CODEWITHSUNDEEP
java
user3135757
user3135757

Reputation: 165

Difference between Object and Class

lets say we have this class without any constructors and instances variables

public class ConsoleWriter{
  public void write(){
    System.out.println("Console Writing...");
  }
}

and you have another class App with 2 variables console and console1

public class App{
   ConsoleWriter console;
   ConsoleWtiter console1=new ConsoleWriter();
}

Now I understand that console1 is an object of ConsoleWriter while console is just a reference type to ConsoleWriter.

When having such case (No constructors, No instance Variable)

what is the difference ?

or how would it be useful to create the Object console1 if we could just do it using the console.

console.write();//would output Console Writing...
console1.write();//would output Console Writing...

Upvotes: 0

Views: 167

Answers (8)

user207421
user207421

Reputation: 311055

Now I understand that console1 is an object of ConsoleWriter

No. console1 is a reference to ConsoleWriter which isn't null, because you initialized it.

while console is just a reference type to ConsoleWriter.

Correct. console1 is a reference to ConsoleWriter which is null, because you didn't initialize it.

what is the difference ?

The difference is that you didn't initialise one of them.

Upvotes: 1

Suresh Atta
Suresh Atta

Reputation: 122026

You really need to know the difference between Declaring, Instantiating and Initializing an Object

ConsoleWriter console;
ConsoleWiter console1=new ConsoleWriter();

No, the below lines are wrong 

console.write();//would output Console Writing...
console1.write();//would output Console Writing...

You face a NullPointerException in first line.

You have'nt to initialize it and now it's holding default value null.

Where as second line executes the write() method. In order to use it further you have to initialize it.

Recomminding : Declaring, Instantiating and Initializing an Object

Upvotes: 1

TheLostMind
TheLostMind

Reputation: 36304

"Now I understand that console1 is an object of ConsoleWriter while console is just a reference type to ConsoleWriter." - Wrong. Both console and console1 are references.. The difference is that console1 points to a ConsoleWriter object whereas console doesnt point to anything.

So. console.anyFunction() will give you an exception (NulPointerException) because console is not initialized whereas console1.someFunction() will work as console1 points to a ConsoleWriter Object.

Upvotes: 2

Aniket Thakur
Aniket Thakur

Reputation: 69035

console1 is an object of ConsoleWriter

Incorrect it is a reference that will point to Object of type ConsoleWriter. Since uninitialized it will point to null. Also you cannot call methods on this as it is uninitialized.

console.write();//would output Console Writing...

This is also incorrect. Must be initialized first.

ConsoleWtiter console1=new ConsoleWriter();

This is equivalent to

ConsoleWtiter console1;

which creates a reference which will point to Objects/instances of class ConsoleWriter. Since it is an instance variable it is assigned default value i.e null.

console1 = new ConsoleWriter();

At this point the reference console actually points to an Object of class ConsoleWriter. It is only after this(initialization) you can call methods of the class.

Upvotes: 4

Simulant
Simulant

Reputation: 20140

Any class without a constructor specified always has the default constructor you can use. In your case this is:

public class ConsoleWriter{
  public ConsoleWriter(){
  }
}

Upvotes: 0

user2357112
user2357112

Reputation: 282006

console isn't usable. It's not a ConsoleWriter; it's null. If you try to call its methods, you'll get a NullPointerException. This isn't C++, where

ConsoleWriter console;

constructs a ConsoleWriter.

Upvotes: 1

SpringLearner
SpringLearner

Reputation: 13854

console.write(); will give you runtime exception most probably null pointer exception

But

 ConsoleWtiter console1=new ConsoleWriter();
console1.write();

would give you

Console Writing...

Upvotes: 1

Maroun
Maroun

Reputation: 96016

See the JLS:

For all reference types (§4.3), the default value is null.

So when you write:

ConsoleWriter console;
console1.write();

It's like writing:

null.write();  //Will throw NullPointerException

On the other hand, when you do:

ConsoleWriter console1=new ConsoleWriter();

Then you are constructing a new object of type ConsolWriter.

Upvotes: 1

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