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Java Java Data Structures - Retired Getting There Type Casting

jinhwa yoo
jinhwa yoo
10,042 Points
Posted by jinhwa yoo
jinhwa yoo
10,042 Points

help me ...

as i expect I don't know.. hahah

3 Answers

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

Hey jinhwa,

you will make it ... just believe it :)

Here my code proposal:

  public static String getTitleFromObject(Object obj) {
    //this method takes an Object as argument
   // not a String or BlogBost just an object

    String result = "";
    BlogPost blog = null;
   // here we initialize (same as declaring) an empty String and an empty BlogPost
   // empty BlogPost is "null" and empty String is " "
  // we need them to ibe nitialized, so they can exist inside the if statements

   if (obj instanceof String) {
  // instanceof method is analizing the Object "obj" we passed in as argument 
  // if this Object is  instance of a String or BlogPost (instance is same as kind or type)
  // we need to cast the Object that we passed in
  // into a String (String) or BlogPost (BlogPost)
  // this is important because the compiler doesn,´t know wich kind of object is the argument
 // only we know that this is a String or BlogPost
 // don´t forget ... the compiler is stupid :)

      result = (String) obj;
 // casting obj into String
      return result;
    }
    if (obj instanceof BlogPost) {
      blog = (BlogPost) obj;
   // casting obj into BlogPost  
    }
    return blog.getTitle();
 // after typecasting we return String or BlogPost
 // if it is BlogPost we return the title of the blog
  }

Does it make sense?

Grigorij
jinhwa yoo
jinhwa yoo
10,042 Points
jinhwa yoo
jinhwa yoo
10,042 Points

What is instance??????

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

Instance is the same as object.

If you create a class somewhere like a BlogPost class, where you define methods like getTitle() and so on. You need to create an object of that BlogPost class to be able to use that methods. We call this "object creation" instantiation. Or making an instance of a class. Making an object is the same but in other words ...

So instantiation look like this:

BlogPost blog = new BlogPost;
// this line creates a new instance/object of the BlogPost class

Then you can call a method from the BlogPost class using the name of the instance "blog" and the dot (.) operator to do this:

blog.getTitle();
// blog is the object/instance of the BlogPost
// so it can access methods and variables of BlogPost

Does it make sense?

Grigorij

jinhwa yoo
jinhwa yoo
10,042 Points
jinhwa yoo
jinhwa yoo
10,042 Points

if (obj instanceof BlogPost) { blog = (BlogPost) obj;---->()why use this?
// casting obj into BlogPost
} return blog.getTitle();---->where is getTitle ();????

blog = (BlogPost) obj;---> BlogPost blog = new BlogPost; is it the same meaning?

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

Hey,

so let see the first question:

if (obj instanceof BlogPost) { blog = (BlogPost) obj;---->()why use this?

In this case you use the parenthesis () for casting "obj" (your argument) into BlogPost. You need to do so because obj is an Object and you need a BlogPost.

So you write "(BlogPost) obj" and tell the comiler that "blog" is not an object anymore but a BlogPost. This all crazy stuff is made for one purpose ... to use the getTitle() method.

And this is your second question. The getTitle method is inside the BlogPost class. So you need a BlogPost object = same as instance to access the getTitle method.

The Object "obj" that was your argument of the getTitleFromObject() method can´t access methods in BlogPost class. And this is the reason we need to cast it into BlogPost using "blog = (BlogPost) obj"

BlogPost blog = new BlogPost;
// making an instance
// we don´t need to make it in this challenge

Is not the same as 

blog = (BlogPost) obj;
// converting Object "obj" into a BlogPost

Don´t hesitate to ask if something is not clear, the topic is not easy to understand

Grigorij

jinhwa yoo
jinhwa yoo
10,042 Points
jinhwa yoo
jinhwa yoo
10,042 Points

instanceof String ---> what does this mean???

Grigorij Schleifer
Grigorij Schleifer
10,365 Points
Grigorij Schleifer
Grigorij Schleifer
10,365 Points

This line is comparing the type of the argument with a String type. If you give a String as argument to the getTitleFromObject like this:

String argument = "bla bla bla";
public static String getTitleFromObject(//here you give a String  "argument" as argument) {
if (string instanceof String) {
// is my argument a String or not?
// code
}

If your argument is a String you need to convert "obj" into String. Because the method takes an Object and not a String explicitly ...