Basic Java 1-7

Printing in Java

  • System.out.print(x); prints x to the console
  • System.out.println(x); prints x and starts a new line in the console after
    • Useful if you want to view results easier
  • Escape characters:
    • “\n” makes the rest of the output go on a new line
      • System.out.print(x + “\n”) is the same as System.out.println(x)
    • “\t” makes a tab
    • “\” makes a \, “\’” makes a ‘, “\”” makes a “

Variables

  • Items that hold stuff

  • Rules for naming variables:

    • All variable names must begin with a letter of the alphabet, an underscore, or ( _ ), or a dollar sign ($).

    • After the first letter, variable names can also contain letters and the digits 0 to 9. No spaces or special characters.

    • You cannot use a java keyword

      • Ex: boolean if = true;

User Input

    import java.util.Scanner
    
    Scanner scan = new scanner(System.in);
    int x = scan.nextInt();
  • x is now equal to whatever the user inputted into the console
  • Works with all data types, just change scan.nextInt to scan.nextDouble or scan.nextChar based on what data type you are accepting

Arithmetic Expression

  • ”+” - Add
  • ”-“ - Subtract
  • “*” - Multiply
  • ”/” - Divide

  • ”%” - Mod

  • If you mix integers and reals in an operation, the result is a real. For example, 3*5.0 is 15.0, not 15.
  • You can assign an integer value to a real variable. For example, double x=5 sets x to 5.0.
  • You cannot assign a real value to an integer variable. For example, both int i=5.8 and int i=5.0 are illegal. This is to protect you from accidentally losing the fractional part. However, if you use a cast to reassure Java that you really mean it, then it’s legal. For example, int i=(int)5.8 is legal,and gives i the value 5.

Casting

  • Upcasting
    • If you want to raise a value up to a larger data type, that is always allowed.

    • Example:

        double x = 33;
  • Downcasting
    • Bringing something to a lower data type is harder, you need to declare the cast explicitly

    • this always truncates (brings down) the decimal, rounding use:

        Math.round(x);
  x--;

Booleans

Image result for light switch

  • A boolean is a light switch.
  • it can either be true or false.
  • These translate to…

    • On or off

      or…

    • 1 or 0

Logical Operators

  • ”   ” - or
  • ”==” - is equal to
  • ”!=” - is not equal (! = is not, can be applied to more things)
  • “&&” - and
  • One expression at a time: If the first one doesn’t satisfy the condition, it doesn’t evaluate the second.

Random CS Fact

  • The first “bug” was actually a moth in someone’s computer!