Static

Up to now, we've seen the keyword static in front of all of our methods.

public static void convertTemperature(double temp, char celsOrFahr) {
  //...
}

public static void isOddOrEven(int number) {
  if (number % 2 == 0) {
    System.out.println(number + " is even");
  } else {
    System.out.println(number + " is odd");
  }
}

There are also static fields. * The class below has a static field for PI.

public class Sphere2 {
  public static double PI = 3.14159;

  public double radius;

  public double getVolume() {
    double vol = 4.0 / 3.0 * PI  * radius * radius * radius;
    return vol;
  }
}

static means a method or field is part of the class (blueprint) itself, and not tied to a specific instance.

Each instance of a class knows about the blueprint used to create it, so it can refer to statics.

Practice Exercise

Java stores static methods and fields in the definition of the class, separate from each instance. It's as if statics are part of the blueprint itself.

This is different than an instance field because we need an actual object to hold an instance field.

Notes on static

• There is only one of each static field that all instances share.
• Any instance of the class can call a static method or refer to a static field.

statics are different than an instance method or field, which can only be used when you have an instance of an object.

This class has a static field for PI and a static method to calculate a circumference.

public class Sphere3 {
  public static double PI = 3.14159;

  public double radius;

  public double getVolume() {
    double vol = 4.0 / 3.0 * PI  * radius * radius * radius;
    return vol;
  }

  public static double calculateCircumference(double r) {
    return PI * 2 * r;
  }
}

Using Static Fields and Methods

Normally we refer to static fields and call static methods by using the class name, rather than using an instance variable.

Sphere3 sp = new Sphere3();
System.out.println("PI is " + sp.PI);         // NO (though it's legal)
System.out.println("PI is " + Sphere3.PI);    // YES

double circ = sp.calculateCircumference(5.0); // NO (though it's legal)
circ = Sphere3.calculateCircumference(5.0);   // YES

Using an instance variable to refer to a static field or method will compile and run, but other developers do not write code this way.

Practice Exercise

We can directly refer to static fields from instance methods, but not instance fields from static methods.

This is because instance fields are always part of an actual instance of a class, while static methods are part of the class definition.

double rad = Sphere3.radius;  // WILL NOT COMPILE
                              // Can't have a radius without an actual Sphere object.

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