Editing JavaLanguage (section)

====ctors====

 ['''public'''|'''private'''|'''protected'''| <ClassName>(<ParameterList>)
 {
   ['''this('''<ParameterList>''');'''|'''super('''ParameterList''');''']
   <statements>}
 }

A ctor has no return value and it is not allowed to use the return statement with an argument. The compiler decides depending on the parameters which base class ctor is used. If a default ctor (without parameters) is missing, the compiler adds on. If an explicit ctor call for the superclass or for another ctor is missing ‘super();’ is inserted implicitly at the first line, so all ctors are chained. If the base class does not have a ctor without an argument this will lead to a compile error. A call super ( ... ); to a superclass constructor, whether it actually appears as an explicit constructor call statement or is implicitly assumed, performs an additional implicit action after a normal return of control from the superclass constructor: all the instance variables that have initializers are initialized at that time. More precisely: for every instance variable declared in the class containing the call, taken in the order in which the instance variables appear in the class declaration: if that variable has an initializer and either the initialization expression is not a constant expression or its value is not the standard default value for the variable, then the initialization expression is executed and its value is assigned to the instance variable. It is a compile-time error for instance variable initializations to have a forward dependency. Wenn man fälschlicherweise eine Methode mit gleichem Namen wie die Klasse und einem return type definiert, so handelt es sich um eine normale Methode und nicht um einen ctor.

Private ctors verhindern, dass eine Klasse angelegt wird (s. Example (w)).

A special ctor feature is the possibility to call a different ctor by

 ctor1(a,b,c)
 {
   '''this'''(new X(), a, b, c);  // it must be the first line of a ctor 
 }

which also must be the first statement in the ctor. A call this(... ); to another constructor in the same class does not perform the mentioned additional implicit action.

Bei ctor design ist generell zu beachten, daß subclasses damit zurechtkommen. Man ruft am besten keine privaten Methoden auf, und schreibt am besten immer auch einen parameterlosen ctor und eine Menge von u.U. private Methoden, die die Einzelschritte implementieren (s. 24 page 26).