When to Write Static Methods

• Consider whether the action taken by the method is one that should "belong" to an object
• Is it an action that an object itself would take?
  
• Or is this an action that should be taken even outside the context of the class?
  
• For example, you might have a Temperatures class with a method called convertToFahrenheit.
  
• Because you might want to calculate a conversion between a Fahrenheit and Celsius without constructing an object of the Temperature class, you could make this method static.
  
• In the below example, we don't want to construct an object of the Temperature class (with all of the "baggage" that goes along with it, such as a Patient object and hospital ID) just to convert between Celsius and Fahrenheit.
• Thus, we define the conversion method to be static
  

• Alternately:

• The following shows static methods defined in a class and a test class that calls the static methods

Example Defining and calling static Methods

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import java.util.Scanner;

public class BankAccount {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		double amount, interest;
		String response;
		
		System.out.println("Welcome to Wells Fraudster Bank!\n");
		System.out.println("Ready to open an account?");
		System.out.printf("We're offering an amazing annual interest rate of %.2f percent!\n\n",
				+ (Account.interestRate * 100)); //accessing static variable of Account class
		System.out.print("Enter the amount you are considering investing: $");
		amount = input.nextDouble();
		
		//calling static method of Account class
		interest = Account.calcYearlyInterest(amount);
		System.out.printf("In one year, you could earn: $%.2f\n\n", interest);
		System.out.print("Would you like to create an account (yes/no): ");
		response = input.next();
		if (response.equalsIgnoreCase("yes")) {
			input.nextLine();
			System.out.print("Enter your name: ");
			String name = input.nextLine();
			Account newAccount = new Account(name, amount);
			System.out.println("Your new account number is: " + newAccount.getAccountNum());
		} else {
			System.out.println("Goodbye!");
		}
		input.close();
	}
}

class Account {
	public static final double interestRate = 0.015;
	private static int nextAccountNum = 10000;
	
	private String name;
	private int accountNum;
	private double balance;
	
	Account(String name, double balance) {
		this.name = name;
		this.accountNum = nextAccountNum++;;
		this.balance = balance;
		
	}	

	public static double calcYearlyInterest(double dollars) {
		//I = Principle x rate x time
		return dollars * interestRate * 1;
	}
	
	public void updateBalance(double newBalance) {
		balance = newBalance;
	}
	
	public String getName() {
		return name;
	}
	
	public int getAccountNum() {
		return accountNum;
	}
	
	public double getBalance() {
		return balance;
	}
	
}

Static Variables

• A class can have static variables as well as static methods
• Like static methods, a static variable belongs to the class as a whole and not to objects individually
• Another way to think of it: the static variable is shared among all objects of the class, rather than belonging to just one, individual object.
  
• With instance variables, each object has its own copies
• However, with static variables all objects share the variable

Writing Static Variables

• You write static variables like instance variables, but with the keyword static added
• For example:

  private static int numObjs;
  

• If you do not initialize static variables, then Java initializes them just like instance variables
• Numerical values are initialized to 0, boolean values to false and objects to null
• Like instance variables, all static variables should be declared private except for constants
• Oftentimes you declare constant variables static (and public)
• For example:

  public static final double interestRate = 0.015;
  

Accessing Static Variables

• Any method can access the value of a static variable
• This includes static methods as well as instance methods

• A classic example of using static fields and methods is to keep track of how many objects of a class are constructed
• We first add an object count variable to the class:

  private static int numObjs = 0;
  

• We need to increment the variable whenever a new object is instantiated
• Thus we increment the static variable in the every constructors

  public ConstructionCounter() {
      numObjs++;
  }
  

• Also need a method to retrieve the count

  public static int getNumObjs() {
      return numObjs;
  }
  

• Since the getNumObjs() method is static, we can call it without instantiating an object:

  System.out.println("Initial count: " + getCount());
  

• After instantiating several objects, we call the method again to retrieve the count of objects instantiated
• We can experiment with the following example code

Example Using static Methods and Variables

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public class ConstructionCounter {
    private static int numObjs = 0;

    public ConstructionCounter() {
        numObjs++;
    }

    public static int getCount() {
        return numObjs;
    }

    // For testing
    public static void main(String[] args) {
        System.out.println("Initial count: "
            + getCount());
        ConstructionCounter c1 =
            new ConstructionCounter();
        ConstructionCounter c2 =
            new ConstructionCounter();
        System.out.println("Number constructed: "
            + getCount());
    }
}

Static Dos and Don'ts:

• Open up your Person.java class from the previous activity.
• Add a static variable to the top of your Person class:

• Next, add two static methods to access and update the value of the numPersons variable:

• Write a static access method as follows:
• This accessor method is named getNumPersons
• It is a static method
• It returns an int
  
• The body of the method returns the value of the numPersons variable
• Write a static mutator method as follows:
• This mutator method is named updateNumPersons
• It is a static method
• It returns nothing
• The body of the method increments the value of numPersons by 1
  

Sample Output (Note that user input may vary):