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Why is subclass field initialized to its default value within super constructor? [duplicate]

I ran into an interesting problem yesterday and while the fix was quite simple, I'm still a bit fuzzy on the "why" of it.
I have a class that has a private member variable that is assigned when it is instantiated, however if it is used in an abstract function that is called by the super class's constructor, the variable does not have a value. The solution to the problem was quite simple, I simply had to declare the variable as static and it was assigned correctly. Some code to illustrate the problem:
class Foo extends BaseClass
{
private final String bar = "fooBar!";
public Foo()
{
super();
}
#Override
public void initialize()
{
System.out.println(bar);
}
}
And the base class:
abstract class BaseClass
{
public BaseClass()
{
initialize();
}
public abstract void initialize();
}
In this example, when we call new Foo(); it will output (null) instead of the expected fooBar!
Since we're instantiated an object of type Foo, should its members not be allocated and assigned prior to calling its (and consequently its super class's) constructor? Is this specified somewhere in the Java language or is it JVM specific?
Thanks for any insight!

The assignment of bar = "fooBar!"; is inlined into the constructor during compile time.
The superclass constructor runs before the subclass constructor, hence it would only be natural that the statement is executed afterwards.
Generally though, it's bad practice to call overridable methods from a constructor.

It is as defined by the Java Language Specification. Changing it to static will almost never be and acceptable solution in real world situation.
See JLS 4.12.5 Initial Values of Variablesand JLS 8.3.2 Initialization of Fields
Overall, it is bad practice to call a non-final method from a constructor. the reason being that it could (and if the method is abstract then definitely does) call method in the class that has not yet been initialized: When new Foo() is executed, the BaseClass initializer (constructor) gets called before the Foo constructor, so Foo.initialize is essentially working on an Object that has not been fully constructed yet.

There's just one thing I would like to add to the accepted answer, because I don't entirely agree with his conclusion.
We've all done this.
class Engine {
public Engine() {
init();
}
void init() {
lockDoors();
releasePressure();
tightenSeatbelts();
launchRocket();
}
...
}
Now the question is, which access modifier should we add to our init() function. Should it be private or protected.
make it private <-- keeps subclasses out
make it protected <-- allows subclasses in
Before you make a choice
Now first of all, you should realize that (almost) all code in the Engine class can be replaced by a subclass.
code in a public function, can easily be overridden
code in a protected function, can easily be overridden
code in a private function, can be replaced by overriding all methods that call it.
Well, there is just one exception:
you can never modify the code of a constructor
you can never avoid a private method being called from the constructor of a super class.
(and of course, you cannot replace a final method)
Protected init() - the wrong way
Let's say the init() method is protected there is indeed a pitfall. It is tempting to override it to add features as follows. That would indeed be a mistake.
class SubEngine extends Engine {
int screws = 5;
void init() {
tightenScrews();
super.init();
}
void tightenScrews() {
// this won't print 5, but it will print 0.
System.out.println("tightening " + screws + " screws");
}
}
Protected init() - the right way
So, basically, you should just disable the parents code and postpone execution to your own constructor instead.
class SubEngine extends Engine {
int screws = 5;
public SubEngine() {
initSubEngine();
}
void init() {
// disable parent code
}
void initSubEngine() {
tightenScrews();
super.init();
}
void tightenScrews() {
// this will print 5 as expected
System.out.println("tightening " + screws + " screws");
}
}
Private init() - you may need a phonecall
Now, what if the init() method is private ?
Like mentioned above, there is no way to disable the code of a parent constructor. And if init() is private you simply cannot disable it.
You'll end up copying the entire Engine class, perhaps just to add 1 line of code.
And that may not be the end of it. Even after copying your class, your copied object won't be an Engine meaning that you won't be able to use your EngineUtil#inspectEngine(Engine engine) function.
Perhaps somebody knew this in advance and made an IEngine interface. Then you can get away with it.
In practice it means you'll have to take your phone, and call to that other department that made the Engine class, and ask them to change their code a little to take away some restrictions.
Intelligent design
There is another way. Constructors are for setting variables. They shouldn't activate anything. Everytime you see a class creating a new Thread from their constructor (or through a private method) that should be a red flag.
class Engine {
public Engine() {
}
public void init() {
lockDoors();
releasePressure();
tightenSeatbelts();
launchRocket();
}
// and you probably also want one of these
public void shutdown() { ... }
...
}
Intention
Of course, your intention may very well be not to open up your code. Perhaps you really don't want to allow others to extend your classes. There certainly can be cases where you want to lock people out.
Be aware that it will also make it harder to write tests for your code.
Anyway that's a different scenario.

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();
}

Why are member objects initialized after the super class's constructor?

I ran into an interesting problem yesterday and while the fix was quite simple, I'm still a bit fuzzy on the "why" of it.
I have a class that has a private member variable that is assigned when it is instantiated, however if it is used in an abstract function that is called by the super class's constructor, the variable does not have a value. The solution to the problem was quite simple, I simply had to declare the variable as static and it was assigned correctly. Some code to illustrate the problem:
class Foo extends BaseClass
{
private final String bar = "fooBar!";
public Foo()
{
super();
}
#Override
public void initialize()
{
System.out.println(bar);
}
}
And the base class:
abstract class BaseClass
{
public BaseClass()
{
initialize();
}
public abstract void initialize();
}
In this example, when we call new Foo(); it will output (null) instead of the expected fooBar!
Since we're instantiated an object of type Foo, should its members not be allocated and assigned prior to calling its (and consequently its super class's) constructor? Is this specified somewhere in the Java language or is it JVM specific?
Thanks for any insight!

The assignment of bar = "fooBar!"; is inlined into the constructor during compile time.
The superclass constructor runs before the subclass constructor, hence it would only be natural that the statement is executed afterwards.
Generally though, it's bad practice to call overridable methods from a constructor.

It is as defined by the Java Language Specification. Changing it to static will almost never be and acceptable solution in real world situation.
See JLS 4.12.5 Initial Values of Variablesand JLS 8.3.2 Initialization of Fields
Overall, it is bad practice to call a non-final method from a constructor. the reason being that it could (and if the method is abstract then definitely does) call method in the class that has not yet been initialized: When new Foo() is executed, the BaseClass initializer (constructor) gets called before the Foo constructor, so Foo.initialize is essentially working on an Object that has not been fully constructed yet.

There's just one thing I would like to add to the accepted answer, because I don't entirely agree with his conclusion.
We've all done this.
class Engine {
public Engine() {
init();
}
void init() {
lockDoors();
releasePressure();
tightenSeatbelts();
launchRocket();
}
...
}
Now the question is, which access modifier should we add to our init() function. Should it be private or protected.
make it private <-- keeps subclasses out
make it protected <-- allows subclasses in
Before you make a choice
Now first of all, you should realize that (almost) all code in the Engine class can be replaced by a subclass.
code in a public function, can easily be overridden
code in a protected function, can easily be overridden
code in a private function, can be replaced by overriding all methods that call it.
Well, there is just one exception:
you can never modify the code of a constructor
you can never avoid a private method being called from the constructor of a super class.
(and of course, you cannot replace a final method)
Protected init() - the wrong way
Let's say the init() method is protected there is indeed a pitfall. It is tempting to override it to add features as follows. That would indeed be a mistake.
class SubEngine extends Engine {
int screws = 5;
void init() {
tightenScrews();
super.init();
}
void tightenScrews() {
// this won't print 5, but it will print 0.
System.out.println("tightening " + screws + " screws");
}
}
Protected init() - the right way
So, basically, you should just disable the parents code and postpone execution to your own constructor instead.
class SubEngine extends Engine {
int screws = 5;
public SubEngine() {
initSubEngine();
}
void init() {
// disable parent code
}
void initSubEngine() {
tightenScrews();
super.init();
}
void tightenScrews() {
// this will print 5 as expected
System.out.println("tightening " + screws + " screws");
}
}
Private init() - you may need a phonecall
Now, what if the init() method is private ?
Like mentioned above, there is no way to disable the code of a parent constructor. And if init() is private you simply cannot disable it.
You'll end up copying the entire Engine class, perhaps just to add 1 line of code.
And that may not be the end of it. Even after copying your class, your copied object won't be an Engine meaning that you won't be able to use your EngineUtil#inspectEngine(Engine engine) function.
Perhaps somebody knew this in advance and made an IEngine interface. Then you can get away with it.
In practice it means you'll have to take your phone, and call to that other department that made the Engine class, and ask them to change their code a little to take away some restrictions.
Intelligent design
There is another way. Constructors are for setting variables. They shouldn't activate anything. Everytime you see a class creating a new Thread from their constructor (or through a private method) that should be a red flag.
class Engine {
public Engine() {
}
public void init() {
lockDoors();
releasePressure();
tightenSeatbelts();
launchRocket();
}
// and you probably also want one of these
public void shutdown() { ... }
...
}
Intention
Of course, your intention may very well be not to open up your code. Perhaps you really don't want to allow others to extend your classes. There certainly can be cases where you want to lock people out.
Be aware that it will also make it harder to write tests for your code.
Anyway that's a different scenario.

Java passing subclass instance data to superclass constructors

Does anybody know if there's a way in Java to set the value of an instance variable in a subclass before calling the superclass constructor. I have a brief schematic below of what I'm trying to accomplish -- I need to set up the instance variables defined in the superclass differently depending on the subclass type, but I still want to be able to share common non-constructor code among different instances of the subclass.
Is there any clean way to do this, maybe some sort of coding pattern that I'm missing or something? Thanks in advance for any ideas.
public abstract class ConstraintSatisfactionProblem {
final Set<Variable> variables;
final Set<Constraint> constraints;
public Foo() {
this.variables = setupVariables();
this.constraints = setupConstraints();
}
public abstract Set<Variable> setupVariables();
public abstract Set<Constraint> setupConstraints();
public Map<Variable, Constraint> solve() { ... }
}
public class WordSquare extends ConstraintSatisfactionProblem {
final int size;
final static Set<Character> domain = ...;
public WordSquare() {
super(); // can I simulate calling super() after setting this.value = 4?
this.value = 4;
}
public Set<Variable> setupVariables() {
this.variables = new HashSet<Variable>();
for(int row = 0; row < size; ++row) {
for(int col = 0; col < size; ++col) {
variables.add(new Variable<Pair, Character>(new Pair(row, col), domain);
}
}
return this.variables;
}
public Set<Constraint> setupConstraints() {
// setup code specific to this problem
}
}
public class Cryptarithmetic extends ConstraintSatisfactionProblem {
final String problem;
public Cryptarithmetic(String problem) {
super();
this.problem = problem;
}
public Set<Variable> setupVariables() {
this.variables = new HashSet<Variable>();
for(char c : problem.toCharArray()) {
variables.add(new Variable<Character, Integer>(c, getDomain());
}
}
return this.variables;
}
public Set<Constraint> setupConstraints() {
// setup code specific to this problem
}
}

Firstly, please don't.
Secondly, really it's a really bad idea. Don't. Think about what you are trying to do in a broader context.
If you absolutely must do, you can stash it in a ThreadLocal. You can call a (non-instance) method by evaluating an expression the result of which is passed to a super() or this() (possibly the only reason why you need a second, private constructor that possibly takes a Void (capital 'V') argument). It's so evil, I am not going to even write the code down.
In your edited example code, just pass the sets into a protected constructor. If you have many arguments possibly some subclasses being special about some arguments, you might want to wrap all the arguments into a single argument object.
There is another really hacky approach, so long as you have -target 1.4 or later (which you should do!). Make the subclass an inner class (possibly anonymous). The references to the outer this and other captured variables are available before calling the super constructor.
public class Outer {
// What a hack!
private static abstract class Base {
Base() {
hello(); // Calling a virtual method in a constructor - poor form.
}
abstract void hello();
}
public static void main(String[] args) {
// Do not do this.
final String hi = "Hi!";
new Base() {
void hello() {
// Really, don't do it.
System.err.println(hi);
}
};
}
}

Place the common code you want to run in a protected method instead of in the constructor. Call that method when you wish.

You should never call any "alien" method (ie. overridable method of this class, or any method from any other class) form within a constructor. As long as the object is not fully initialized, you may have side-effects like the one you see.
In your case, in the subclass constructor, super() is called even before the "value" is set to 4. This means, the superclass constructor is called, then calls the "setup" method, while the "value" is still at 0.
Only once the superclass constructor returns, the "value" is set to 4. And it's too late then.
What I would recommend, is to set the "o1" variable to protected, so that subclasses can set its value themselves.

In Java, if you want to call a base class's constructor, you have to do it on the first line of your sub-class's constructor. So the answer is no, you can't set this.value before calling the super class's constructor.
But your sub-class's setup() method is already called in the super's constructor. Why don't you set your value there?
UPDATE:
Sorry, I didn't pay attention that your 'setup()' method returns a value. What you could do is make an abstract init() method in your super class, and call it in your super constructor before you call the setup() method. This way sub-classes will be forced to implement init(), and you would know that that is the place to initialize any sub-class's members before they are used in your super-class.
That being said, this approach does not force safety on you. When you call the super constructor from your sub-constructor, the sub-class instance is just starting to get created. It still needs to run the rest of the code in the sub-constructor before the object is safely created.
And in this situation, the super-constructor goes and calls the init() method on your just-in-the-process-of-creation sub-class. This means that if you go with approach, you have to be extra careful about what you do in the init() class.

Like others have said, don't do this. If you want to share some code between these classes, try containment/encapsulation instead of inheritance.
public class Foo {
private final Object o1;
public Foo(Object o) {
o1 = o;
}
public void complexMethodCommonToAllSubclassesOfFoo() { ... }
}
public class Bar {
private final int value;
private final Foo foo;
public Bar() {
super();
this.value = 4;
this.foo = new Foo( new Object() ); // or whatever
}
// If you need to expose complexMethodCommonToAllSubclassesOfFoo to clients of this class, just add the method and delegate to foo like this
public void complexMethodCommonToAllSubclassesOfFoo() {
foo.complexMethodCommonToAllSubclassesOfFoo();
}
}

I need to set up the instance variables defined in the superclass differently depending on the subclass type, but I still want to be able to share common non-constructor code among different instances of the subclass.
In that case, create a protected constructor in the superclass, and pass all of the customized values to it when you construct the subclass.