CASTING - What exactly does it do? What is the aim?

Question

Good evening coders!
I am just a begginer and I like to understand the logic behind the things that I learn...
And honestly, I can't see what I am doing here!
In the case of the tweet for instance...it says something like:
"If it is a string, then cast it to string" EING? If it IS an instance of string, why should I need to cast it to string? Isn't it already a String?? 
Or it is not a String but it has the "shape" of a String and casting it to a String we are saying Java that we want to be able to use the String method because that's, in fact, a string?
Please, some good teacher here explain what's the logic behind casting...
I really think is a fundamental stone in our learning and is quite confusily explained!
Thank you!

Mohammed AlShaiban · Answer

We use casting when going from a general type to a more specific one. For example, you can store an Integer as a Number since Integers are numbers. 
We can also use casting when we only want to output an integer, but we have a double.
Let's say we have a double 
Java 
double d = 9.99;

However, we only want to print it as integer, we can cast double to integer
Java
System.out.println(d);                   //Will output 9.99
System.out.println((int) (d));        //Will output 9

It is important to note that casting will only work if the object is an instanceof the class we are casting to. 
If it is not, we will get ClassCastException

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Ricardo Sala

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CASTING - What exactly does it do? What is the aim?

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Mohammed AlShaiban

Mohammed AlShaiban