Creating an instance using the class name and calling constructor

Question

Is there a way to create an instance of a particular class given the class name  dynamic  and pass parameters to its constructor   Something like   Object object   createInstance  mypackage MyClass   MyAttributeValue      Where  MyAttributeValue  is an argument to the constructor of MyClass

User · Answer

Very Simple way to create an object in Java using Class lt   gt  with constructor argument s  passing   Case 1 - Here  is a small code in this Main class   import java lang reflect Constructor  import java lang reflect InvocationTargetException   public class Main        public static void main String args    throws ClassNotFoundException  NoSuchMethodException  SecurityException  InstantiationException  IllegalAccessException  IllegalArgumentException  InvocationTargetException               Get class name as string          String myClassName   Base class getName               Create class of type Base          Class lt   gt  myClass   Class forName myClassName              Create constructor call with argument types          Constructor lt   gt  ctr   myClass getConstructor String class              Finally create object of type Base and pass data to constructor          String arg1    My User Data           Object object   ctr newInstance new Object     arg1                Type-cast and access the data from class Base          Base base    Base object          System out println base data              And  here is the Base class structure   public class Base        public String data   null       public Base                  data    default           System out println  Base                 public Base String arg1            data   arg1          System out println  Base   arg1                  Case 2 - You  can code similarly for constructor with multiple argument and copy constructor  For example  passing 3 arguments as parameter to the Base constructor will need the constructor to be created in class and a code change in above as   Constructor lt   gt  ctr   myClass getConstructor String class  String class  String class   Object object   ctr newInstance new Object      Arg1    Arg2    Arg3         And here the Base class should somehow look like   public class Base        public Base String a  String b  String c              This constructor need to be created in this case               Note - Don t forget to handle the various exceptions which need to be handled in the code

User · Answer

You can use reflections  return Class forName className  getConstructor String class  newInstance arg

User · Answer

You want to be using java lang reflect Constructor newInstance Object

User · Answer

If class has only one empty constructor  like Activity or Fragment etc  android classes    Class lt   gt  myClass   Class forName  com example MyClass        Constructor lt   gt  constructor   myClass getConstructors   0

User · Answer

Another helpful answer  How do I use getConstructor params  newInstance args    return Class forName   complete classname         getConstructor   here pass parameters passed in constructor         newInstance   here pass arguments       In my case  my class s constructor takes Webdriver as parameter  so used below code     return Class forName  com page BillablePage        getConstructor WebDriver class       newInstance this driver

User · Answer

If anyone is looking for a way to create an instance of a class despite the class following the Singleton Pattern  here is a way to do it      Get Class instance Class lt   gt  clazz   Class forName  myPackage MyClass        Get the private constructor  Constructor lt   gt  cons   clazz getDeclaredConstructor        Since it is private  make it accessible  cons setAccessible true       Create new object   Object obj   cons newInstance      This only works for classes that implement singleton pattern using a private constructor

User · Answer

You can use Class forName   to get a Class object of the desired class   Then use getConstructor   to find the desired Constructor object   Finally  call newInstance   on that object to get your new instance   Class lt   gt  c   Class forName  mypackage MyClass    Constructor lt   gt  cons   c getConstructor String class   Object object   cons newInstance  MyAttributeValue

User · Answer

when using  i e   getConstructor String lang  the constructor has to be declared public  Otherwise a NoSuchMethodException is thrown   if you want to access a non-public constructor you have to use instead  i e   getDeclaredConstructor String lang

User · Answer

You can also invoke methods inside the created object   You can create object instant by invoking the first constractor and then invoke the first method in the created object        Class lt   gt  c   Class forName  mypackage MyClass        Constructor lt   gt  ctor   c getConstructors   0       Object object ctor newInstance new Object    ContstractorArgs         c getDeclaredMethods   0  invoke object Object    MethodArgs

User · Answer

Yes  something like   Class lt   gt  clazz   Class forName className   Constructor lt   gt  ctor   clazz getConstructor String class   Object object   ctor newInstance new Object     ctorArgument       That will only work for a single string parameter of course  but you can modify it pretty easily   Note that the class name has to be a fully-qualified one  i e  including the namespace  For nested classes  you need to use a dollar  as that s what the compiler uses   For example   package foo   public class Outer       public static class Nested        To obtain the Class object for that  you d need Class forName  foo Outer Nested

[java] Creating an instance using the class name and calling constructor

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