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Order of Initialization

Java defines the order in which it initializes fields in a class.

  1. static fields and static initializer blocks, in the order in which they appear in the file.
  2. Instance fields and initializer blocks, in the order in which they appear in the file.
  3. Constructor code.

static fields are initialized once, when the class is first used.

  • Once the class's fields are initialized, Java doesn't need to initialize them again.

Instance fields are initialized each time an object is created.

When Does Java Initialize a Class?

Java will not initialize the static members of a class until a program uses the class.1 This means when:

  • An instance of the class is created.
  • A static method of the class is invoked.
  • A static field of the class is assigned.
  • A non-constant$ static field of the class is used.

$ - constants will be explained soon

Drill

ObjectInitialization/com.example.objs2.drills.InitClassProgram

The classes in this example call static methods to print to the screen. This allows us to see when fields are initialized. * Look at InitClassProgram. It creates instances of InitClassOne and InitClassTwo. Review these to see what each does. * Run the program to see the order of initialization. Is it what you expected? * Create another instance of InitClassOne. Run the program again to see order of initialization.

(Solution: InitClassProgram.java, InitClassOne.java, InitClassTwo.java )

ObjectInitialization/com.example.objs2.drills.OrderOfInitQuiz * Look at OrderOfInitQuiz. In the comments, write what you expect to see printed to the screen, based on order of initialization.

(Solution: OrderOfInitQuiz.java)

[1] https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.4.1

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