by writing its type, its name, and assigning its value. E.g.

    		int myInteger = 5;

    • Here, a variable of type integer (int) named myInteger is declared and set equal to 5

    		String myString = "Hello World!";

    • Here, a variable of type String named myString is declared and set equal to "Hello World"

  • So what does the computer do with this?

    • The computer takes the values we assign, and stores them in the memory
    • Once in the memory, we can use these values to calculate things and much more

  • Most Common Variable Types:

     int
     float
     double
     String

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Operators

• An operator (again, like in math) is used to modify a stored value
  • E.g. consider two integers stored in memory, that we want to add. We would first declare the integers, and then use the + operator to perform addition

    		int x = 5;
    		int y = 10;
    		int sum = 0;
    		sum = x + y;

    • The addition of x and y is stored in the variable sum
  • Java has many operators. The most important ones we will explore are:
    • Addition                        +
    • Subtraction                   -
    • Multiplication               *
    • Division                         /
    • Assignment                   =
    • Function Call                 .

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Classes

• A Java class is a sort of container for your code
  • When a class is declared in java, memory is allocated for this class and whatever may be inside it
• Everything in Java is a class! Literally everything
  • This isn't really the norm though; Most languages offer classes but don't require them
• E.g. consider your classic "Hello World" program

   	class HelloWorld 
   	{
   	    public static void main(String[] args) 
   	    {
   	        System.out.println("Hello World!");
   	    }
   	}

  • Here you can see that the main function of the program is contained within a class, called HelloWorld
    • This file would be saved as HelloWorld.java
• Access Modifiers -- Public & Private
  • Variables and functions either declared as:
    • Public: All functions from any file can access the data/function
    • Private: Only functions within the surrounding class can access them

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Functions

• Functions are called to perform some sort of routine

  • This routine could be adding two numbers, printing a statement, whatever you'd like!

• We create functions so that we don't have to rewrite code

• Functions consist of three main parts:

  • A return type, arguments, and a body
    • The return type tells us what type of variable the function will give us (maybe an integer or a string)
    • The arguments are what we can pass to a function (maybe we pass it one integer, maybe two)
    • The body defines what we do with the arguments, and what we return

• E.g. take a look at the following two functions

   	public static int add(int x, int y)
   	{
   		int sum = x + y;
   		return sum;
   	}

  • Here the return type is integer, the arguments are the two integers x and y, and the body of the function adds the two arguments.

• A function could also not return anything. In that case, we say the return type is void

   	private static void print()
   	{
   		System.out.println("Hello World!");
   	}

  • Note that this function has no return type, nor does it take any arguments! It just prints Hello World! to the terminal.

• Finally, to invoke or call these functions, we would do the following

   	int sum = add(5,10);	// here we assign the return value to the variable sum
   	print();		// here we invoke a void function with no arguments

Conditional Statement and Iterators

• Conditional statment and iterators are tools we use in Java and other languages to control the flow of a program

Conditional Statement

• A conditional statement checks is a conditions is met, and acts based on if it is met or not

  • The condition could be comparing two values, checking if two values are equal, etc.

• The if statement

  • An if statement allows a program to enter a body of code if the condition presented is met
  • E.g. suppose we have the following situation

   	int x = 5;
   	int y = 10; 
   	
   	if(x > y)
   	{
   		// do stuff
   	}
   	if(x < y)
   	{
   		// do more stuff
   	}
   	if(x == y)
   	{
   		// do even MORE stuff
   	}

  • Which of these would execute?

• Conditions we could use include

  • Greater Than                                    >
  • Less Than                                          <
  • Greater Than or Equal to                >=
  • Less Than or Equal to                      <=
  • Equal                                                 ==
  • Not Equal                                          !=

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Iterators

• An iterator makes a body of code loop until a condition is met. There are two types:
• The for loop
  • A for loop makes a body of code execute as many times as we want it to
    • We could make a body of code loop as little as 1 time or as many as 10000000 times
  • E.g., if we wanted to make a loop that runs 100 times and adds 1 to a variable, we could do

   	int x = 0;
   	for(int i = 0; i<100; i++)
   	{
   		x = x+1;
   	}

  • What does the stuff in the parenthesis mean?
    • int i = 0; declares a new variable i
    • i<100 means "Keep looping while i is less than 100"
    • i++ means add 1 to i after every loop
• The while loop
  • A while loop loops forever until a condition is met
    • It's kind of like a for loop, except instead of stopping when a number is reached, the while loop stops when a condition is reached.
  • E.g. if we wanted a while loop to run 100 times and add 1 to a variable, we could say

   	int x = 0;
   	while(x < 100)
   	{
   		x = x+1;
   	}