How to code static fields and methods

• You can use the static keyword to code static fields and static methods.
• Since static fields and static methods belong to the class, not to an object created from the class, they are sometimes called class fields and class methods.
• When you code a static method, you can only use static fields and fields that are defined in the method.
• You can't use instance variables in a static method because they belong to an instance of the class, not to the class as a whole.

How to declare static fields

private static int numberOfObjects = 0;
private static double majorityPercent = .51;
public static final int DAYS_IN_JANUARY = 31;
public static final float EARTH_MASS_IN_KG = 5.972e24F

A class that contains a static constant and a static method

public class FinancialCalculations
{
    public static final int MONTHS_IN_YEAR = 12;

    public static double calculateFutureValue(
        double monthlyPayment, double yearlyInterestRate,
        int years)
    {
        int months = years * MONTHS_IN_YEAR;
        double monthlyInterestRate =
            yearlyInterestRate/MONTHS_IN_YEAR/100;
        double futureValue = 0;
        for (int i = 1; i <= months; i++)
            futureValue = (futureValue + monthlyPayment)
                          * (1 + monthlyInterestRate);
        return futureValue;
    }
}

The Product class with a static variable and a static method

public class Product
{
    private String code;
    private String description;
    private double price;

    private static int objectCount = 0; // declare a static variable

    public Product()
    {
        code = "";
        description = "";
        price = 0;
        objectCount++;                 // update the static variable
    }

    public static int getObjectCount() // get the static variable
    {
        return objectCount;
    }
   ...