Question

public class Person{
private Disease currentDisease;
private String name;
private Point position;
private List<Susceptible> othersInfected;

public Person(Point position,String name){
this.position =position;
this.name = name;
List<Susceptible> othersInfected = new ArrayList<>();
}

public void addToTracing(Susceptible person){
othersInfected.add(person);
}
public List<Susceptible> getOthersInfected(){
return this.othersInfected;
}
}

I got nullpointerException error when using addToTracing method, how do I fix it

9 List<Susceptible> folks = new ArrayList<>(); 10 11 folks.add(new Person (new Point(0,0), "Ann")); 12 folks.add(new Person (new Point(4,23), "Mike")); 13 folks.add(new Person (new Point(0,43), "Larry")); 14 folks.add(new Person(new Point(0,0), "Kacey")); 15 16 for (Susceptible person : folks) { 17 p.addToTracing (person); 18 } 19 20 21 if(!folks.equals(p.getothersInfected ( ) ) ) { System.out.println("We added 4 people to the return false; 22 23 } 24 return true;

Answer 1

Hi There,

When I try to compile the person class i get the below error. This is because Person class in not extending Susceptible class.

By adding the below statement this error was resolved.

public class Person extends Susceptible

Then the above mentioned nullpointerException was observed.

The code below was used to check the issue. it was observed that othersInfected was null.

The Reson for othersInfected to be null is the following statement in constructor.

This is allocating memory to a local variable in the constuctor. changing this to the below code will allocate memory to the member variable and executes without any exceptions.

It can also be changed to the code below to get rid of the null pointer exception.

Summarizing:

1. Exception was due to the fact that memory was allocated to the local variable created in the constructor and not to the member variable of the class.

2. Solution is to allocate memory to the member variable. either by any one of the below methods.

OR

private List<Susceptible> othersInfected = new ArrayList<>();   
public Person(Point position,String name)
{
this.position =position;
this.name = name;
//othersInfected
}

OR

private List<Susceptible> othersInfected;
public Person(Point position,String name)
{
this.position =position;
this.name = name;
othersInfected = new ArrayList<>();
}

Please Comment below if you need more information.