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Member initialization

Java goes out of its way to guarantee that any variable is properly initialized before it is used. In the case of variables that are defined locally to a method, this guarantee comes in the form of a compile-time error. So if you say: 

  void f() {
    int i;
    i++;
  }

You’ll get an error message that says that i might not have been initialized. Of course, the compiler could have given i a default value, but it’s more likely that this is a programmer error and a default value would have covered that up. Forcing the programmer to provide an initialization value is more likely to catch a bug.

If a primitive is a data member of a class, however, things are a bit different. Since any method can initialize or use that data, it might not be practical to force the user to initialize it to its appropriate value before the data is used. However, it’s unsafe to leave it with a garbage value, so each primitive data member of a class is guaranteed to get an initial value. Those values can be seen here: 

//: InitialValues.java
// Shows default initial values

class Measurement {
  boolean t;
  char c;
  byte b;
  short s;
  int i;
  long l;
  float f;
  double d;
  void print() {
    System.out.println(
      "Data type      Inital value\n" +
      "boolean        " + t + "\n" +
      "char           " + c + "\n" +
      "byte           " + b + "\n" +
      "short          " + s + "\n" +
      "int            " + i + "\n" +
      "long           " + l + "\n" +
      "float          " + f + "\n" +
      "double         " + d);
  }
}

public class InitialValues {
  public static void main(String[] args) {
    Measurement d = new Measurement();
    d.print();
    /* In this case you could also say:
    new Measurement().print();
    */
  }
} ///:~

The output of this program is: 

Data type      Inital value
boolean        false
char
byte           0
short          0
int            0
long           0
float          0.0
double         0.0

The char value is a null, which doesn’t print.

You’ll see later that when you define an object handle inside a class without initializing it to a new object, that handle is given a value of null.

You can see that even though the values are not specified, they automatically get initialized. So at least there’s no threat of working with uninitialized variables.

Specifying initialization

What happens if you want to give a variable an initial value? One direct way to do this is simply to assign the value at the point you define the variable in the class. (Notice you cannot do this in C++, although C++ novices always try.) Here the field definitions in class Measurement are changed to provide initial values: 

class Measurement {
  boolean b = true;
  char c = 'x';
  byte B = 47;
  short s = 0xff;
  int i = 999;
  long l = 1;
  float f = 3.14f;
  double d = 3.14159; 
  //. . .

You can also initialize non-primitive objects in this same way. If Depth is a class, you can insert a variable and initialize it like so: 

class Measurement {
  Depth o = new Depth();
  boolean b = true; 
  // . . .

If you haven’t given o an initial value and you go ahead and try to use it anyway, you’ll get a run-time error called an exception (covered in Chapter 9).

You can even call a method to provide an initialization value: 

class CInit {
  int i = f();
  //...
}

This method can have arguments, of course, but those arguments cannot be other class members that haven’t been initialized yet. Thus, you can do this: 

class CInit {
  int i = f();
  int j = g(i);
  //...
}

But you cannot do this: 

class CInit {
  int j = g(i);
  int i = f();
  //...
}

This is one place in which the compiler, appropriately, does complain about forward referencing, since this has to do with the order of initialization and not the way the program is compiled.

This approach to initialization is simple and straightforward. It has the limitation that every object of type Measurement will get these same initialization values. Sometimes this is exactly what you need, but at other times you need more flexibility.

Constructor initialization

The constructor can be used to perform initialization, and this gives you greater flexibility in your programming since you can call methods and perform actions at run time to determine the initial values. There’s one thing to keep in mind, however: you aren’t precluding the automatic initialization, which happens before the constructor is entered. So, for example, if you say:

class Counter {

int i;

Counter() { i = 7; }

// . . .

then i will first be initialized to zero, then to 7. This is true with all the primitive types and with object handles, including those that are given explicit initialization at the point of definition. For this reason, the compiler doesn’t try to force you to initialize elements in the constructor at any particular place, or before they are used – initialization is already guaranteed. [21]

Order of initialization

Within a class, the order of initialization is determined by the order that the variables are defined within the class. Even if the variable definitions are scattered throughout in between method definitions, the variables are initialized before any methods can be called – even the constructor. For example: 

//: OrderOfInitialization.java
// Demonstrates initialization order.

// When the constructor is called, to create a
// Tag object, you'll see a message:
class Tag {
  Tag(int marker) {
    System.out.println("Tag(" + marker + ")");
  }
}

class Card {
  Tag t1 = new Tag(1); // Before constructor
  Card() {
    // Indicate we're in the constructor:
    System.out.println("Card()");
    t3 = new Tag(33); // Re-initialize t3
  }
  Tag t2 = new Tag(2); // After constructor
  void f() {
    System.out.println("f()");
  }
  Tag t3 = new Tag(3); // At end
}

public class OrderOfInitialization {
  public static void main(String[] args) {
    Card t = new Card();
    t.f(); // Shows that construction is done
  }
} ///:~

In Card, the definitions of the Tag objects are intentionally scattered about to prove that they’ll all get initialized before the constructor is entered or anything else can happen. In addition, t3 is re-initialized inside the constructor. The output is: 

Tag(1)
Tag(2)
Tag(3)
Card()
Tag(33)
f()

Thus, the t3 handle gets initialized twice, once before and once during the constructor call. (The first object is dropped, so it can be garbage-collected later.) This might not seem efficient at first, but it guarantees proper initialization – what would happen if an overloaded constructor were defined that did not initialize t3 and there wasn’t a “default” initialization for t3 in its definition?

Static data initialization

When the data is static the same thing happens; if it’s a primitive and you don’t initialize it, it gets the standard primitive initial values. If it’s a handle to an object, it’s null unless you create a new object and attach your handle to it.

If you want to place initialization at the point of definition, it looks the same as for non-statics. But since there’s only a single piece of storage for a static, regardless of how many objects are created the question of when that storage gets initialized arises. An example makes this question clear: 

//: StaticInitialization.java
// Specifying initial values in a
// class definition.

class Bowl {
  Bowl(int marker) {
    System.out.println("Bowl(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

class Table {
  static Bowl b1 = new Bowl(1);
  Table() {
    System.out.println("Table()");
    b2.f(1);
  }
  void f2(int marker) {
    System.out.println("f2(" + marker + ")");
  }
  static Bowl b2 = new Bowl(2);
}

class Cupboard {
  Bowl b3 = new Bowl(3);
  static Bowl b4 = new Bowl(4);
  Cupboard() {
    System.out.println("Cupboard()");
    b4.f(2);
  }
  void f3(int marker) {
    System.out.println("f3(" + marker + ")");
  }
  static Bowl b5 = new Bowl(5);
}

public class StaticInitialization {
  public static void main(String[] args) {
    System.out.println(
      "Creating new Cupboard() in main");
    new Cupboard();
    System.out.println(
      "Creating new Cupboard() in main");
    new Cupboard();
    t2.f2(1);
    t3.f3(1);
  }
  static Table t2 = new Table();
  static Cupboard t3 = new Cupboard();
} ///:~

Bowl allows you to view the creation of a class, and Table and Cupboard create static members of Bowl scattered through their class definitions. Note that Cupboard creates a non- static Bowl b3 prior to the static definitions. The output shows what happens: 

Bowl(1)
Bowl(2)
Table()
f(1)
Bowl(4)
Bowl(5)
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
Creating new Cupboard() in main
Bowl(3)
Cupboard()
f(2)
f2(1)
f3(1)

The static initialization occurs only if it’s necessary. If you don’t create a Table object and you never refer to Table.b1 or Table.b2, the static Bowl b1 and b2 will never be created. However, they are created only when the first Table object is created (or the first static access occurs). After that, the static object is not re-initialized.

The order of initialization is statics first, if they haven’t already been initialized by a previous object creation, and then the non- static objects. You can see the evidence of this in the output.

It’s helpful to summarize the process of creating an object. Consider a class called Dog:

  1. The first time an object of type Dog is created, or the first time a static method or static field of class Dog is accessed, the Java interpreter must locate Dog.class, which it does by searching through the classpath.
  2. As Dog.class is loaded (which creates a Class object, which you’ll learn about later), all of its static initializers are run. Thus, static initialization takes place only once, as the Class object is loaded for the first time.
  3. When you create a new Dog( ) , the construction process for a Dog object first allocates enough storage for a Dog object on the heap.
  4. This storage is wiped to zero, automatically setting all the primitives in Dog to their default values (zero for numbers and the equivalent for boolean and char).
  5. Any initializations that occur at the point of field definition are executed.
  6. Constructors are executed. As you shall see in Chapter 6, this might actually involve a fair amount of activity, especially when inheritance is involved.

Explicit static initialization

Java allows you to group other static initializations inside a special “static construction clause” (sometimes called a static block ) in a class. It looks like this:

class Spoon {

static int i;

static {

i = 47;

}

// . . .

So it looks like a method, but it’s just the static keyword followed by a method body. This code, like the other static initialization, is executed only once, the first time you make an object of that class or you access a static member of that class (even if you never make an object of that class). For example: 

//: ExplicitStatic.java
// Explicit static initialization
// with the "static" clause.

class Cup {
  Cup(int marker) {
    System.out.println("Cup(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

class Cups {
  static Cup c1;
  static Cup c2;
  static {
    c1 = new Cup(1);
    c2 = new Cup(2);
  }
  Cups() {
    System.out.println("Cups()");
  }
}

public class ExplicitStatic {
  public static void main(String[] args) {
    System.out.println("Inside main()");
    Cups.c1.f(99);  // (1)
  }
  static Cups x = new Cups();  // (2)
  static Cups y = new Cups();  // (2)  
} ///:~

The static initializers for Cups will be run when either the access of the static object c1 occurs on the line marked (1), or if line (1) is commented out and the lines marked (2) are uncommented. If both (1) and (2) are commented out, the static initialization for Cups never occurs.

Non-static instance initialization

Java 1.1 provides a similar syntax for initializing non-static variables for each object. Here’s an example: 

//: Mugs.java
// Java 1.1 "Instance Initialization"

class Mug {
  Mug(int marker) {
    System.out.println("Mug(" + marker + ")");
  }
  void f(int marker) {
    System.out.println("f(" + marker + ")");
  }
}

public class Mugs {
  Mug c1;
  Mug c2;
  {
    c1 = new Mug(1);
    c2 = new Mug(2);
    System.out.println("c1 & c2 initialized");
  }
  Mugs() {
    System.out.println("Mugs()");
  }
  public static void main(String[] args) {
    System.out.println("Inside main()");
    Mugs x = new Mugs();
  }
} ///:~

You can see that the instance initialization clause: 

  {
    c1 = new Mug(1);
    c2 = new Mug(2);
    System.out.println("c1 & c2 initialized");
  }

looks exactly like the static initialization clause except for the missing static keyword. This syntax is necessary to support the initialization of anonymous inner classes (see Chapter 7).

[21] In contrast, C++ has the constructor initializer list that causes initialization to occur before entering the constructor body, and is enforced for objects. See Thinking in C++ .

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