Null Pointer exception??

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I'm not sure why I am getting a null pointer exception. Or could someone point me in the right direction where to put a try-catch block perhaps. thanks

import java.util.ArrayList;

public class Solution {
    public static void main(String[] args) {
        //write your code here

        // CREATE OBJECTS
        // three children
        Human zach = new Human("Zach", true, 34);
        Human henry = new Human("Henry", true, 2);
        Human evie = new Human("Evie", false, 4);

        // initialize an array and add kids to it
        ArrayList<Human> grandkids = new ArrayList<Human>();
        grandkids.add(henry);
        grandkids.add(evie);
        grandkids.add(zach);

        // one father
        Human ty = new Human("Ty", true, 34, grandkids);

        // one mother.
        Human kate = new Human("Kate", false, 35, grandkids);

        // initialize array and add kids to it
        ArrayList<Human> maleKids = new ArrayList<Human>();
        maleKids.add(ty);
        ArrayList<Human> femaleKids = new ArrayList<Human>();
        femaleKids.add(kate);

        // two grandfathers
        Human clay = new Human("Clay", true, 70, maleKids);
        Human rich = new Human("Rich", true, 77, femaleKids);

        // two grandmothers
        Human marty = new Human("Marty", false, 78, femaleKids);
        Human debbie = new Human("Debbie", false, 65, maleKids);

        // print out all objects using the toString() method
        System.out.println(zach.toString());
        System.out.println(henry.toString());
        System.out.println(evie.toString());
        System.out.println(ty.toString());
        System.out.println(kate.toString());
        System.out.println(marty.toString());
        System.out.println(rich.toString());
        System.out.println(clay.toString());
        System.out.println(debbie.toString());



    }

    public static class Human {
        //write your code here
        // initialize variables
        String name;
        boolean sex;
        int age;
        ArrayList<Human> children;

        // create constructor for everyone but children
        public Human(String _name, boolean _sex, int _age, ArrayList<Human> _children)
        {
            this.name = _name;
            this.sex = _sex;
            this.age = _age;
            this.children = _children;
        }

        // create constructor for children
        public Human(String _name, boolean _sex, int _age)
        {
            this.name = _name;
            this.sex = _sex;
            this.age = _age;
        }

        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;
        }
    }

}

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; /* Make a family */ import java.util.ArrayList; public class Solution { public static void main(String[] args) { //write your code here // CREATE OBJECTS // three children Human zach = new Human("Zach", true, 34); Human henry = new Human("Henry", true, 2); Human evie = new Human("Evie", false, 4); // initialize an array and add kids to it ArrayList<Human> grandkids = new ArrayList<Human>(); grandkids.add(henry); grandkids.add(evie); grandkids.add(zach); // one father Human ty = new Human("Ty", true, 34, grandkids); // one mother. Human kate = new Human("Kate", false, 35, grandkids); // initialize array and add kids to it ArrayList<Human> maleKids = new ArrayList<Human>(); maleKids.add(ty); ArrayList<Human> femaleKids = new ArrayList<Human>(); femaleKids.add(kate); // two grandfathers Human clay = new Human("Clay", true, 70, maleKids); Human rich = new Human("Rich", true, 77, femaleKids); // two grandmothers Human marty = new Human("Marty", false, 78, femaleKids); Human debbie = new Human("Debbie", false, 65, maleKids); // print out all objects using the toString() method System.out.println(zach.toString()); System.out.println(henry.toString()); System.out.println(evie.toString()); System.out.println(ty.toString()); System.out.println(kate.toString()); System.out.println(marty.toString()); System.out.println(rich.toString()); System.out.println(clay.toString()); System.out.println(debbie.toString()); } public static class Human { //write your code here // initialize variables String name; boolean sex; int age; ArrayList<Human> children; // create constructor for everyone but children public Human(String _name, boolean _sex, int _age, ArrayList<Human> _children) { this.name = _name; this.sex = _sex; this.age = _age; this.children = _children; } // create constructor for children public Human(String _name, boolean _sex, int _age) { this.name = _name; this.sex = _sex; this.age = _age; } 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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Nouser

Level 36 , Germany

22 February 2021, 21:27

zach, henry and evie are created with the constructor taking no list and that constructor also doesn't intitialize the children field. It stays at its default (null). When you now try to print a human (toString) you check the lists size (children.size()). For the tree mentioned humans the list is still null and you try to get the size of null. And that is your NullPointerException

Zach

Level 22 , Fort Collins, United States

23 February 2021, 20:50

Thank you for the concise explanation. This makes perfect sense! Figured it out.

Guadalupe Gagnon

Level 37 , Tampa, United States

22 February 2021, 21:19

Line 85. For each child there is no ArrayList created. This means that the default value will be used. With primitives the default value will be 0. ArrayLists are objects though and the default value will be null. When line 85 is reached for a child object it tries to call the method .size() on its "children" field. Whenever you call a method on a null object a NullPointerException will be thrown. To fix this you can either: a) ensure that all fields are initialized when a new object is created, or b) only proceed to lines 92 and below if the children field is initialized (it does not equal null)

Zach

Level 22 , Fort Collins, United States

23 February 2021, 20:51

Thank you for the explanation. I understand it great now!

The Chief

Level 18 , United States

31 March 2021, 23:19

G, I solved this problem by doing option a. But how would you do option b?

Guadalupe Gagnon

Level 37 , Tampa, United States

1 April 2021, 00:54

if(children != null) {
   // ...
}