initializers

[arivu mathi]

class Super {
Super() {
printVariable("Super");
}
public void printVariable(String name) {}
}

class Sub extends Super
{
int y;

// The following block is an instance initializer
{
printVariable("Instance Initializer(1)");
y = 10;
}

int x = 1; // Variable Initialzer
Sub() {
printVariable("Sub");
}
public void printVariable(String name) {
System.out.printf("(%s) x = %d, y = %d\n", name, x, y);
}

// The following block is an instance initializer separated by some methods
{
printVariable("Instance Initializer(2)");
y = 30;
}

public static void main(String[] args) {
new Sub();
}
}



can anyone explain the above code clearly.....

[Keith Lynn]

There are some key points.

1. A superclass will be initialized before a subclass.
2. The instance initializers in a class will finish before the constructor does.

So when you create a new instance of Sub, the first thing that happens is Super is initialized.

Super doesn't contain any initializers, and since Sub does not contain an explicit call to a constructor of Super, Super() is called.

Super() calls printVariable(). Since this method is overridden in Sub, the constructor of Super() will make a call to the overridden method in the subclass even though the subclass has not finished initializing.

So the first thing that prints is Super 0 0.

Next the instance initializers in Sub are executed in textual order.

The first thing that executes is a call to printVariable with the parameter Instance Initializer(1).

So Instance Initializer(1) 0 0 prints.

Remember that x and y haven't been initialized yet.

Next y is initialized to 10 and then x is initialized to 1.

Next is a call to printVariable with the parameter Instance Initializer(2).

So Instance Initializer(2) 1 0 prints.

Next y is assigned to 30.

The last thing that happens is the constructor finishes executing.

The constructor calls the method printVariable.

So the last thing that prints is Sub 1 30.