Question

I have been banging my head for a while now, understanding java language. I have been familiar with classes but i always get confused with object? I know the question sounds very stupid but I am still not able to figure out what a java object do and why do we need it. How different is it from a plain variable. How to call it. How can I use a java object defined in one class into another? These questions are mocking my head for a while now.
Could someone explain object in simple sentences?
Any help with some simple example would be very much helpful.

Answer 1

A class defines the data and behavior that all the instances (objects) of this class will have. For example, the class Dog has a name and color fields, and a bark() method. Now if you want to make an application with three dogs, you will create three instances of the Dog class:

Dog blacky = new Dog("Blacky", "black");
Dog whity = new Dog("Whity", "white");
Dog rooky = new Dog("Rooky", "red");

Once you have a dog you can ask it to bark:

blacky.bark();

You could also imagine a House class, containing dogs:

public class House {
    private Set<Dog> dogsInTheHouse = new HashSet<Dog>();

    public void addDog(Dog dog) {
        this.dogsInTheHouse.add(dog);
    }

    public void someOneIsAtTheDoor() {
        for (Dog dog : dogsInTheHouse) {
            dog.bark();
        }
    }
}

So you could create a house:

House myHouse = new House();

add dogs in the house:

myHouse.add(blacky);
myHouse.add(whity);
myHouse.add(rooky);

And when you come at the door of the house, all its dogs start barking:

myHouse.someOneIsAtTheDoor();

So an object can be composed of other objects, and objects can call each others methods to assemble a whole application.