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Java Java Data Structures - Retired Getting There Type Casting

Posted by Bruce Gordon
Bruce Gordon
6,850 Points

Need help understanding typecasting in Java

I watched the video twice and don't understand the question and don't understand how typecasting works.

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 = "";
    return result;
  }
}

2 Answers

Chase Marchione
Chase Marchione
155,055 Points
Chase Marchione
Chase Marchione
155,055 Points

Hi Bruce,

Within the getTitleFromObject method, we could create two if statements that operate independently of each other. The reason for this is because we know that the object could hold String or BlogPost data. By splitting most of this method's body into two if statement branches, we now account for both of these possibilities--and since we're returning result either way, we'll have a return statement that is outside of the if blocks.

As for casting itself, we use parentheses as a casting operator. You'll see that in both of these if statement blocks we cast the value of 'obj', so that we can convert it to String and store it in our String result variable.

For additional notes on casting, you can check out the comments in my sample code.

  public static String getTitleFromObject(Object obj) {
    String result = ""; // We'll eventually use result to store obj's value and have it returned as a string.

    if (obj instanceof String) { // In the event that obj's data is of the String type...
      result = (String) obj;    // ... we'll cast obj to String, so it can be stored in result.
    }

    if (obj instanceof BlogPost) { // In the event that obj's data is of the BlogPost type...
      BlogPost bp = (BlogPost) obj;  // ... we'll cast obj to BlogPost, store it in a variable...
      result = bp.getTitle();   // ... and use the getTitle() method, since getTitle() returns a String.
    }

    return result;  // We now return the obj value, which is stored in result.

Hope this helps!

Will Rodgers
Will Rodgers
3,005 Points

Nice notes, very helpful!

ericarmando
ericarmando
848 Points
ericarmando
ericarmando
848 Points

thanks, i was lost too i think that for beginners like us they should explain better the objectives.