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Thinking in Java
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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 InitialValues are changed to provide initial values:

class InitialValues {
  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 nonprimitive objects in this same way. If Depth is a class, you can create a variable and initialize it like so:

class Measurement {
  Depth d = new Depth();
  // . . .

If you haven’t given d an initial value and you 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 InitialValues will get these same initialization values. Sometimes this is exactly what you need, but at other times you need more flexibility.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire