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Java Java Data Structures Getting There Type Casting

Ignacio Martinez de Salinas
Ignacio Martinez de Salinas
1,389 Points

What is the question?

I dont understand what type of variable is com.example.BlogPost, I thought it was BlogPost but then you wouldnt have to casted into BlogPost. Also, if in the first task we have to return a String, how are we going to pass a BlogPost now?

com/example/BlogPost.java
package com.example;

import java.util.Date;

public class BlogPost {
    private String mAuthor;
    private String mTitle;
    private String mBody;
    private String mCategory;
    private Date mCreationDate;

    public BlogPost(String author, String title, String body, String category, Date creationDate) {
      mAuthor = author;
      mTitle = title;
      mBody = body;
      mCategory = category;
      mCreationDate = creationDate;
    }

    public String getAuthor() {
      return mAuthor;
    }

    public String getTitle() {
      return mTitle;
    }

    public String getBody() {
      return mBody;
    }

    public String getCategory() {
      return mCategory;
    }

    public Date getCreationDate() {
      return mCreationDate;
    }
}
TypeCastChecker.java
import com.example.BlogPost;

public class TypeCastChecker {
  /***************
  I have provided 2 hints for this challenge.
  Change `false` to `true` in one line below, then click the "Check work" button to see the hint.
  NOTE: You must set all the hints to false to complete the exercise.
  ****************/
  public static boolean HINT_1_ENABLED = false;
  public static boolean HINT_2_ENABLED = false;

  public static  String getTitleFromObject(Object obj) {
    // Fix this result variable to be the correct string.
    String result=""; 
    if(obj instanceof Object){
      String obj2 = (String) obj;
      if( obj2 instanceof String){
        result=obj2;
      }
    }else{
      if(obj instanceof BlogPost){
       String obj3 = (String) obj;
        result=obj3;
      }
    }
    return result;
  }
}

1 Answer

Kourosh Raeen
Kourosh Raeen
23,732 Points

The type of the parameter obj passed in the definition of getTitleFromObject is Object so inside that method obj will be of type Object no matter what the original type of the parameter passed in. For example, if you make the following call:

String title = getTitleFromObject(someBlogPost)

where someBlogPost is an instance of BlogPost, then inside the method getTitleFromObject the variable obj will be of type Object. Now if you made the following call:

String title = getTitleFromObject(someStringTitle)

where someStringTitle is of type String, then inside the method getTitleFromObject the variable obj will still be of type object because that is what the method signature says: public static String getTitleFromObject(Object obj)

So once you determine the actual type of the object passed in you need to cast it to that type, say BlogPost, before you can call any methods of the associated class on it.

Try the following code:

public static String getTitleFromObject(Object obj) {
    // Fix this result variable to be the correct string.
    String result = "";
    if (obj instanceof String) {
      result = (String) obj; 
    }
    if (obj instanceof BlogPost) {
      result = ((BlogPost) obj).getTitle(); 
    }
    return result;
  }
Ignacio Martinez de Salinas
Ignacio Martinez de Salinas
1,389 Points

Thank you! I've had to change Blogpost into com.example.Blogpost but I understood the problem!