In intelij it validates besides for the second last condition and the message the mentor gives is: If the input string is "moon", assign the corresponding object to the variable thePlanet.

Question about the task

Reinforce the singleton pattern

Java Core, Level 5, Lesson 12

Resolved

Happens to be that I also can't seem to understand what is wrong with my moon? maybe it's an eclipse :-)

1. Find an example of the singleton pattern used for lazy initialization. Use your favorite search engine (such as Google).
2. In separate files, create three singleton classes in its likeness and image: Sun, Moon, and Earth.
3. Implement the Planet interface in the Sun, Moon, and Earth classes.
4. In Solution class's static block, call the readKeyFromConsoleAndInitPlanet method.
5. Implement the readKeyFromConsoleAndInitPlanet method:
5.1. Read one String value from the console.
5.2. If the value is equal to one of the Planet interface's constants, create a corresponding object and assign it to Planet thePlanet, otherwise set the Planet thePlanet to null.

Requirements:

The Sun class should prevent Sun objects from being created outside the class.
The Sun class must have a private static Sun field named instance.
The Sun class must implement the public static getInstance method, which returns the value of the field instance.
The Sun class's getInstance method must ALWAYS return the same object.
The field instance must be initialized after the first call to the getInstance method, but not before.
The Moon class should prevent Moon objects from being created outside the class.
The Moon class must have a private static Moon field instance.
The Moon class must implement the public static getInstance method, which returns the value of the field instance.
The Moon class's getInstance method must ALWAYS return the same object.
The field instance must be initialized after the first call to the getInstance method, but not before.
The Earth class should prevent Earth objects from being created outside the class.
The Earth class must have a private static Earth field instance.
The Earth class must implement the public static getInstance method, which returns the value of the field instance.
The Earth class's getInstance method must ALWAYS return the same object.
The field instance must be initialized after the first call to the getInstance method, but not before.
The readKeyFromConsoleAndInitPlanet method must be called in the Solution class's static block.
The readKeyFromConsoleAndInitPlanet method should read one line from the keyboard.
The readKeyFromConsoleAndInitPlanet method must correctly update the value of the variable thePlanet according to the task conditions.
The Sun, Moon and Earth classes must be in separate files.

package com.codegym.task.task15.task1522; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; /* Reinforce the singleton pattern */ public class Solution { public static void main(String[] args) { } public static Planet thePlanet; // Add static block here static { try { readKeyFromConsoleAndInitPlanet(); } catch (IOException e) { e.printStackTrace(); } } public static void readKeyFromConsoleAndInitPlanet() throws IOException { try { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); String key = reader.readLine(); if (key.equals(Planet.EARTH)) { thePlanet = Earth.getInstance(); } if (key.equals(Planet.MOON)) { thePlanet = Moon.getInstance(); } if (key.equals(Planet.SUN)) { thePlanet = Sun.getInstance(); } else { thePlanet = null; } }catch (IOException e) { thePlanet = null; } // Implement step #5 here } }

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Thomas

Level 41 , Bayreuth, Germany

19 June 2020, 11:54

You have declared a field instance. By default not initialized fields are null. So when you call the getInstance() method you return null instead of returning a valid object. I'd suggest to check within the getInstance() method if the instance field is null and in case initialize the field with a Moon object. Then return the instance field.

public static Moon getInstance() {
    if (instance == null) instance = new Moon();
    return instance;
}

yehuda b

Level 23 , Beersheba, Israel

20 June 2020, 21:26

Thanks for responding! I understood your answer and I edited the code, but it still wouldn't validate, besides I wonder why only the moon input isn't validating, I believe it's some type of technical issue with how the validator expects the solution to be implemented, because I see no reason even with my previous code why it didn't work (I did initialize it b4, with instance == moon, although I edited the code afterwards as u wrote and it still won't work)

yehuda b

Level 23 , Beersheba, Israel

20 June 2020, 22:53

solution

Ok, So I tried many different solutions from the help section, and in the end the only thing that worked was changing the "if" conditionals to if, else if (line 35), else if (line 39), else thePlanet == null (instead as the above code is with 3 if's and then else, WEIRD!!

Nikitata

Level 22 , Ba Sing Se, United States

31 July 2020, 22:45

useful

if (key.equals(Planet.EARTH)) {
                thePlanet = Earth.getInstance();
            }
            if (key.equals(Planet.MOON)) {
                thePlanet = Moon.getInstance();
            }

            if (key.equals(Planet.SUN)) {
                thePlanet = Sun.getInstance();
            }
            else {
                thePlanet = null;
            }

In the code above, the last else statement belongs only to the previous if statement. When we change the if's to else if like this:

if (key.equals(Planet.EARTH)) {
                thePlanet = Earth.getInstance();
            }
            else if (key.equals(Planet.MOON)) {
                thePlanet = Moon.getInstance();
            }

            else if (key.equals(Planet.SUN)) {
                thePlanet = Sun.getInstance();
            }
            else {
                thePlanet = null;
            }

There is now one large if statement and the code carries out what it was meant to do.

yehuda b

Level 23 , Beersheba, Israel

1 August 2020, 20:09

I understand your point and thanks for responding, although I agree that the way you coded it in the second piece is more correct and sensible, I still believe the code in the first piece would've gotten the same result in either way, though you explained well why it would be a requirement to implement the solution the way you coded it, Thanks, and correct me if I'm wrong please.

Nikitata

Level 22 , Ba Sing Se, United States

7 August 2020, 12:49

Hi @yehudab To water it down, you are looking to make one large if-loop that cycles through all of the possibilities. In the first bit of code, there were 3 different if loops, the last -else statement belonging to the if-loop directly before it. However, since you would need a large if loop for this code, we want to connect the if loop. We can do that with else if statements. In the second bit of code, I have now woven together each separate if loop by adding the else-if.