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Initialization and class loading


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Initialization and
class loading

In many more traditional languages, programs are loaded all at once as part of the startup process. This is followed by initialization, and then the program begins. The process of initialization in these languages must be carefully controlled so that the order of initialization of statics doesn't cause trouble. C++, for example, has problems if one static expects another static to be valid before the second one has been initialized.

Java doesn't have this problem because it takes a different approach to loading. Because everything in Java is an object, many activities become easier, and this is one of them. As you will learn in the next chapter, the code for each object exists in a separate file. That file isn't loaded until the code is needed. In general, you can say that until an object of that class is constructed, the class code doesn't get loaded. Since there can be some subtleties with static methods, you can also say, "Class code is loaded at the point of first use."

The point of first use is also where the static initialization takes place. All the static objects and the static code block will be initialized in textual order (that is, the order that you write them down in the class definition) at the point of loading. The statics, of course, are initialized only once.

Initialization with inheritance

It's helpful to look at the whole initialization process, including inheritance, to get a full picture of what happens. Consider the following code:


// The full process of initialization.

class Insect

static int x1 =

prt('static Insect.x1 initialized');

static int prt(String s)

public class Beetle extends Insect

static int x2 =

prt('static Beetle.x2 initialized');

static int prt(String s)

public static void main(String[] args)

The output for this program is:

static Insect.x initialized

static Beetle.x initialized

Beetle constructor

i = 9, j = 0

Beetle.k initialized

k = 63

j = 39

The first thing that happens when you run Java on Beetle is that the loader goes out and finds that class. In the process of loading it, the loader notices that it has a base class (that's what the extends keyword says), which it then loads. This will happen whether or not you're going to make an object of that base class. (Try commenting out the object creation to prove it to yourself.)

If the base class has a base class, that second base class would then be loaded, and so on. Next, the static initialization in the root base class (in this case, Insect) is performed, and then the next derived class, and so on. This is important because the derived-class static initialization might depend on the base class member being initialized properly.

At this point, the necessary classes have all been loaded so the object can be created. First, all the primitives in this object are set to their default values and the object handles are set to null. Then the base-class constructor will be called. In this case the call is automatic, but you can also specify the constructor call (as the first operation in the Beetle( ) constructor) using super. The base class construction goes through the same process in the same order as the derived-class constructor. After the base-class constructor completes, the instance variables are initialized in textual order. Finally, the rest of the body of the constructor is executed.

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