通过将对象作为参数传递在方法中使用对象

Question

I'm facing a situation that involves class and methods.

To make it simple, imagine you have 2 classes: A and B

In the class A, I have a method called, lets say, "jumpOnOtherClass". In the class B, I have another method called "doThings".

I'd like the class A method to take as argument any object of any class, so that it first checks if the object is null (so it can creates a new instance of the object passed), and then execute a method of the object's class.

Here is an example code:

class MyMainClass
{
  private A objectA = new A();
  private B objectB;

  public void main()
  {
    this.objectA.jumpOnOtherClass(this.objectB,"doThings");
  }
}

class A
{
  public void jumpOnOtherClass(Object objectFromAnyClass, String methodToInvoke)
  {
    if(objectFromAnyClass == null) objectFromAnyClass = new DefaultConstructorOfB();

    objectFromAnyClass.getMethod(methodToInvoke);
  }
}

class B
{
  public void doThings()
  {
    //Do some stuff here
  }
}

I know there are exception to take in consideration but for the example we'll assume that the class, and the methods exist and are found :)

I tried things with "java.lang.reflect" but it seems not to be the good way to achieve what I try to do...

Any help would be really nice!

Thanks and have a nice day!

Answer 1

I'm not entirely sure I understand the question, but it looks like you may be looking for a way to differentiate between classes initially passed as Objects? You could use the instanceof keyword, and then risk some casting exceptions

class MyMainClass
{
private A objectA = new A();
private B objectB;

public void main()
{
   this.objectA.jumpOnOtherClass(this.objectB,"doThings");
}
}

class A
{
  public void jumpOnOtherClass(Object objectFromAnyClass, String methodToInvoke)
  {
    if(objectFromAnyClass == null) objectFromAnyClass = new DefaultConstructorOfB();
    else if (objectFromAnyClass instanceof B) {
         if (methodToInvoke.equals("doThings")) 
            ((B) objectFromAnyClass).doThings();
    }
  }
}

class B
{
  public void doThings()
  {
    //Do some stuff here
  }
}

Answer 2

The only problem is that if the reference is null, you cannot really know the class of the "object" at runtime, so you will need to fix this by providing additional information.

I suggest this approach (it does involve reflection):

public <T> void jumpOnOtherClass(T objectFromAnyClass, Class<T> classType, String methodToInvoke)
            throws
            NoSuchMethodException,
            IllegalAccessException,
            InvocationTargetException,
            InstantiationException
    {
        if(objectFromAnyClass == null) 
            objectFromAnyClass = classType.getDeclaredConstructor().newInstance();

        classType.getMethod(methodToInvoke).invoke(objectFromAnyClass);
    }

and use it like this:

jumpOnOtherClass(this.objectB, B.class, "doThings");

Answer 3

Ok! I get it works finally! So here is the final code:

class A
{
  public <T> void jumpOnOtherClass(T objectFromAnyClass, Class<T> classType, String methodToInvoke) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException
  {
    if(objectFromAnyClass == null)
    {
      println("instantiating");
      objectFromAnyClass = classType.newInstance();
    }

    println("getting the method");
    classType.getMethod(methodToInvoke).invoke(objectFromAnyClass);

    println("It works :D");
  }
}

static class B
{  
  public void doThings()
  {
    println("message from class B");
  }
}

The problem was actually coming from the class nesting made by processing (and in general, any class nesting). I removed the "getConstructor()" from the line where it was checking if the object b was null.

Maybe someone could explain under this post why does removing this method made everything working... I'm gonna try to understand and if I have something about that, it'll be posted down here!

Again, thanks a lot to you guys for helping :D

Have a nice day!

Larry.K