AP Computer Science --- Haas --- StaticExamples

/**
 * >>>>>>>>>>>>>>>>>>>>  static fields <<<<<<<<<<<<<<<<<<<
 * When you declare a field variable of a class as static, 
 * that field variable is instantiated only once, no matter how many instances 
 * of the class are created. 
 * In other words, a static field variable is a class 
 * variable, and, not an instance variable. If the value of a static field 
 * variable of a class is changed by an instance of that class, the value of 
 * that field variable changes for all instances of that class.
 * 
 * >>>>>>>>>>>>>>>>>>>>  static methods <<<<<<<<<<<<<<<<<<<
 * When you declare a method of a class as static, that method becomes a class 
 * method operating in the class itself rather than in instances of the class. 
 * A static method can only refer to static field variables and static methods 
 * of the class. A static method is implicitly final, i.e. it cannot be overriden 
 * by subclasses of the class.
 * 
 * >>>> Copy into BlueJ and run, make sure you understand how it works! <<<<
 */
class Stock {
    private static double pricePerShare;
    private int numberOfShares;
    private double totalValue;
    
    Stock (int shares, double pps)   {
        numberOfShares = shares;    
        pricePerShare = pps;
    }
 
    public double getNetWorth()  {
        return numberOfShares * pricePerShare;
    }
    
    public static double getPricePerShare() {
        return pricePerShare;
    }
 }

public class StockTester {
    public static void main (String[] argsIn) 
    {
        Stock joesStocks = new Stock(10,15);
        System.out.println("joes net worth = " + joesStocks.getNetWorth());
        System.out.println("joes price per share = " + joesStocks.getPricePerShare());
       
        Stock jimsStocks = new Stock(2,20);
        System.out.println("jims net worth = " + jimsStocks.getNetWorth());
        System.out.println("joes net worth = " + joesStocks.getNetWorth());
        
        System.out.println("jims price per share = " + jimsStocks.getPricePerShare());
        System.out.println("joes price per share = " + joesStocks.getPricePerShare());
        System.out.println("price per share = " + Stock.getPricePerShare());
    }
}

/**
 * A class with Instance and Static methods.
 *
 * Instance methods rely on the state of a specific object instance. 
 * Instance methods are tied to a particular instance because the 
 * behavior that the method invokes relies upon the state of that 
 * particular instance.
 * 
 * When you declare a method as static, you define that method as 
 * being a class method. A class method applies to the class as opposed 
 * to any particular instance. The behavior instigated by a class method 
 * does not rely on the state of a particular instance. In fact, a static 
 * method cannot rely on an instance's state since static methods lack
 * access to this reference. Instead, the behavior of a class method 
 * either depends on a state that all objects share at the class level,
 * or is independent of any state at all. 
 */
public class MrHappy
{
    private String mood = "happy";
    private static int happyCount = 0;

    public MrHappy() {
      happyCount++;
    }

    public static int instances() { 
      return happyCount;
    }

    public String getMood() {
      return mood;
    }
    
    public void receivePinch()  {
        if(mood.equals("happy")) {
            mood = "annoyed";
        } 
        else { 
            mood = "angry";
        }
     }
    
    public void receiveHug() {
        if(mood.equals("angry")) {
            mood = "annoyed";
        } 
        else {
            mood = "happy";
        }
    }
}

/***********************************************************
 *                      MrHappyTester 
 **********************************************************/
public class MrHappyTester
{
   public static void main(String[] args)
   {

     MrHappy obj1 = new MrHappy();
     System.out.println( "\nFirst MrHappy");
     System.out.println( "I am " + obj1.getMood());
     obj1.receiveHug();
     System.out.println( "After Hug I am " + obj1.getMood());
     
     MrHappy obj2 = new MrHappy();
     System.out.println( "\nSecond MrHappy");
     System.out.println( "I am " + obj2.getMood());
     obj2.receivePinch();
     System.out.println( "After Pinch I am " + obj2.getMood());

     System.out.println( "\nMrHappy Count = " + obj1.instances());
     System.out.println( "\nMrHappy Count = " + obj2.instances());
     System.out.println( "\nMrHappy Count = " + MrHappy.instances());
    }
}