Creating an instance from String in Java

Question

If I have 2 classes, "A" and "B", how can I create a generic factory so I will only need to pass the class name as a string to receive an instance?

Example:

public static void factory(String name) {
     // An example of an implmentation I would need, this obviously doesn't work
      return new name.CreateClass();
}

Thanks!

Joel

score 25 · Accepted Answer · edited Oct 03 '21 at 12:00

25

Class c= Class.forName(className);
return c.getDeclaredConstructor().newInstance();//assuming you aren't worried about constructor .

javadoc

For invoking constructor with argument

 public static Object createObject(Constructor constructor,
      Object[] arguments) {

    System.out.println("Constructor: " + constructor.toString());
    Object object = null;

    try {
      object = constructor.newInstance(arguments);
      System.out.println("Object: " + object.toString());
      return object;
    } catch (InstantiationException e) {
      //handle it
    } catch (IllegalAccessException e) {
      //handle it
    } catch (IllegalArgumentException e) {
      //handle it
    } catch (InvocationTargetException e) {
      //handle it
    }
    return object;
  }
}

have a look

edited Oct 03 '21 at 12:00

boardtc

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answered Jan 22 '11 at 09:26

jmj

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How can I send arguments to the constructor? – Joel Jan 22 '11 at 09:36
2

`System.out.println(e);` Please, NEVER! Why don't you simply declare the exception? When you *really* feel like being able to handle it by reporting, use `e.printStackTrace()`. – maaartinus Jan 22 '11 at 10:10
newInstance() is deprecated now, replaced with clazz.getDeclaredConstructor().newInstance() – boardtc Oct 01 '21 at 21:06

score 5 · Answer 2 · answered Jan 22 '11 at 09:24

You may take a look at Reflection:

import java.awt.Rectangle;

public class SampleNoArg {

   public static void main(String[] args) {
      Rectangle r = (Rectangle) createObject("java.awt.Rectangle");
      System.out.println(r.toString());
   }

   static Object createObject(String className) {
      Object object = null;
      try {
          Class classDefinition = Class.forName(className);
          object = classDefinition.newInstance();
      } catch (InstantiationException e) {
          System.out.println(e);
      } catch (IllegalAccessException e) {
          System.out.println(e);
      } catch (ClassNotFoundException e) {
          System.out.println(e);
      }
      return object;
   }
}

Creating an instance from String in Java

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