instanceof

Question

instanceof BlogPost problem

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 = true;
  public static boolean HINT_2_ENABLED = false;

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

Answer 1 · 2015-10-20T18:22:11Z

October 20, 2015 6:22pm

Hey Jianxun,

I posted my solution on your other question but I will add it here also

Let's first see what we're given:

Challenge.java

public static String getTitleFromObject(Object obj) {
    // Fix this result variable to be the correct string.
    String result = "";

   //code to input here

    return result;
  }

What we want to do is first check if the object we are passing in is a string. We can pass any object into this method. Remember, as an object is the parent class for literally everything, this means everything is an object. For this, we check if our object is a string, and then we downcast the object to a string. Do you remember how to do this?

We use the instanceof keyword. This will check if our object can be downcasted to a String.

instanceof.java

 if (obj instanceof String) {

        //downcast here

    }

So in this if statement, if we indeed have a string object, then we can downcast it.

downcast.java

result = (String) obj;

This will make our result variable equal to our downcasted string object

In the end it should look like this:

Answer.java

public static String getTitleFromObject(Object obj) {
    // Fix this result variable to be the correct string.
    String result = "";

    if (obj instanceof String) {
      result = (String) obj;
    }

    return result;
  }

Hope that solved your question!

Best of luck and happy coding,

Kevin

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