simple objects and methods

Program is to add and average and list amount of times the "addNumber" method is called.

I can make the Amount work, but nothing else

public class Main {
    public static void main(String[] args) {
        NumberStatistics stats = new NumberStatistics();
        stats.addNumber(3);
        stats.addNumber(5);
        stats.addNumber(1);
        stats.addNumber(2);
        System.out.println("Amount: " + stats.amountOfNumbers());
        System.out.println("sum: " + stats.sum());
        System.out.println("average: " + stats.average());
    }

public class NumberStatistics {
    private int amountOfNumbers;
    private int addNumber; 
    private double average; 
    private int sum; 

public NumberStatistics() {
    this.amountOfNumbers=amountOfNumbers; 
    this.average=0; 
    this.sum=0;
}

public void addNumber(int number) {
    number=addNumber; 
    addNumber++; 
    // code here
}
public int amountOfNumbers() {
    return addNumber; 
    // code , here
}
public int sum() {
    return this.addNumber++; 
}

public double average() {
    return sum() / addNumber; 
}

My incorrect output:

Amount: 4 sum: 4 average: 0.0