Constants public class Car { //This class produces cars with 18 MPG private String color; private String make; private String model; private double gas; public Car (String col, String mk, String mod, double galofgas) { …….. } public void moveForward(double miles) { gas = gas - miles/18.0; } public void moveBackward(double miles) { gas = gas - miles/18.0; }

public class Car { private final double MPG = 18.0; //value cannot change //adds readablility to code private String make; private String model; private double gas; public Car (String col, String mk, String mod, double galofgas) { …….. } public void moveForward(double miles) { gas = gas - miles/MPG; } public void moveBackward(double miles) { gas = gas - miles/MPG; }

Static Methods If a method is declared as static, it may be called without using an object of the class. For example: we called all Car class methods in the form CarObject.methodName because none of these methods were static If a method is declared as static, it may be called without an object ClassName.methodName Example: Math.sqrt(4) The sqrt method is a static method defined in the Math class.

Static Variables If an instance variable is declared as static, it is shared by ALL OBJECTS of the class. ( eg. Each object created from that class DOES NOT have its own storage location allocated for the variable … all the objects use the one variable location.) When do you use a static instance variable?? * when information needs to be shared between objects and a method calls does not make design sense *class constants should be static. A constant memory location cannot be changed, so it makes sense to just allocate one

Class Constants should be static In a method: final typeName variableName= expression ; In a class: accessSpecifier static final typeName variableName = expression; Why?? Because, each object of class does not need it’s own copy of a constant (each Car object does not need it’s own memory location to store MPG)

Simple example public class Player{ private static int topScore = 0; private int myScore = 0; private static final int winScore = 21; public Player() { …………….} public void gotPoint () { myScore = myScore + 1; if (myScore > topScore) topScore = myScore; } public static boolean winner( ) { return (topScore >= winScore) ; }

public class Game{ public static void main (String [] args){ Player player1, player2, player3; player1 = new Player (); player2 = new Player (); while ( Player.winner() == false) { //code to play game }

Arithmetic Operators +, -, * are used for addition, subtraction and multiplication / is the division operator If both arguments are integers, the result is an integer. The remainder is discarded For example: int val1 = 7; double value = 7.0; value = value / 4; //assigns 1.75 val1 = val1 / 4; //assignes 1 value = val1 / 4; //assigns 1.0

Arithmetic Operators Get the remainder with % (pronounced "modulo") For example: int val1 = 7; val1 = val1 / 4; //assigns 1 val1 = val1 % 4; //assigns 3

Mathematical Functions Math.sqrt(x)square root Math.pow(x, y)power x y Math.exp(x)exex Math.log(x)natural log Math.sin(x), Math.cos(x), Math.tan(x) sine, cosine, tangent (x in radian) Math.round(x)closest integer to x The Math class is a class which contains static methods which perform mathmatical functions.

Type Conversion Java will only allow an assignment if NO VALUE will be lost and types are compatible double total = "a lot"; //not compatible int ww = 5.67; //value would be lost Use “cast” to force conversion from one type to another (if it is possible): int ww = (int) 5.67; //discards fractional part int ww = Math.round(ww); //rounds to 6.0, then //truncates to 6

Reading Input The simplest (and prettiest) way to read input into a Java program is to use a method provided by the JOptionPane class (java.lang package). The call: JOptionPane.showInputDialog(“prompt”) causes a dialog box to be displayed with the prompt and a textbox for the user to type his/her response. Whatever the user types into the box is returned to the program (in String form). Note: showInputDialog is a static method of the JOptionPane class

An Input Dialog

int val; String in; in = JOptionPane.showInputDialog(“Enter a positive number”); Suppose we want to add the user input to the value 45. we cannot say: val = in + 45; because in is NOT of a numeric type (such as int or double). We must convert our string to type int. The Integer class provides a static method called parseInt which accepts a String, and returns its int equivalent. int size = Integer.parseInt(in); val = size + 45;

When using Integer.parseInt (or Double.parseDouble): Conversion throws an exception if user doesn't supply a number (ie. Program will crash if user provides non numerical input) JOptionPane also provides a method for output JOptionPane.showMessageDialog(null,”output string”); Add System.exit(0) to the end of any main method of any program that uses JOptionPane method