Require default constructor

Although I doubt there is, I would like to know if there is a way to require subclasses to implement the default constructor. I am retrieving String class names from a data file and creating objects with the combination of Class.forName(...) and newInstance() (also from Class class). I just wanted to create them without having to deal with various combinations of constructors.

CharanZ wrote:
Java Creates a default constructor for every class you create, Unless you override it.Except that a) C'tors are not inherited and thus cannot be overridden, and b) If you explicitly define any c'tor, then the compiler no longer provides the default.
So no need to worry about it.Except that there's no way to do what the OP asks, except, as already pointed out, by documentation and trusting the subclass author to follow it.

Similar Messages

Always create any Object even without default constructor?

hi!
i'm wondering if there's an easy way to construct an object of a given class?
let's assume i've got a class
class MyClass
public MyClass( OtherClass c)
}now when i get the constructors for its Class-object with
clazz.getDeclaredConstructor()
it will return null;
ok now i know ther's no default/empty constructor.
does it make sense to then search for the next constructor with n parameters?
and then invoke it with nonsense values?
what happens if the parameter classes also have no default constructor to create parameter objects? like this:
class OtherClass
public OtherClass ( MyClass c)
}now there's a loop between the two constructors and i can't create empty parameter values to invoke any of them!
i wonder how the serialization engine works then?
or did i miss something trivial (it's really late here ;) )

What's the problem? To create a MyClass object, you
don't need any OtherClass objects. You just do the
equivalent ofnew MyClass(null)in your
reflective code.
slap forehead with toilet
thanks for the tip.
as i said there was something absolutely easy, it's tough not seeing the forest for the trees ;/
But I agree with jschell, if your requirements are to
create objects of arbitrary classes and call
arbitrary methods with arbitrary parameter lists,
then whoever did your design didn't think for long
enough.WELL. you're absolutely right. but when it's time to process ANY given object there's no reliability on whatever design was chosen. or, would you insist the developer had to provide standatized formats when you write a debugger? no it has to work with any. so must i!

How can i set a breakpoint on a default constructor

I'd like to catch all instances where a specific class is constructed but only the default constructor exists and it is implicitly defined. How can I break in the default constructor?
thanks,

There is no code where you want a breakpoint, so I'm not sure what instruction you'd like the debugger to break your program at. ???
Since your goal is only to "catch" all instances where a specific class is constructed I think the only thing is to modify your code to do so. I can't think of an approach that doesn't require a code modification and recompile. Especially
since it's not clear where such classes were inlined in already compiled code. And because of that alone, I'm
fairly certain you have to instrument your code (modify your code by inserting something special) to catch it.
So can you modify the class you want to catch? Just add a default constructor and set your breakpoint in there.
Alternatively if you really want to try to do what you are doing, you can step into any code that creates an instance of your class in disassembly and set a breakpoint in the disassembled code for the constructor (assuming you can locate it, and it even
exists), then rerun your program.

How to implement public default constructor for type

i copy the servicestack dll from old project for redis 2.6, without using nuget
i add new() but can not compile
without new(), it has error , requires public default constructor
type LogicDataBase =
{ mutable BitKeyFunctionValue1 : Dictionary<String, Dictionary<expr, expr>>;
mutable PathList1 : Dictionary<expr list, expr list>
mutable AllImplicationKeyList1 : expr list
new() = LogicDataBase(BitKeyFunctionValue1, PathList1, AllImplicationKeyList1)
let redisClient1 = new RedisClient("localhost")
let d1 = {
BitKeyFunctionValue1 = BitKeyFunctionValue
AllImplicationKeyList1 = AllImplicationKeyList
PathList1 = PathList
redisClient1.Store<LogicDataBase>(d1)
redisClient1.Save()
let redisClient2 = redisClient1.As<LogicDataBase>()
let rediskeys : List<String> = redisClient2.GetAllKeys()
for kk3 in rediskeys do
let d2 : LogicDataBase = redisClient2.[kk3]
BitKeyFunctionValue <- d2.BitKeyFunctionValue1
AllImplicationKeyList <- d2.AllImplicationKeyList1
PathList <- d2.PathList1
let debugBitKeyFunctionValue2 = BitKeyFunctionValue
let debugAllImplicationKeyList2 = AllImplicationKeyList
let debugPathList2 = PathList
computing nightmare

is there an example about this for this case?
i see that type is also a class, where is wrong?
type LogicDataBase =
{ mutable BitKeyFunctionValue1 : Dictionary<String, Dictionary<expr, expr>>;
mutable PathList1 : Dictionary<expr list, expr list>
mutable AllImplicationKeyList1 : expr list
new() = LogicDataBase(BitKeyFunctionValue1, PathList1, AllImplicationKeyList1)
computing nightmare

De-serialization not calling default constructor ??

Hi,
I have a strange problem with serialization (de-serialization, actually):
I have a bunch of classes that represent the model for my application.
These classes are organized as a complex tree and come in three flavors:
public abstract class AbstractIOCLeaf implements IOCElement {
     private String name;
     private transient boolean changed = false;
     private transient LinkedList<ChangeListener> changeListeners;
     protected AbstractIOCLeaf() {
          super();
          name = null;
          changed = false;
          changeListeners = new LinkedList<ChangeListener>();
     } //needed for Serialzation
     protected AbstractIOCLeaf(String name) {
          this();
          this.name = name;
...this class is a leaf: it cannot contain other sub-elements.
public abstract class AbstractIOCList<T extends IOCElement> extends AbstractIOCNode implements ComboBoxModel {
     protected LinkedList<T> list = null;
     protected transient List<ListDataListener> listListeners;
     protected abstract T newElement(String name);
     protected AbstractIOCList() { super(); listListeners = new LinkedList<ListDataListener>(); }
     public AbstractIOCList(String name) {
          super(name);
          list = new LinkedList<T>();
          listListeners = new LinkedList<ListDataListener>();
... This class holds a list of elements that are all equal.
and finally:
public abstract class AbstractIOCNode extends AbstractIOCLeaf implements ChangeListener, ListDataListener {
     protected AbstractIOCNode() { super(); }
     protected AbstractIOCNode(String name) {
          super(name);
... This class holds elements that are all different.
The actual classes extends one of these following the pattern:
public class StateMachine extends AbstractIOCNode {
     private StateList states = null;;
     private EventQueue fEventQueue = null;
     private StateMachine() { super(); }
     private StateMachine(String name) {
          super(name);
          states = StateList.newInstance(this);
          changed = false;
     public static StateMachine newInstance(String name) {
          StateMachine sm = new StateMachine(name);
          sm.initialize();
          return sm;
public class StateList extends AbstractIOCList<State> {
     private StateMachine sm;
     private StateList() { super("StateList"); sm = null; }
     private StateList(StateMachine sm) {
          this();
          this.sm = sm;
     public static StateList newInstance(StateMachine sm) {
          StateList list = new StateList(sm);
          list.initialize();
          return list;
...etc. etc.
I do serialization calling ObjectOutputStream.writeObject on the root object and (obviously) deserialize using ObjectOutputStream.readObject.
The process works, but it seems that the default constructors in particular AbstractIOCLeaf() is never called while deserializing. First hint to something amiss was the fact that I always had the transient field changeListeners remaining in its default null state.
Further investigation involving debugging and breakpointing confirmed no default constructor is called in spite of the super(); calls.
What am I doing wrong??
Did I miss something about serialization (apparently so, but I cannot understand what!)?
Side issue:
I tried substituting ObjectOutputStream.writeObject with XMLEncoder.writeObject, but then I get the error: "Class sun.reflect.misc.Trampoline can not access a member of class com.softin.IOCbuilder.model.IOController with modifiers "private"".
Aren't those classes supposed to be equivalent?
Is there any (fast) way to desume the offending member?
Excuse me for the length of the post and
Thanks in Advance
Mauro

Oops, nevermind. Sorry.

Creating an instance of a class with no default constructor

Hello gurus,
I wrote my own serialization and RMI protocol for both C++ and Java that follows closely what the default Java version does. I'm trying to recreate an object on the Java side that was sent over the wire. The first step is to create an instance of the class. How do I create an instance of a class that has no constructor (i.e. the only instances are static, created by the class itself and returned by static methods) or one that has no default constructor (like Integer)? The Java serialization seems to support it but the reflection API doesn't seem to have any support for this (i.e. Class::newInstance() and Constructor::newInstance()). It seems that through the standard API you can only create an object via one of its constructors. There must be a "hidden" method somewhere that allows the Java serialization to create an object without calling a constructor - where is it?
Dominique

There must be a "hidden" method
somewhere that allows the Java serialization to create
an object without calling a constructor - where is
it?You are correct, the way in which the Serialization creates Objects is "hidden" deep within the runtime.
If it were not hidden, you would be able to find it, and use it to violate the integrity of the VM.

ORA-02315: incorrect number of arguments for default constructor

I was able to register the XML schema successfully by letting Oracle creating the XML Types. Then when I try to execute the create view command the ORA-02315: incorrect number of arguments for default constructor is always raised.
I tried using the XMLTYPE.createXML but it gives me the same error.
Command:
CREATE OR REPLACE VIEW samples_xml OF XMLTYPE
XMLSCHEMA "http://localhost/samplepeak4.xsd" ELEMENT "SAMPLE"
WITH OBJECT ID (ExtractValue(sys_nc_rowinfo$, '/SAMPLES/SAMPLE/SAMPLE_ID')) AS
SELECT sample_t(s.sample_id, s.patient_info, s.process_info, s.lims_sample_id,
cast (multiset(
SELECT peak_t(p.peak_id, p.mass_charge, p.intensity, p.retention_time,
p.cleavage_type, p.search_id, p.match_id, p.mass_observed,
p.mass_expected, p.delta, p.miss, p.rank, p.mass_calculated,
p.fraction)
FROM peak p
WHERE s.sample_id = p.sample_id) AS PEAK107_COLL))
FROM sample s;
Can someone help me.
Thanks
Carl

This example runs without any problems on 9.2.0.4.0. Which version are you running? And which statement causes the error message?

Selective member import/export using dllexport and default constructor

We can only export some methods in the class(https://msdn.microsoft.com/es-es/library/8d7d1303-b9e9-47ca-96cc-67bf444a08a9%28v=vs.100%29)
What if we export only some methods in the class and not exported the default constructor(Will it get generated by the compiler in external app which use this dll?).The class may have some private data member as well which are not exposed in the published
header file(Which we distribute with DLL).
How the memory gets allocated to those private data members which are not exposed as the default constructor is not exported?
Niranjan

We can only export some methods in the class(https://msdn.microsoft.com/es-es/library/8d7d1303-b9e9-47ca-96cc-67bf444a08a9%28v=vs.100%29)
What if we export only some methods in the class and not exported the default constructor(Will it get generated by the compiler in external app which use this dll?).The class may have some private data member as well which are not exposed in the published
header file(Which we distribute with DLL).
How the memory gets allocated to those private data members which are not exposed as the default constructor is not exported?
Niranjan

Failing with 'no-arg default constructor' on @XmlTransient

Hello,
I have a simple exception I want to serialize to xml like this:
public class Main {
@XmlRootElement
static public class SomeException extends RuntimeException {
private Integer someAdditionalInformation;
public SomeException() {
public SomeException(Integer someAdditionalInformation) {
this.someAdditionalInformation = someAdditionalInformation;
public Integer getSomeAdditionalInformation() {
return someAdditionalInformation;
public void setSomeAdditionalInformation(
Integer someAdditionalInformation) {
this.someAdditionalInformation = someAdditionalInformation;
@XmlTransient
@Override
public StackTraceElement[] getStackTrace() {
return super.getStackTrace();
public static void main(String[] args) {
try {
JAXBContext jaxbCtx = JAXBContext.newInstance(SomeException.class);
Marshaller m = jaxbCtx.createMarshaller();
m.marshal(new SomeException(5), System.out);
} catch (JAXBException ex) {
Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
But I get the following exception:
com.sun.xml.internal.bind.v2.runtime.IllegalAnnotationsException: 1 counts of IllegalAnnotationExceptions
java.lang.StackTraceElement does not have a no-arg default constructor.
this problem is related to the following location:
at java.lang.StackTraceElement
at public java.lang.StackTraceElement[] java.lang.Throwable.getStackTrace()
at java.lang.Throwable
at java.lang.Exception
at java.lang.RuntimeException
at jaxbtest.Main$SomeException
Does this seam like a JAXB bug ?
@XmlTransient should make JAXB just skip that property.
Is there ant way to workaround this ?
PS. Where did the JAXB team fly away ? jaxb.dev.java.net is blank

Looking at your log it is clear that, you are populating Xellerate Type=null. This is mandatory field and can't be null. However, when you create user using UI, the default value "End-Users" is being passed by default, because we have the corresponding field "Design Console" access check box at oim user profile.
Just map the constant value for trusted recon
Xellerate Type=End-Users
--nayan

No appropriate default constructor c++

Hi, have been trying to read up on it but I need some help to understand it, here's my code. What should I do to make it work and how should I try to think about it in the future?
cheers
Law.h
#include <iostream>
#include <string>
using namespace std;
class Law
private:
int k;
int x;
int F;
int m;
const int g;
void init();
public:
void Introduce();
int getMass();
int getLenght();
Law.ccp
#include "Law.h"
int Law::getMass()
return m;
int Law::getLenght()
return x;
void Law::Introduce()
cout << "\nWelcome to a very simplified Hooke's Law Calculator" << endl;
cout << "\nplease type in your mass: ";
main.ccp
#include <iostream>
#include <string>
#include "Law.h"
using namespace std;
int main()
Law hLaw;
hLaw.Introduce();
system("pause");
return 0;

On 1/11/2015 8:50 AM, RonnerBBQ wrote:
class Law
private:
const int g;
Law hLaw;
The variable hLaw needs to be initialized - with a default constructor in this case. Class Law doesn't have any user-defined constructors. Normally, a compiler would implicitly define a default constructor, equivalent to Law::Law() {} . But in
this case, it can't - Law has a const data member, which must be initialized in constructor (it being const, there won't be any other chance), and the compiler doesn't know what value to initialize it to.
You can 1) explicitly define a default constructor, and initialize g there, or 2) drop "const" from g (or drop g entirely, seeing as you aren't using it), or 3) give g an initializer, as in "const int g = 42;"
Igor Tandetnik

Super class default constructor

Hello,
I want to clear some confusion. I am studying for the exam. In this particular book an example shows that
Super class has 2 constructor
public abc() and public abc(int n)
Sub class has 2 constructor
public xyz() and public xyz(int n)
now when an instance is created for the subclass
xyz t = new xyz(1)
It will invoke the super class no argument constructor eventhough a default constructor exist in subclass?
Regards,
adil

Here are the rules for constructors--"ctors" because I'm lazy. Also, because I'm lazy, "super(...)" and "this(...)" mean any super or this call, regardless of how many args it takes, including those that take no args.
1) Every class has at least one ctor.
1.1) If you do not define an explicit constructor for your class, the compiler provides a implicit constructor that takes no args and simply calls super().
1.2) If you do define one or more explicit constructors, regardless of whether they take args, then the compiler no longer provides the implicit no-arg ctor. In this case, you must explicitly define a public MyClass() {...} if you want one.
1.3) Constructors are not inherited.
2) The first statement in the body of any ctor is either a call to a superclass ctor super(...) or a call to another ctor of this class this(...) 2.1) If you do not explicitly put a call to super(...) or this(...) as the first statement in a ctor that you define, then the compiler implicitly inserts a call to super's no-arg ctor super() as the first call. The implicitly called ctor is always super's no-arg ctor, regardless of whether the currently running ctor takes args.
2.2) There is always exactly one call to either super(...) or this(...) in each constructor, and it is always the first call. You can't put in more than one, and if you put one in, the compiler's implicitly provided one is removed.

What is the need for calling default constructor by JVM?

What is the need for calling default constructor by JVM? why the JVM should intiializes default values to the data
fields if the constructor is not there in our class?

mahar wrote:
What is the need for calling default constructor by JVM? Huh? The JVM does not need to call the default constructor. It needs to call a constructor.
You decide which one by the way you use "new".
why the JVM should initialize default values to the data fieldsHuh?
... if the constructor is not there in our class?Huh? The default constructor is always there. It may be private but it is still there.

Will 1.5 compiler tag default constructor as synthetic?

Hi,
This is a question about the synthetic attribute for the default constructor. This attribute has been added at the moment the inner classes have been introduced in the language. I think its purpose is to distinguish methods added by the compiler (like the access methods) to handle the inner classes.
Synthetic basically means something that has been added by the compiler and that has no corresponding source code.
Following this, the default constructor (added when the class has no constructor) should also be tagged with a synthetic attribute, shouldn't it?
Javac 1.4.2 doesn't add this attribute for the default constructor and I was wondering if the version 1.5 would do it.
Thanks,
Olivier

As far as I tested the jdk 1.5 compiler, I found that
it already tags default constructors with isSynthetic
attribute. Default constructors are compiler generated
and I am not sure how it affects the api, if they are
tagged isSynthetic.No, the 1.50 beta 31 compiler does not tag default constructors with Synthetic attributes.

Create POJO instance in fxml with no default constructor

My question is that how to create POJO instance in fxml that has no default constructor. I am creating pie chart data object in fxml like this
*<fx:define>*
*<PieChart.Data fx:id="data" >*
*<name>java</name>*
*<pieValue>20.2</pieValue>*
*</PieChart.Data>*
*</fx:define>*
since there is no default constructor how to create this object fxml?
Edited by: 988476 on Feb 16, 2013 6:21 AM

There must be a "hidden" method
somewhere that allows the Java serialization to create
an object without calling a constructor - where is
it?You are correct, the way in which the Serialization creates Objects is "hidden" deep within the runtime.
If it were not hidden, you would be able to find it, and use it to violate the integrity of the VM.

Dynamic classloading without default constructor

Just wondering if anyone has done dynamic classloading using a constructor other than the default. I know that newInstance() calls the default constructor. But how would you do it without that? Thanks for the help.

If you know that there is a constructor that takes,
for example, a single String argument you can use
something like this:
Class clazz = ...; // class to instantiate
String stringArg = ...; // argument to pass to constructor
Constructor c = clazz.getConstructor( new Class[] { String.class } );
Object value = c.newInstance( new Object[] { stringArg > } );
And even better, when JDK 1.5 is released, you'll be able to write:
    Class clazz = ...;
    String stringArg = ...;
    Constructor c = clazz.getConstructor(String.class);
    Object value = c.newInstance(stringArg);Geoff

Require default constructor

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