Java中的抽象类和反射

Question

I have an abstract class which is mostly instantiated as anonymous inner classes, with abstract method implemented there. These instances get passed around, and so at a different place in the code I would like to get a copy of one of these instances, a new instance but with the method implemented the same way. Here is an example of my code:

public abstract class AbstractClass {
String id;
Entity owner;
public AbstractClass(String id){
    this.id=id;
} 
public Mover(){
    id="This is an id";
}
abstract void update();
}

I instantiate it like this:

AbstractClass instance= new AbstractClass("This is a test"){
void update(){
//do stuff
}
}

Later, I want a copy, not a reference, of that instance, where update() does the same stuff, but owner will be a different entity. I've tried to use reflection, (.getClass.newInstance()), but I get a java.lang.InstantiationException. Why doesn't this work and is there a better way to do what I'm doing?

Answer 1

You can't instantiate anonymous inner classes through newInstance(). Non-static inner classes hold a reference to their container Object, you need that Object to create them.

See Java Tutorial -> Nested Classes

Answer 2

That exception is thrown in two cases (from docs ):

• the class object represents an abstract class, an interface, an array class, a primitive type, or void
• the class has no nullary constructor

Based on a simple test I did , in your case, the problem is that the resulting anonymous class doesn't have a nullary constructor.

Test code:

import java.util.*;
import java.lang.*;

class Main
{
        public static void main (String[] args) throws java.lang.Exception
        {
            {
            Test thing = new Test(){ public void update(){ id="updated"; } };

            thing.update();

            Test thang = thing.getClass().newInstance();

            System.out.println( thing.id+" "+thang.id );
            }

            {
            Test2 thing = new Test2("name"){ public void update(){ id="updated"; } };

            thing.update();

            Test2 thang = thing.getClass().newInstance();

            System.out.println( thing.id+" "+thang.id );
            }

        }
}

abstract class Test{
    public String id;
    Test(){ id = "Empty"; }
    abstract public void update();
}

abstract class Test2{
    public String id;
    Test2(String me){ id = me; }
    abstract public void update();
}

The first test (with Test ) runs just fine, the second (with Test2 ) fails on the newInstance call with that exception.

Answer 3

Sounds like you want to clone the object, implement the interface Cloneable and add in a clone method to define what you need to do to clone said object. In this simple case it would just mean instantiating a new object and setting the id, but I assume your actual implementation is a little more advanced.

Answer 4

Anonymous inner classes require NO NAME! You shouldn't create an object with the name 'instance' Whenever you need a copy of your abstract class, for example, as an argument for a method, go ahead and use it directly instead of creating and using 'instance' :

someObject.methodToBeUsed(new AbstractClass("This is a test"){
   void update(){
   //do stuff
});