unsure what to do

Question about the task

HashMap of Objects

Java Syntax, Level 8, Lesson 3

Resolved

see code

There is a HashMap<String, Object>. 10 different object pairs have been added to it.
Display the contents of the collection, each element on a new line.

Example output: (here we just show one line):
Sim - 5

Requirements:

The program must create a HashMap variable with (String, Object) elements. The variable must be initialized immediately.
The program should not read values from the keyboard.
The program should add 10 different objects to the collection in accordance with the conditions.
The program should display the contents of the collection on the screen, each pair on a new line.

package com.codegym.task.task08.task0806; import java.util.HashMap; import java.util.Map; /* HashMap of Objects */ public class Solution { public static void main(String[] args) throws Exception { HashMap<String, Object> map = new HashMap<String, Object>(); map.put("Sim", 5); map.put("Tom", 5.5); map.put("Arbus", false); map.put("Baby", null); map.put("Cat", "Cat"); map.put("Eat", new Long(56)); map.put("Food", new Character('3')); map.put("Gevey", '6'); map.put("Hugs", 111111111111L); map.put("Comp", (double) 123); for (Map.Entry<String, Object> pair : map.entrySet()) { String key = pair.getKey(); // Key String value = pair.getValue().toString(); // Value System.out.println(key + ":" + value); } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

7 May 2019, 15:30

#1 You are getting a null pointer reference by trying to call the toString() method on null (in the map, the pair ["baby", null], then line 29 is where toString is called). That method requires a valid object to be called, which 'null' is not. The println method can handle this for you and I recommend instead of converting to strings just put the getkey and getvalue methods directly in the println method. Example:

System.out.println(pair.getKey() + ":" + pair.getValue());

#2 you should be outputting according to the task requirements: correct: Sim - 5 incorrect: Sim:5

Steven Mcilhone

Level 17 , Belfast, United Kingdom

7 May 2019, 15:35

thanks, however how would i get value without using println? even if it was null

Guadalupe Gagnon

Level 37 , Tampa, United States

7 May 2019, 16:18

just like I showed with that line of code above. Put that in to replace lines 28-30 in your code (make sure to also fix the mistake in it I pointed out in tip #2). The println method is fairly complex behind the scenes. It can handle a lot of situations that programmers throw at it. It will automatically call the .toString() method on any object passed to it, and handle most all exceptions that come up. You can pass it Strings, ints, doubles, custom classes, streams, and any etc object you can think of and it will print it. It also concatenates strings for you almost unlimitedly (only restrained by the computer's memory). Have you thought about how complex the code is for that method to be able to do all these things?

Ed Maphis

Level 20 , Painesville, United States

21 May 2019, 18:37

I would guess "toString" doesn't handle 'null' values while string concatenation does.

dean

Level 8 , Deutschland, Germany

27 May 2019, 16:19

hey i passed this just 2 related questions: 1. why the for each loop syntax uses object.entrySet() and not object.entryMap() which seems a much better fit in this case..? 2. what is the "Object" which is the second parameter in the map object means (on lines 13&26)?

dean

Level 8 , Deutschland, Germany

27 May 2019, 16:19

Guadalupe Gagnon

Level 37 , Tampa, United States

27 May 2019, 17:36

1) there is no entryMap() method in HashMaps. 2) Object is the base class of everything in Java. Because of inheritance and polymorphism, Object can hold any class. In the above code the hash map is holding values that are int, double, boolean, String, Long, Character, char, long. This is possible because the map uses Object as the variable type of the value set.