Instantiation

Constructors

• Every class - including Abstract classes - MUST have a constructor.
• Constructor chaining is built in. Every class constructor will call its default superclass constructor using super().
• Every constructor as its first statement has a call to this() or super().
• Compiler will only automatically insert call to no-arg super() or this().
• CANNOT call an instance method or access instance variables till the super constructor has run.
• Only Static variables can be accessed as a call to super() or this().

Field Initializers

• Initialization of fields can be done through field declaration statements.
• Declaration of field must occur before its usage in any initializer if it is used on the RHS of an assignment. This will always result in a compiler error. Example:

public class Init4 {
	
	int length = 10;
	
	int area = length * width; //Illegal since width is a forward reference being used on the RHS.
	
	int width = 20;

}

• Initializer expression must not result in an uncaught checked exception. Compiler will throw an error.

public class Init4 {
	
	int length = 10;

        int width = 20;
	
	int area = getArea(); //Compiler error - uncaught exception.
	
        private int getArea() throws Exception {
            return length * width;
        }
}

Initialization Blocks

• A static initialization block runs when the class is first loaded.(Runs only once)
• An instance init block runs every time a new instance is created.
• An instance init block runs right after the call to super() in a constructor (ie. after all the super-constructors have run).
• In case of multiple init blocks (either instance and static) they run in the order in which they are defined.
• Exceptions in init blocks can be handled using try-catch mechanism.
• A java.lang.ExceptionInInitializerError is thrown to indicate that an exception occurred during evaluation of a static initializer block or the initializer for a static variable.