Java Java Data Structures Getting There Type Casting

rodnel lighty
rodnel lighty
1,090 Points

I am stuck on task 2 of type casting and could use some help. Could anybody help clarify?

I am not sure where to begin with this. Any explanation would be very much appreciated.

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) {
    String result = "";
    if (obj instanceof String) {
      result = (String)obj;
    } else (obj instanceof BlogPost){
      result = (String)BlogPost.getTitle();
    }
    // Fix this result variable to be the correct string.
    return result;
  }
}

2 Answers

Steven Parker
Steven Parker
171,342 Points

You're pretty close, here's a few hints:

  • a plain "else" doesn't take a conditional expression, perhaps you meant "else if"
  • you may need to cast the object into a BlogPost (but not a BlogPost into a String)
  • you might need extra parentheses to control the order of evaluation

I'll bet you can get it now without an explicit "spoiler". :wink:

Just to illuminate what Steven is saying. Your

else (obj instanceof BlogPost) ...

will throw a syntax error because a pure else statement is not followed by parenthesis but by braces. So it should be

else if (obj instanceof BlogPost) {....

On your statement:

result = (String)BlogPost.getTitle();

you will get an error again, because you are calling the getTitle() method on a class and not on the instance of the class. The getTitle() method is not a static method, therefore you can't access it directly from the class. The solution is to cast the obj into a BlogPost and then access the getTitle() from obj, which by the way is an instance of BlogPost. For explanatory purposes, here is the step by step way. Will give the nicer way in the final code below.

Casting obj to BlogPost

obj = (BlogPost) obj;

Accessing the getTitle() method

result = obj.getTitle();

All things put together and refactored

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) {
    String result = "";
    if (obj instanceof String) {
      result = (String)obj;
    } else if (obj instanceof BlogPost){
      result = ((BlogPost)obj).getTitle();
    }
    // Fix this result variable to be the correct string.
    return result;
  }
}

Happy coding!!