Lesson03

Lesson Three

Table of Contents

The Lesson
The Program

The Lesson

• In this lesson you can see how to do some simple math, and use if/else statements
• if statements:
  • If statements are a very simple feature of Java that can do some really amazing and complicated things.
    • If there is anything you ever see in an if statement that confuses you just ask.
  • In general, if statements test whether a boolean is true or false.
    • You can pass a boolean variable or literal into an if statement to test it.
      • if (hi) {
        • Keep in mind hi is a variable that we are pretending we declared elsewhere.
      • if (true) {
        • This is used to create an infinite and unending loop.
    • You can also pass in logical expressions which are evaluated as if they were booleans.
      • Think of them as methods that return booleans.
      • if (6 > 7) {
      • if (i > j) {
        • Keep in mind i and j are variables that we are pretending we declared elsewhere.
    • You can also pass in methods that actually do return booleans.
      • if (person.isAwake()) {
        • Keep in mind person is a variable that we are pretending we declared that is of some Class with an isAwake() method.
• else statements:
  • If statements are usually followed by else statements that are obviously used for if the if statement failed.
    • Remember that if statements will fail when what is passed in is false and not true.
  • You can put another if statement after an else statement to make your code more legible.
    • This is called an "else if" or "elseif" statement.
    • } else if (...) {
• Nested if statements:
  • You can put if, else, and elseif statements anywhere in your code including within the bodies of other if and else statements.
  • This is called nesting if statements.
• Comparing two variables, objects, or literals:
  • There are two ways to compare things in Java.
  • Double Equals Sign (==)
    • This is used when comparing two literals or two variables/objects that are the exact same.
      • When I say exact same I mean LITERALLY AND EXACTLY the same hence why we use this with literals.
        • This means that they must be in the same exact spot in memory so == is usually a very dangerous thing to use.
          • You should pretty much only use this with literals or when you want to know if two things are the exact same.
  • .equals()
    • This is a special method that will check to see if two variables or objects are equal to each other.
      • We will learn more about this method later so for now just remember that it allows you to compare two Strings.
• Operators:
  • There are many types of operators but for now we will only focus on three of them.
  • Assignment Operator:
    • = is the only assignment operator and we already talked about what it does when discussing variables.
  • Arithmetic Operators:
    • +, -, *, /, and % are the only arithmetic operators and they are all self explanatory except for %.
    • % is called the Modulus or Modulo and it gives you the remainder of the division of two numbers.
      • 6 % 2 = 0
      • 7 % 2 = 1
      • It is useful for determining whether numbers are odd or even.
    • If you ever want to compress the idea of adding a number to a variable (var = var + NUM) you can write this:
      • "var += NUM" which directly adds NUM to the var and then sets it equal to var.
      • You can do the same with all of the arithmetic operators:
        • var /= num, var -= num, var %= num, etc...
  • Equality and Relational Operators:
    • == and != are used for testing for equality with != meaning "not equal".
      • Just because == is an operator doesn't mean you should only use it. Don't forget .equals()!

    • and < are used for seeing whether something is greater or less than another.

      • You can also use >= and <= for greater than or equal to and less than or equal to.

The Program

• In this program we create a fairly simple calculator that allows the user to choose what operation they wish to perform on two numbers.