Why am I getting "java.lang.NullPointerException" w/ my ArrayList?

I am making a class to deal with a friend list scenario using an ArrayList and I'm not sure what I have done wrong. The "java.lang.NullPointerException" occurred the moment I called the addFriend method and I can't seem to troubleshoot exactly why this is happening. Please give me some hints in the right direction!

public class Person {
    private String name; 
    private ArrayList<String> friends;


    public Person(String name) {
        this.name = name;
        this.friends = friends;
    }

    public String getName() {
        return this.name;
    }

    public void addFriend(String friend) {
        friends.add(friend);     
    }

    public boolean hasFriend(String name) {
        for(String friend : this.friends) {
            if(name.equals(friend)) {
                return true;
            }
        }
    return false;   
    }

    public String getFriends() {
        String stringOfFriends=friends.toString();
        return stringOfFriends;
    }

    public String unfriend(String friend) {
        if (friends.contains(friend)) {
            friends.remove(friend);
        }
    return friends.toString();    
    }



}

Copyright License：
Author:「hattic」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/42549371/why-am-i-getting-java-lang-nullpointerexception-w-my-arraylist