We Keep Coding

Calling extended's class method from parent class with extended class keeping instances of parent's variables

I'm implementing a visitor pattern for a particular domain, where I have some BaseVisitor, ie:
public class BaseVisitor {
someC someInstance;
visitA(...) {
...
}
visitB(...) {
...
}
}
and a class that changes one particular functionality, ExtendedVisitor, ie:
public class ExtendedVisitor extends BaseVisitor {
visitA(...) {
...
}
}
This ExtendedVisitor has a different implementation of visitA.
What I want to do is that when I'm in visitB of BaseVisitor, in special case I want to use the method of the ExtendedVisitor (visitA) as opposed to the regular visitA of the BaseVisitor itself. This works fine, ie.:
visitB(...) {
if (...)
new ExtendedVisitor().visitA();
else
visitA();
}
Now obviously, in the BaseVisitor there are many visit methods, and so the visitA of ExtendedVisitor will call them (ie. the original implementation of those methods - in BaseVisitor). The problem is that at this point I lost the instance of someInstance (ie. it is null). Is there a way for the two classes to share the variables? Ie. let the child use parent's variables?

Since you are calling to new ExtendedVisitor() you are creating a new instance of that class and of course someInstance will be null. You could create a constructor like
public ExtendedVisitor(someC someInstance ){
this.someInstance = someInstance
}
But it doesn't sound a great idea...
With your design you are forcing your parent class to know the functionality of its children classes. I see a coupling issue here. Probably you should rethink your code and use inheritance and polimorfism in a better way.

How to prevent sublasses from default implementing a method of super class?

I have a method which adds Objects to an static list like this:
#PostConstruct
protected void registerToTransactionList() {
TransactionValidator.registerTransactionList(this);
}
registerTransactionList method just adds "this" to the static list, this method is in BalanceTransactionValidator class which extends TransactionValidator (owner of static list),the problem is all subclasses of BalanceTransactionValidator class are added to static list either,and if I override registerToTransactionList method in them like this:
#Override
#PostConstruct
protected void registerToTransactionList() {
}
It doesn't add subclasses but doesn't add BalanceTransactionValidator either. Can anybody help me on this? Please notice sublasses are overriding this method by default.

make the method private to block the visibility
private void registerToTransactionList() {
}
or make the method final to block it from been override
protected final void registerToTransactionList() {
}

There are two ways of achieving that:
Keep your method as it is; but then you have to actively check for the type of your objects before externally calling that method
Change your whole logic and make that method private
It won't help to make the method final as suggested in one of the comments - your problem is not that subclasses are overwriting that method; in essence, you have a design problem: you wish that subclasses should not invoke that method at all.
So, the only real option that makes sense here is "2.". You see, by having public method on a class that you want to be extended you are implicitly saying: it is perfectly fine to call that method; on any object that is instance of the base class (or child class!).
And in your case, that is not true: you actually do not want that the code behind this method runs for child classes. Then you shouldn't put that method in the list of public/protected methods of your base class!
Finally: you might want to step back and do some reading about good OO design. Class hierarchies do not fall from the sky: you willfully design them for a certain purpose. In other words: there is more to inheritance than just putting some "A extends B" on your class declaration. You have to understand each and every method on your B class; and how your child classes should deal with them!
EDIT: after some more thinking, I guess you are doing things "the wrong way", like:
class BaseClass {
public final void doRegistration() {
BaseClass toRegister = getObjectForRegistration();
if (toRegister != null) { ... register toRegister ...
}
protected BaseClass getObjectForRegistration() {
return null;
}
With that code, you could then put
protected BaseClass getObjectForRegistration() {
if (this instanceof ClassThatShouldBeRegistered) {
return this;
}
return null;
}
into that one class that wants to be registered. Probably there could be even nicer ways of doing so; but after some thinking I don't see how we could avoid the instanceof. But the above code should work; and it only requires specific code only in your base class and in that one class that wants to register something.

Constructor invoke overridable method in Java [duplicate]

I have a Wicket page class that sets the page title depending on the result of an abstract method.
public abstract class BasicPage extends WebPage {
public BasicPage() {
add(new Label("title", getTitle()));
}
protected abstract String getTitle();
}
NetBeans warns me with the message "Overridable method call in constructor", but what should be wrong with it? The only alternative I can imagine is to pass the results of otherwise abstract methods to the super constructor in subclasses. But that could be hard to read with many parameters.

On invoking overridable method from constructors
Simply put, this is wrong because it unnecessarily opens up possibilities to MANY bugs. When the #Override is invoked, the state of the object may be inconsistent and/or incomplete.
A quote from Effective Java 2nd Edition, Item 17: Design and document for inheritance, or else prohibit it:
There are a few more restrictions that a class must obey to allow inheritance. Constructors must not invoke overridable methods, directly or indirectly. If you violate this rule, program failure will result. The superclass constructor runs before the subclass constructor, so the overriding method in the subclass will be invoked before the subclass constructor has run. If the overriding method depends on any initialization performed by the subclass constructor, the method will not behave as expected.
Here's an example to illustrate:
public class ConstructorCallsOverride {
public static void main(String[] args) {
abstract class Base {
Base() {
overrideMe();
}
abstract void overrideMe();
}
class Child extends Base {
final int x;
Child(int x) {
this.x = x;
}
#Override
void overrideMe() {
System.out.println(x);
}
}
new Child(42); // prints "0"
}
}
Here, when Base constructor calls overrideMe, Child has not finished initializing the final int x, and the method gets the wrong value. This will almost certainly lead to bugs and errors.
Related questions
Calling an Overridden Method from a Parent-Class Constructor
State of Derived class object when Base class constructor calls overridden method in Java
Using abstract init() function in abstract class’s constructor
See also
FindBugs - Uninitialized read of field method called from constructor of superclass
On object construction with many parameters
Constructors with many parameters can lead to poor readability, and better alternatives exist.
Here's a quote from Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters:
Traditionally, programmers have used the telescoping constructor pattern, in which you provide a constructor with only the required parameters, another with a single optional parameters, a third with two optional parameters, and so on...
The telescoping constructor pattern is essentially something like this:
public class Telescope {
final String name;
final int levels;
final boolean isAdjustable;
public Telescope(String name) {
this(name, 5);
}
public Telescope(String name, int levels) {
this(name, levels, false);
}
public Telescope(String name, int levels, boolean isAdjustable) {
this.name = name;
this.levels = levels;
this.isAdjustable = isAdjustable;
}
}
And now you can do any of the following:
new Telescope("X/1999");
new Telescope("X/1999", 13);
new Telescope("X/1999", 13, true);
You can't, however, currently set only the name and isAdjustable, and leaving levels at default. You can provide more constructor overloads, but obviously the number would explode as the number of parameters grow, and you may even have multiple boolean and int arguments, which would really make a mess out of things.
As you can see, this isn't a pleasant pattern to write, and even less pleasant to use (What does "true" mean here? What's 13?).
Bloch recommends using a builder pattern, which would allow you to write something like this instead:
Telescope telly = new Telescope.Builder("X/1999").setAdjustable(true).build();
Note that now the parameters are named, and you can set them in any order you want, and you can skip the ones that you want to keep at default values. This is certainly much better than telescoping constructors, especially when there's a huge number of parameters that belong to many of the same types.
See also
Wikipedia/Builder pattern
Effective Java 2nd Edition, Item 2: Consider a builder pattern when faced with many constructor parameters (excerpt online)
Related questions
When would you use the Builder Pattern?
Is this a well known design pattern? What is its name?

Here's an example which helps to understand this:
public class Main {
static abstract class A {
abstract void foo();
A() {
System.out.println("Constructing A");
foo();
}
}
static class C extends A {
C() {
System.out.println("Constructing C");
}
void foo() {
System.out.println("Using C");
}
}
public static void main(String[] args) {
C c = new C();
}
}
If you run this code, you get the following output:
Constructing A
Using C
Constructing C
You see? foo() makes use of C before C's constructor has been run. If foo() requires C to have a defined state (i.e. the constructor has finished), then it will encounter an undefined state in C and things might break. And since you can't know in A what the overwritten foo() expects, you get a warning.

Invoking an overridable method in the constructor allows subclasses to subvert the code, so you can't guarantee that it works anymore. That's why you get a warning.
In your example, what happens if a subclass overrides getTitle() and returns null ?
To "fix" this, you can use a factory method instead of a constructor, it's a common pattern of objects instanciation.

Here is an example that reveals the logical problems that can occur when calling an overridable method in the super constructor.
class A {
protected int minWeeklySalary;
protected int maxWeeklySalary;
protected static final int MIN = 1000;
protected static final int MAX = 2000;
public A() {
setSalaryRange();
}
protected void setSalaryRange() {
throw new RuntimeException("not implemented");
}
public void pr() {
System.out.println("minWeeklySalary: " + minWeeklySalary);
System.out.println("maxWeeklySalary: " + maxWeeklySalary);
}
}
class B extends A {
private int factor = 1;
public B(int _factor) {
this.factor = _factor;
}
#Override
protected void setSalaryRange() {
this.minWeeklySalary = MIN * this.factor;
this.maxWeeklySalary = MAX * this.factor;
}
}
public static void main(String[] args) {
B b = new B(2);
b.pr();
}
The result would actually be:
minWeeklySalary: 0
maxWeeklySalary: 0
This is because the constructor of class B first calls the constructor of class A, where the overridable method inside B gets executed. But inside the method we are using the instance variable factor which has not yet been initialized (because the constructor of A has not yet finished), thus factor is 0 and not 1 and definitely not 2 (the thing that the programmer might think it will be). Imagine how hard would be to track an error if the calculation logic was ten times more twisted.
I hope that would help someone.

If you call methods in your constructor that subclasses override, it means you are less likely to be referencing variables that don’t exist yet if you divide your initialization logically between the constructor and the method.
Have a look on this sample link http://www.javapractices.com/topic/TopicAction.do?Id=215

I certainly agree that there are cases where it is better not to call some methods from a constructor.
Making them private takes away all doubt: "You shall not pass".
However, what if you DO want to keep things open.
It's not just the access modifier that is the real problem, as I tried to explain here. To be completely honest, private is a clear showstopper where protected usually will still allow a (harmful) workaround.
A more general advice:
don't start threads from your constructor
don't read files from your constructor
don't call APIs or services from your constructor
don't load data from a database from your constructor
don't parse json or xml documents from your constructor
Don't do so (in)directly from your constructor. That includes doing any of these actions from a private/protected function which is called by the constructor.
Calling an start() method from your constructor could certainly be a red flag.
Instead, you should provide a public init(), start() or connect() method. And leave the responsibility to the consumer.
Simply put, you want to separate the moment of "preparation" from the "ignition".
if a constructor can be extended then it shouldn't self-ignite.
If it self-ignites then it risks being launched before being fully constructed.
After all, some day more preparation could be added in the constructor of a subclass. And you don't have any control over the order of execution of the constructor of a super class.
PS: consider implementing the Closeable interface along with it.

In the specific case of Wicket: This is the very reason why I asked the Wicket
devs to add support for an explicit two phase component initialization process in the framework's lifecycle of constructing a component i.e.
Construction - via constructor
Initialization - via onInitilize (after construction when virtual methods work!)
There was quite an active debate about whether it was necessary or not (it fully is necessary IMHO) as this link demonstrates http://apache-wicket.1842946.n4.nabble.com/VOTE-WICKET-3218-Component-onInitialize-is-broken-for-Pages-td3341090i20.html)
The good news is that the excellent devs at Wicket did end up introducing two phase initialization (to make the most aweseome Java UI framework even more awesome!) so with Wicket you can do all your post construction initialization in the onInitialize method that is called by the framework automatically if you override it - at this point in the lifecycle of your component its constructor has completed its work so virtual methods work as expected.

I guess for Wicket it's better to call add method in the onInitialize() (see components lifecycle) :
public abstract class BasicPage extends WebPage {
public BasicPage() {
}
#Override
public void onInitialize() {
add(new Label("title", getTitle()));
}
protected abstract String getTitle();
}

Questions on classes extending from a base class (Java)

I'm a beginner in Java trying to write a system of party quests for a game that I'm currently writing and I have a few questions I'd like to be answered. I've already gone around and asked other people, but they're not familiar in Java.
In the past I'd tried making a bunch of classes and accessing them with multiple get methods. I found that incredibly tedious to write and thought I could unify them under an abstract class/implemented class. Thus, the code looked more like this ...
DynastyPQInterface pq = new CustomPQ // or ....
DynastyPQInterface pq = new OtherCustomPQ
Of course, this presented difficulties such as being only able to use implemented methods. It didn't allow me to access the class' exclusive methods that I might want to use later on.
Ultimately, what I want to do is to be able to use a single get method to return any of these derived classes, but still retain the ability to just universally use the get method to call methods that they have in common, such as execute, create, end, while simultaneously allowing me to reach out to their exclusive methods specifically. Is there a way to do that, or is it impossible?
If it's still not clear ...
The code I have write now is a base class that is extended to the other classes in the manner ...
DynastyPQ (base) -> methods include (run(), execute(), end())
CustomAPQ (inherited from DynastyPQ) -> (has exclusive methods like getPoints())
CustomBPQ (inherited from DynastyPQ) -> (has exclusive methods like revivePlayer())
I want to write a get method so to rid myself of multiple. What I have right now is ...
DynastyPQ dynastyPQ;
DynastyPQ getPQ() {
return dynastyPQ;
}
void setPQ(DynastyPQ pq) {
dynastyPQ = pq;
}
Doing this ...
DynastyPQ pq = new CarnivalPQ();
I can only access DynastyPQ's methods rather than Carnival's methods.
Is there a way to access the exclusive methods while universally being able to execute the four base functions without regard to the type of class, or did I miss something earlier?
tl;dr -> I want one get method that universally returns all classes that inherit from class X; however, I want to be able to access each class's exclusive methods.

You can probably just cast the object to the derived class:
DynastyPQ pq = new CustomAPQ();
((CustomAPQ)pq).customAPQmethod();
If you don't know what is the dynamic type (the type you used after the new operator), you can use the instanceof keyword:
DynastyPQ pq = getPQ();
if (pq instanceof CustomAPQ) {
CustomAPQ a = (CustomAPQ)pq;
a.customAPQmethod();
} else if (pq instanceof CustomBPQ) {
CustomBPQ b = (CustomBPQ)pq;
b.customBPQmethod();
} else {
// Neither a CustomAPQ nor a CustomBPQ.
}
If you don't want to do that, you can use polymorphism:
class DynastyPQ {
final void run() {
// code.
}
final void execute() {
// code.
}
final void create() {
// code.
}
void specific1() {}
void specific2() {}
}
class CustomAPQ extends DynastyPQ {
#Override
void specific1() {
// do stuff specific to CustomAPQ.
}
#Override
void specific2() {
// do stuff specific to CustomAPQ.
}
}
class CustomBPQ extends DynastyPQ {
#Override
void specific1() {
// do stuff specific to CustomBPQ.
}
#Override
void specific2() {
// do stuff specific to CustomBPQ.
}
}
Now, you can do:
DynastyPQ pq = new CustomAPQ();
pq.specific1();
The called method will be CustomAPQ::specific1(). If specific1() was not declared in CustomAPQ, then, it will just do nothing.

Other than #CelineNOEL suggested it is not possible. Because you declared a class of type DynastyPQ, you can call only methods defined inside that class. In the moment you want to call specific method, not shared one, you know from which class it is and you can use casting to call that specific method.
((CustomAPQ)pq).customAPQmethod()
Shared methods you are using in code, when you don't know which class should execute same peace of code(or you want it to execute it different if you override share methods in every sub-class), and you delegate it to be resolved in runtime. So reconsider your design and in base class put methods that needs to be called dynamically. All other methods you are sure are specific for one class put only in that class. On that way your code will be cleaner and you will not mess thing that should be separated.

is this possible in java or any other programming language

public abstract class Master
{
public void printForAllMethodsInSubClass()
{
System.out.println ("Printing before subclass method executes");
System.out.println ("Parameters for subclass method were: ....");
}
}
public class Owner extends Master {
public void printSomething () {
System.out.println ("This printed from Owner");
}
public int returnSomeCals ()
{
return 5+5;
}
}
Without messing with methods of subclass...is it possible to execute printForAllMethodsInSubClass() before the method of a subclass gets executed?
update:
Using AspectJ/Ruby/Python...etc
Would it also be possible to print the parameters? Above code formatted below:
public abstract class Master
{
public void printForAllMethodsInSubClass()
{
System.out.println ("Printing before subclass method executes");
}
}
public class Owner extends Master {
public void printSomething (String something) {
System.out.println (something + " printed from Owner");
}
public int returnSomeCals (int x, int y)
{
return x+y;
}
}

AspectJ can provide this functionality for you, but it's a separate compilation step and some extra libraries involved.
public aspect ServerLogger {
pointcut printSomething ();
before(): printSomething()
{
(Master)(thisJoinPointStaticPart.getTarget()).printForAlMethodsInSubClass();
}
}
The Eclipse Project provides a great implementation of AspectJ that integrates nicely with Eclipse and Maven. There's a boatload of great documentation available for it, and a lot of really good material for it here on StackOverflow.
[update]
To access parameter info, you can use the
thisJoinPoint.getSignature();
method to access information about the function being called if the returned Object is an instance of MethodSignature, you can use Signature.getParameterNames() to access the parameters to the function being called. You'd have to use a bit of reflection to actually get at the values, I think - AspectJ doesn't seem to handle this for you. I'd have to actually do some experimentation to get some working code for you.

To answer the "any other programming language": It's easily possible in Ruby:
class Master
REDEFINED = []
def printForAllMethodsInSubClass
puts 'Printing before subclass method executes'
end
def self.method_added(meth)
if self < Master and not Master::REDEFINED.include? meth
new_name = "MASTER_OVERRIDE_#{meth}".intern
Master::REDEFINED.push meth, new_name
alias_method new_name, meth
define_method(meth) {|*args| printForAllMethodsInSubClass; send(new_name, *args)}
end
end
end
You could also make a proxy declaration method to use in subclasses:
class Master
def printForAllMethodsInSubClass
Printing before subclass method executes
end
def self.master_method(name)
define_method(name) {|*args| printForAllMethodsInSubClass; yield *args}
end
end
class Owner
master_method(:print_something) do
puts "This was printed from Owner"
end
end
(This approach would also translate very naturally to Python decorators.)

This is possible in aspect-oriented programming languages, such as AspectJ.

In Python you can accomplish this using meta classes, here's a small example. You can probably make it more elegantly but it is just to make the point
import types
class PrintMetaClass(type):
def __init__(cls, name, bases, attrs):
# for every member in the class
override = {}
for attr in attrs:
member = attrs[attr]
# if it's a function
if type(member) == types.FunctionType:
# we wrap it
def wrapped(*args, **kwargs):
print 'before any method'
return member(*args, **kwargs)
override[attr] = wrapped
super(PrintMetaClass, cls).__init__(name, bases, attrs)
for attr in override:
setattr(cls, attr, override[attr])
class Foo:
__metaclass__ = PrintMetaClass
def do_something(self):
print 2
class Bar(Foo):
def do_something_else(self):
print 3
In this example, the PrintMetaClass gets in the way of the creation of the Foo class and any of its subclasses redefining every method to be a wrapper of the original and printing a given message at the beginning. The Bar class receives this aspect-like behavior simply by inheriting from Foo which defines its __metaclass__ to be PrintMetaClass.
Metaclasess in OOP:
http://en.wikipedia.org/wiki/Metaclass
Metaclasses in python:
http://www.python.org/doc/essays/metaclasses/
http://www.ibm.com/developerworks/linux/library/l-pymeta.html

Besides aspect oriented programming have a look at Template Method Pattern, http://en.wikipedia.org/wiki/Template_method_pattern.
In short: the parent class have an abstract method, which subclasses have to implement, this abstract method is called by a method in the parent class where put your printouts or whatever necessary statements.