default-constructor

Why do we need a default no argument constructor in many Java related APIs? Like as a general rule all java bean classes or entity classes (JPA etc) or JAX-WS implementation classes require a explicit no argument constructor. If by default Java provides a no argument constructor then why most of these standards require a explicit constructor? Java only provides a default no-argument constructor if no other constructors are defined . Thus, if you have other constructors you must explicitly define a no-arg constructor yourself. These frameworks use the reflection API and look at method names to

I have a class which has not default constructor. And I need a way to get 'blank' instance of this class. 'blank' means that after instantiation all class fields should has default values like null, 0 etc. I'm asking because I need to be able serialize/desirialize big tree of objects. And I have no access to sources of this objects classes and classes has neither default constructors nor implements serializable. It is likely not very good idea to try to serialize such structure but the alternative is to convert it to something more easily serializable. With standard reflection, no, but there

I recently noticed a class in C++0x that calls for an explicit default constructor. However, I'm failing to come up with a scenario in which a default constructor can be called implicitly. It seems like a rather pointless specifier. I thought maybe it would disallow Class c; in favor of Class c = Class(); but that does not appear to be the case. Some relevant quotes from the C++0x FCD, since it is easier for me to navigate [similar text exists in C++03, if not in the same places] 12.3.1.3 [class.conv.ctor] A default constructor may be an explicit constructor; such a constructor will be used to

I recently grew tired of constantly having to know String keys to pass arguments into Bundles when creating my Fragments . So I decided to make constructors for my Fragments that would take the parameters I wanted to set, and put those variables into the Bundles with the correct String keys, therefore eliminating the need for other Fragments and Activities needing to know those keys. public ImageRotatorFragment() { super(); Log.v(TAG, "ImageRotatorFragment()"); } public ImageRotatorFragment(int imageResourceId) { Log.v(TAG, "ImageRotatorFragment(int imageResourceId)"); // Get arguments passed

I've looked at a few other questions about this, but I don't see why a default constructor should even be called in my case. I could just provide a default constructor, but I want to understand why it is doing this and what it affects. error C2512: 'CubeGeometry' : no appropriate default constructor available I have a class called ProxyPiece with a member variable of CubeGeometry.The constructor is supposed to take in a CubeGeometry and assign it to the member variable. Here is the header: #pragma once #include "CubeGeometry.h" using namespace std; class ProxyPiece { public: ProxyPiece

I don't want to create a default constructor for my auditRecord class. But Spring seems to insist on it: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'auditRecord' defined in ServletContext resource [/WEB-INF/applicationContext.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.bartholem.AuditRecord]: No default constructor found; nested exception is java.security.PrivilegedActionException: java.lang.NoSuchMethodException: com.bartholem.AuditRecord Is this

What is the difference between this: TestClass t; And this: TestClass t = TestClass(); I expected that the second might call the constructor twice and then operator=, but instead it calls the constructor exactly once, just like the first. TestClass t; calls the default constructor. TestClass t = TestClass(); is a copy initialization . It will call the default constructor for TestClass() and then the copy constructor (theoretically, copying is subject to copy elision ). No assignment takes place here . There's also the notion of direct initialization : TestClass t(TestClass()); If you want to

As JPA requires, @Entity classes should have a default (non-arg) constructor to instantiate the objects when retrieving them from the database. In Kotlin, properties are very convenient to declare within the primary constructor, as in the following example: class Person(val name: String, val age: Int) { /* ... */ } But when the non-arg constructor is declared as a secondary one it requires values for the primary constructor to be passed, so some valid values are needed for them, like here: @Entity class Person(val name: String, val age: Int) { private constructor(): this("", 0) } In case when