Question

I have an issue with Java Reflection. Here’s my code very much simplified.

Let’s say we have this class :

public class MyClassA {
    private MyClassB myPropertyB_1;
    private MyClassB myPropertyB_2;

    ...
}

And this class (which is used as a property for MyClassA :

public class MyClassB {

    private int myProperty;

    public MyClassB(){
    }

    ...
}

I would like to use Java Reflection to do something like this, in MyClassA :

    public void methodThatUsesReflectionOnProperty(int id){
        // 1. Get the field using id
        // 2. Instantiate this field dynamically (i.e. new MyClassB() )
        // 3. Invoke some methods on this field dynamically (setters, addlisteners...)
}

How could I do this ?

Answer 1

You could get a field using:

classObject.getDeclaredField(String fieldName);

You can get a method using:

classObject.getDeclaredMethod(String methodName, Class<?> ... parameterTypes);

And you can instantiate it with newInstance() of course, if there is a default constructor.

For instance:

ClassA aObj = new ClassA();

// field access
Field field = ClassA.class.getDeclaredField("objectB");
Object bObj = field.getType().newInstance();
field.setAccessible(true); // as it is private it needs to be accessible first
field.set(aObj, bObj); // set bObj into field from aObj

// invoking method
Method method = bObj.getClass().getDeclaredMethod("methodB", String.class);
method.invoke(bObj, "reflection test!");

This will print:

B wrote: reflection test!