String to Class: ClassNotFoundException

Aleksi

Hi there.

As many before have no doubt tried, I'm attempting to create a class instance from a string. I've read that it's possible, and even seems fairly straightforward, but it's not working for me. Any help appreciated! Here's the code:

import java.lang.Class;

String model = "Ribbon";

void setup(){
  noLoop(); 

  // test 1 -- successfully prints the instance of a Ribbon class
  Object ribbon1 = new Ribbon();
  println(ribbon1.getClass());

  // test 2 -- uncommenting the following line gives a ClassNotFoundException
  //Object ribbon2  = Class.forName(model);
}

class Ribbon {
  Ribbon(){
    println("New Instance of Ribbon!");
  }
}

GoToLoop

1. Java already imports the package java.lang automatically.
   That import statement is completely unnecessary!
2. As it is the case for all reflexive operations, Class.forName():
   https://Docs.Oracle.com/javase/9/docs/api/java/lang/Class.html#forName-java.lang.String-
   Demands to be inside a try {} / catch () {} structure:
   https://Processing.org/reference/try.html
   B/c it may throw a ClassNotFoundException:
   https://Docs.Oracle.com/javase/9/docs/api/java/lang/ClassNotFoundException.html
3. Also, the String parameter from method Class.forName() gotta be a fully qualified name for the target class. That includes its fully package name besides its name.
   However, b/c your target class Ribbon is also a nested inner class, all of its enclosing classes must be included in its fully qualified name too!
   Take a look at the sketch example from this old forum thread link below:
   https://Forum.Processing.org/two/discussion/7036/multiple-screens#Item_9

Aleksi

Once again I thank GoToLoop for the quick reply!

1) Didn't know that.

2) Didn't know that it was mandatory.

3) Ok, so now I've learned that Ribbon is a nested inner class of Sketch, I assume. The example was helpful, but I don't understand how, given my lone string variable, I can pass it as an arg to Class.forName() as a fully qualified name.

So, to be blunt, what does the line look like?

GoToLoop

/**
 * Reflexive Inner Class Instantiation (v1.1)
 * GoToLoop (2018/Mar/24)
 *
 * Forum.Processing.org/two/discussion/27164/
 * string-to-class-classnotfoundexception#Item_3
 */

void setup() {
  Class<?> appCls = getClass(), innerCls = null;
  try {
    innerCls = Class.forName(appCls.getName() + "$Inner");
    //innerCls = Class.forName(appCls.getDeclaredClasses()[0].getName());
  }
  catch (final ClassNotFoundException ex) {
    System.err.println(ex);
  }
  println(innerCls);

  Inner inner = null;
  try {
    // Works only if Inner is a static class:
    //inner = (Inner) innerCls.newInstance();

    // If Inner is an inner class:
    inner = (Inner) innerCls.getDeclaredConstructor(appCls).newInstance(this);
  }
  catch (final ReflectiveOperationException ex) {
    System.err.println(ex);
  }
  println(inner);

  exit();
}

//static
class Inner {
  @ Override String toString() {
    return getClass().getName();
  }
}

Aleksi

Wow, I can't wrap my head around that, I'm not there yet. 8-}

Ok, since you were so immensely helpful to me last time, let me tell you what my goal is in case you have a simpler solution.

A csv file has a field name that contains a string of a class I want to instantiate (along with its parms). I can parse it in a giant switch block to instantiate the right class, but after reading about Java's reflexive capabilities I thought it might be possible to do it that way. Well, perhaps it might be, but the hoops to jump through seem too onerous to negotiate...

GoToLoop

/**
 * Reflexive Inner Class Instantiation (v2.0)
 * GoToLoop (2018/Mar/24)
 *
 * Forum.Processing.org/two/discussion/27164/
 * string-to-class-classnotfoundexception#Item_5
 */

void setup() {
  final Class<?> appCls = getClass(), innerCls = appCls.getDeclaredClasses()[0];
  println(innerCls);

  Inner inner = null;
  try {
    inner = (Inner) innerCls.getDeclaredConstructor(appCls).newInstance(this);
  }
  catch (final ReflectiveOperationException ex) {
    System.err.println(ex);
  }
  println(inner);

  exit();
}

class Inner {
  @ Override String toString() {
    return getClass().getName();
  }
}

GoToLoop

... but the hoops to jump through seem too onerous to negotiate...

In Java, the official & preferable way to instantiate classes & interfaces is via the operator new:
https://Processing.org/reference/new.html

In Java, all of its members should be known at compile-time.

Reflective operations is an advanced & complicated way to inquire whether some member exists at runtime.

GoToLoop

... in case you have a simpler solution.

W/o knowing your ".csv" file, the classes you made to represent them, how similar those classes are to each other, there's no way to tailor the most possible simplest solution. :-@

Aleksi

But the try block still needs to cast inner as (Inner). If I have many inner classes, wouldn't each inner class need its own try block to properly cast the assignment? That rather defeats the point of a generalized solution. (I've gotten too accustomed to a simple eval() to do such things. I must jettison the idea that I can use such handy tricks in Java, it appears.)

It seems that a switch block is probably the simplest and most versatile solution. Oh well...

Thank you for your responses, as always.

GoToLoop

The try/catch blocks are always mandatory in order to call newInstance(), no matter whether it's a regular, nested or inner class! =;

You can of course place the whole try/catch blocks inside some function which accepts a Class argument. *-:)

Then you can (cast) the returning Object to its actual datatype. :-bd

But reflective operations should be used as a last recourse only, when the regular means aren't enough! L-)

GoToLoop

I've gotten too accustomed to a simple eval() to do such things.

https://Forum.Processing.org/two/discussion/7147/some-simple-pulse-equations-using-trig-#Item_8

Aleksi

Cool! Thx!

Aleksi

Think I found the answer I'm looking for...

Aleksi

Proof of concept test, in case anyone else encounters the same problem:

String[] sa = {"Ribbon","Box","Circle","Ribbon"};

void setup(){

  Shape[] shapes = new Shape[sa.length];

  int i = 0;
  for (String s : sa){
    if (s.equals("Ribbon")){
      shapes[i] = new Ribbon();
    }
    if (s.equals("Box")){
      shapes[i] = new Box();
    }
    if (s.equals("Circle")){
      shapes[i] = new Circle();
    }
    i++;
  }

  for (Shape s : shapes){
    s.test();
  }
}


interface Shape {
  void test();
}

public class Ribbon implements Shape {
   void test(){ 
     println("Hello Ribbon.test!"); 
   }
}

public class Circle implements Shape {
   void test() {
    println("Hello Circle.test!");
  }
}

public class Box implements Shape {
   void test() {
    println("Hello Box.test!");
  }
}

Aleksi

The answer I originally sought can be found here.