Static Initializers

When a program runs and uses a class, Java initializes the class's static fields.

• We can accept the default values for fields (null, 0.0, etc.), or initialize them ourselves.

public class ParkingLot {
  private static int MAX_CARS = 100;

  private Car[] cars;
  private int numberOfCars;

  // fields for name and location of this parking lot
  private String lotName;
  private String lotLocation;

  // ...
}

The simplest way to initialize a field is to provide an initial value in its declaration.

public class ParkingLot {
  private static int MAX_CARS = 100;
  // ...
}

Sometimes initialization might require error checking or logic we cannot do in a simple declaration, so instead we can use a static initialization block.

• This is a block of code { } preceded by the static keyword.

static {
  // code to do initialization
}

Using a static initialization block, our class would look like:

public class ParkingLot2 {
  private static int MAX_CARS;

  static {
    MAX_CARS = 100;
  }

  private Car[] cars;
  private int numberOfCars;

  // fields for name and location of this parking lot
  private String lotName;
  private String lotLocation;

  // ...
}

A class can have many static initialization blocks, and they can appear anywhere in the class body (though we usually put them toward the top).

Note: these are also called static initializer blocks or just static initializers.

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