Setting an object to an unknown class type in Java

The ideal solution is to create your own superclass, say Animal which has declares abstract methods. For example, you can do:

abstract class Animal {
    public abstract void speak();
}

Then Cat and Dog each provide their own implementations for the abstract method:

class Cat {
    public void speak() {
        System.out.println("Meow!");
    }
}

class Dog {
    public void speak() {
        System.out.println("Woof!");
    }
}

Now you can simply declare your array as

Animal[][] myAnimal = new Animal[4][5];

Place some animals in the array:

myAnimal[0][0] = new Dog();
myAnimal[0][1] = new Cat();

And then tell the animals to speak:

myAnimal[0][0].speak();
myAnimal[0][1].speak();

I have left out the syntax for main() here, but I hope you can put this together yourself. The concepts used here are called "polymorphism". I strongly suggest you study up on it to help you use this crucial OO concept when designing your code.

Well, assuming that your array is filled with instances of one of the two classes, you could do something like:

if(myAnimal[x][x] instanceof Dog) //it is an instance of the Dog class 

You really shouldn't do things like this especially as a beginner though. You said that you're trying to select a random dog or cat? The better way would be to keep two separate arrays, one for dogs and one for cats. Then you can select a random element from the two arrays and you'll know what class you're dealing with.

Visitor pattern would be really good approach to consider in such situation. Let say, you are making a game that need to react differently for different object (Cat, Dog, Money, etc). So do this:

Make an interface IAnimal that has following definition:

interface IAnimal {
    void visit(IAnimalVisitor visitor, Object param);
}

class Cat implements IAnimal {
    void visit(IAnimalVisitor visitor, Object param) {
        visitor.visit(this, param);
    }
}

class Dog implements  IAnimal {
    void visit(IAnimalVisitor visitor, Object param) {
        visitor.visit(this, param);
    }
}

The IAnimalVisitor will contain one visit() method for each animal type defined. So it will be like:

interface IAnimalVisitor {
    public void visit(Cat c, Object param);
    public void visit(Dog c, Object param);
}

Then you could use put your logic to deal with cat and dog as per you want. For example:

class AnimalFeeder implements IAnimalVisitor {
    public void visit(Cat c, Object param) {
        c.feed(milk);
        c.feed(cat_food);
    }
    public void visit(Dog d, Object param) {
        d.feed(dog_food);
    }
}

Then you can use above food feeding class to feed to you IAnimal array like this:

IAnimal[] animals = new IAnimal[] { new Cat(), new Dog(), 
    new Dog(), new Cat() };

IAnimalVisitor feeder = new AnimalFeeder();
for(IAnimal animal : animals) {
    animal.visit(feeder, null); 
}

You can achieve full freedom to deal with any level of hierarchy within classes. Just put the visit() for each animal class type within IAnimalVisitor.

I think you have to use reflection api, if you want that class instance , its necessary to check any object at runtime ... i think your question is you don't want to put condition static and how to put condition dynamically for different different animal, today its dog,tiger tomrrow its cow or etc... so plz use refelction api