CodeGym/Help with Java Tasks/What's wrong?

Level 14

Mumbai

11.12.2018
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9comments

What's wrong?

Question about the task

Make a family

Java Syntax, Level 8, Lesson 11

Resolved

I have tried everything. Finally I created 3 lists .Or do I have to create another constructor for storing objects.

1. Create a Human class with String name, boolean sex, int age, and ArrayList<Human> children fields.
2. Create and populate objects so that we end up with: two grandfathers, two grandmothers, one father, one mother, and three children.
3. Display all the Human objects (Hint: use the Human class's toString() method).

Requirements:

The program should display text on the screen.
The Human class must have four fields.
The Human class must have one method.
The Solution class must have one method.
The program should create objects and fill them with data to get two grandfathers, two grandmothers, one father, one mother, and three children. Then it should display all the Human objects on the screen.

package com.codegym.task.task08.task0824; import java.util.*; /* Make a family */ public class Solution { public static void main(String[] args) { //write your code here ArrayList<Human> children1 = new ArrayList<>(); ArrayList<Human> children2 = new ArrayList<>(); ArrayList<Human> children3 = new ArrayList<>(); Human child1 = new Human("Child1", true, 8, null); Human child2 = new Human("Child2", false, 5, null); Human child3 = new Human("Child3", true, 2, null); children1.add(child1); children1.add(child2); children1.add(child3); Human father = new Human("Father", true, 56, children1); Human mother = new Human("Mother", false, 50, children1); children2.add(father); children3.add(mother); Human grandF1 = new Human("Gf1", true, 90, children2); Human grandF2 = new Human("Gf2", true, 92, children3); Human grandM1 = new Human("Gm1", false, 87, children2); Human grandM2 = new Human("Gm2", false, 89, children3); System.out.println(grandF1); System.out.println(grandF2); System.out.println(grandM1); System.out.println(grandM2); System.out.println(father); System.out.println(mother); } public static class Human { //write your code here String name; boolean sex; int age; ArrayList<Human> children; public Human(String name, boolean sex, int age, ArrayList<Human> children) { this.name = name; this.sex = sex; this.age = age; this.children = children; } public String toString() { String text = ""; text += "Name: " + this.name; text += ", sex: " + (this.sex ? "male" : "female"); text += ", age: " + this.age; int childCount = this.children.size(); if (childCount > 0) { text += ", children: " + this.children.get(0).name; for (int i = 1; i < childCount; i++) { Human child = this.children.get(i); text += ", " + child.name; } } return text; } } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

11 December 2018, 14:33

useful

You are not displaying the three children as required (only mention them as part of the mother and father objects). Add three sys.out statements for child1, child2, and child3.

Akhilesh

Level 14 , Mumbai, India

12 December 2018, 12:59

But what about the children field of grandfather and grandmother ? Do I have to pass null or create separate arraylist for mother and father and store them in the grandfather and grandmother field?

Guadalupe Gagnon

Level 37 , Tampa, United States

12 December 2018, 14:36

no. just add sys.out.println for the three children.

Akhilesh

Level 14 , Mumbai, India

13 December 2018, 07:27

It's still not working System.out.println(grandF1); System.out.println(grandF2); System.out.println(grandM1); System.out.println(grandM2); System.out.println(father); System.out.println(mother); System.out.println(child1); System.out.println(child2); System.out.println(child3);

Guadalupe Gagnon

Level 37 , Tampa, United States

13 December 2018, 14:26

solution

Did you initialize the ArrayList 'children' of the three child objects? If you run the code without doing that you will get a null pointer exception. Make sure that you add this before calling the sys.out(child#). Example:

child1.children = new ArrayList<>();

Akhilesh

Level 14 , Mumbai, India

14 December 2018, 08:25

Thanks for the help!! It worked!

Picvajzmen

Level 20 , Belgrade, Serbia

23 May 2019, 08:44

Hey Guadalupe, can you tell me why is this the case? why do you get null pointer exception?

Guadalupe Gagnon

Level 37 , Tampa, United States

23 May 2019, 13:25

All of this is in regards to the original code: ======================================================= Lines 15-17: Human child1 = new Human("Child1", true, 8, null); Human child2 = new Human("Child2", false, 5, null); Human child3 = new Human("Child3", true, 2, null); You are passing in null for the 4th argument that activates the constructor on line 51: public Human(String name, boolean sex, int age, ArrayList<Human> children) Which sets the ArrayList contained within the Human class to null on those three objects. Line 64, inside the toString() method: int childCount = this.children.size(); calls the .size() method on the ArrayList children for each object that toString() is called on. Because the objects 'child1', 'child2', and 'child3' had their respective internal 'children' ArrayLists set to null, this .size() is being called on a non existent object.... which in turn throws a NullPointerException ======================================================== Now we fixed it by creating an actual object for the children variable by setting the 'children' object in each child equal to a new ArrayList. We could have just as easily tested for null in the toString method and replaced line 64 with this:

int childCount;
if(children == null)
    childCount; = 0;
else
    childCount; = children.size();

This would only call the children.size() method if the children variable actually had an object and was not null.

Tom ford

Level 15

17 October 2020, 19:51

Hello Guadalupe, child1.children = new ArrayList<>(); is this another way of adding an element to the children array that is in the human class? I am trying to understand what is exactly going on because my code is a bit different but the solution was the same.