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Java Java Data Structures - Retired Getting There Type Casting

Jef Davis
Jef Davis
29,162 Points
Posted by Jef Davis
Jef Davis
29,162 Points

BlogPost.getTitle()

I've run into this problem a few times in the past and I've always managed to "fix" it without truly understanding the solution. I've just encountered this error again in trying to complete this activity and the compiler error I get is the following: ./TypeCastChecker.java:18: error: non-static method getTitle() cannot be referenced from a static context result = BlogPost.getTitle(); ^

So, I get what a static and non-static method is, I'm just not sure how it can best be resolved here (and the why of the whole process). Thanks in advance.

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 == true) {
      result = (String) obj;
    }
    else if (obj instanceof BlogPost == true) {
      result = BlogPost.getTitle();
    }
    return result;
  }
}

2 Answers

Gloria Dwomoh
Gloria Dwomoh
13,116 Points

I believe you need to make an instance of the class since your method is non static.

ex.

BlogPost news = new BlogPost();
news.setPost("Yes");

instead of

BlogPost.setPost("Yes");

which would have been okay if your method was static.

I hope this helps.

Jef Davis
Jef Davis
29,162 Points
Jef Davis
Jef Davis
29,162 Points

Thanks for the quick reply. I think you're right that I need to create a new instance of BlogPost here to access the getTitle() method inside a static method here; however, I don't want to create a new instance, but access information from obj passed into the getTitleFromObject method in the TypeCastChecker class. In the first part of the task I completed code in getTitleFromObject to check if obj was a String and return the variable result. The instructions for this part of the exercise are as follows:

Now make sure that if a com.example.BlogPost is passed in for obj that you can cast it to a BlogPost. Return the results of the getTitle method on the newly type-casted BlogPost instance.

I think I need to take obj and downcast as a BlogPost to use the method getTitle, which returns a string.

Jef Davis
Jef Davis
29,162 Points
Jef Davis
Jef Davis
29,162 Points

I tried this: 

  public static String getTitleFromObject(Object obj) {
    String result;
    if (obj instanceof String == true) {
      result = (String) obj;
    }
    else if (obj instanceof BlogPost == true) {
      BlogPost title = (BlogPost) obj; 
      result = title.getTitle();
    }
    return result;
  }

I get the following error:

./TypeCastChecker.java:21: error: variable result might not have been initialized return result;

So, that line worked in the first half of the exercise when I initialized is on the first line inside the method. Any ideas why initializing the BlogPost title might have caused problems for that?

Gloria Dwomoh
Gloria Dwomoh
13,116 Points

Okay thanks. The problem with that code is "results" variable is not initialized. What happens if the if-else ends up being false in both blocks? Then your return will return what?

From Initial Values of Variables (make sure you read that).

"A local variable (§14.4, §14.14) must be explicitly given a value before it is used, by either initialization (§14.4) or assignment (§15.26), in a way that can be verified using the rules for definite assignment (§16). "

The editor says

// Fix this result variable to be the correct string.
    String result = "";

so you should assign in the correct string to that variable and your latest code should be working :)

Jef Davis
Jef Davis
29,162 Points
Jef Davis
Jef Davis
29,162 Points

Oh! Of course, I can't believe I forgot that... it's always the simple things that cause such big problems! Thanks!