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Can you instantiate another class within the constructor?

By : Quí Lục Văn
Date : October 16 2020, 06:10 PM
help you fix your problem Yes, but let me clarify, as you appear to be a beginner (welcome aboard, by the way): you instantiate an object of Class1. A class cannot be instantiated. In your example, that object is a local variable which will be destroyed once all the code inside the constructor is executed. Practically, this means you cannot use c1 anywhere but inside the constructor. You can make it a member (a field, specifically) of Class2 by declaring it in the class body and bound its existence to that of an object of Class2, for example:
code :
public class Class1() { }

public class Class2() 
    Class1 c1;

    public Class2() 
       c1 = new Class1();

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java.lang.RuntimeException: Unable to instantiate service {....} can't instantiate class; no empty constructor

By : user3330073
Date : March 29 2020, 07:55 AM
I hope this helps you . First, you cannot have a Service that is an ordinary nested class. It would have to be a static nested class.
Second, having a Service that is a static nested class of an Activity is very strange.

Js es6 class constructor function run before the constructor instantiate

By : jj451120
Date : March 29 2020, 07:55 AM
it helps some times The problem has nothing to do with classes, ES6 or Babel. Here is a simplified version of your problem:
code :
var foo = {
  bar: 42,
  baz: foo.bar * 2
this.userSelections = {
  types: this.getTypes()
// now that `this.userSelections` exists, we can call `this.getProvider` without problems
this.userSelections.providers = this.getProvider();
class Foo {
  constructor() {
    let types = this.getTypes();
    this.userSelection = {
      providers: this._getProvider(types)

  _getProvider(types) {
    var activeType = types.some(( type ) => {
      return type.active;
    // ...

  getProvider() {
    return this._getProvider(this.userSelection.types);

How does default constructor used to instantiate object if it's super class no argument constructor has empty body?

By : Michael Jones
Date : March 29 2020, 07:55 AM
hop of those help? default constructor invoke no_argument constructor in super class reference and this last constructor used to instantiate object from this class,How ? i mean what does superclass constructor body exactly do ? ,
The syntax for calling a superclass constructor is
code :
super(parameter list);
Child a = new Child() ;

C++ Instantiate, inside another class, a class-type variable from a group of classes with same constructor

By : Olle Kanko
Date : March 29 2020, 07:55 AM
To fix the issue you can do I am not advanced in C++. Suppose I have a group of classes, from A to whatever (the number will grow in time), that use the same type of constructor. Suppose it looks like this: , You could have an interface class with those common member variables
code :
class I
    virtual ~I() = default;
    double m_x, m_y;
class A : public I
    A(const double &x, double &y, const short &n) { ... };
class Bla
    std::unique_ptr<I> m_class;
    Bla(const double &x, const double &y, const double &z, const short &i)
        switch (i)
        case 1: m_class = std::unique_ptr<I>(new A(...)); break;
        case 2: m_class = std::unique_ptr<I>(new B(...)); break;

Need to instantiate the child class members from parent class constructor using reflection and annotations

By : user2072131
Date : March 29 2020, 07:55 AM
it should still fix some issue First I thought that it won't be possible to modify a child class in a parent class constructor because the initialization of the child class it not completed, but then I wasn't sure and had to try it out. So I found that it is possible and learned something today :)
To answer your question, yes it is possible, here is my test code, it will help you solving your particular problem:
code :
@Retention( RetentionPolicy.RUNTIME )
@Target( ElementType.FIELD )
public @interface Annotation {
    // empty
public class Child extends Parent {

    private @Annotation String test;

    public Child() {
        System.out.println( this.test );

    public static void main( final String[] args ) {
        new Child();

public class Parent {

    private @Annotation String testOfParent;

    public Parent() {
        for ( int i = 0; i < this.getClass().getDeclaredFields().length; i++ ) {
            Field field = this.getClass().getDeclaredFields()[i];
            System.out.println( field );

            for ( int j = 0; j < field.getDeclaredAnnotations().length; j++ ) {
                java.lang.annotation.Annotation annotation = field.getDeclaredAnnotations()[j];
                System.out.println( annotation );

            try {
                field.setAccessible( true );
                field.set( this, "Hello World!" );
            } catch ( IllegalArgumentException | IllegalAccessException ex ) {
                // TODO Auto-generated catch block

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