Instance Initializers

We often put code to initialize instance fields in constructors, or when we declare a field.

This class initializes its Car[] in both constructors, and sets numberOfCars when the field is declared:

public class ParkingLot3 {
  private static int MAX_CARS = 100;

  private Car[] cars;
  private int numberOfCars = 0; // initialize to 0

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

  public ParkingLot3(String lotName, String lotLocation) {
    cars = new Car[MAX_CARS]; // create array
    this.lotName = lotName;
    this.lotLocation = lotLocation;
  }

  public ParkingLot3(String lotName) {
    cars = new Car[MAX_CARS]; // create array
    this.lotName = lotName;
    this.lotLocation = "DEFAULT_LOCATION";
  }
  // ...
}

An alternative is to use initializer blocks (sometimes called instance initializers).

• These are like static initializer blocks, but without the static keyword.

{
  // initialization code
}

The advantage with initializer blocks is they execute regardless of which constructor is used, and execute before any constructor's code.

Practice Exercise

Initializer blocks are a great place to do initialization that would otherwise end up in every constructor.

We can make a new version of the ParkingLot3 class that takes advantage of this.

public class ParkingLot4 {
  private static int MAX_CARS = 100;

  private Car[] cars;
  private int numberOfCars = 0; // initialize to 0

  private String lotName;
  private String lotLocation;

  {
    cars = new Car[MAX_CARS];  // create array in initializer block
  }

  public ParkingLot4(String lotName, String lotLocation) {
    this.lotName = lotName;
    this.lotLocation = lotLocation;
  }

  public ParkingLot4(String lotName) {
    this(lotName, "DEFAULT_LOCATION");
  }
  // ...
}

Now every instance will have a cars array, even though we don't initialize it in a constructor.

Practice Exercise

The Java compiler copies initializer blocks into every constructor, so initializer blocks will always execute before constructor code.¹ If there are multiple instance initializers, they are copied in the order they appear in the code.

[1] https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html

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