Implementing compareTo() and Comparable-interface

Question

I'm trying to implement Comparable and compareTo(), but can't seem to get it to work. I've been trying different methods but I don't really get it. I know I'm supposed to implement the comparable interface, and that I need to create the method compareTo() before using it (strange for me, coming from python to object-oriented programming).

I would like it to compare the ages of two humans, so I tried writing the code as seen below, but it seems I can't use compareTo(). I'm getting the error message: "This method must return an int type", but as I see it, I am only returning 1, -1 and 0, which are int?

Also, I know the print-row in the end is wrong. How can I change it so it says for example: "Name1, 25 yrs old, is older than Name2, 21 yrs old". In python I could extract 2 specific values from a list and compare them with some given method; I'm not sure how to do the extraction for 2 different values in Java.

import java.util.ArrayList;
import java.util.Random;

class Human implements Comparable<Human>{

    int age;
    String name;

    public Human (int myAge, String myName) {
        name = myName;
        age = myAge;

    }

    public Human() {
        this(randomAge(),randomName());
    }

    public int compareTo(Human o) {
        if (this.age > o.age) {
            return 1;
        }
        if (this.age < o.age) {
            return -1;
        }
        if (this.age == o.age) {
            return 0;
        }
    }

    protected static int randomAge() {
        Random randomGenerator = new Random();      
        return randomGenerator.nextInt(99);
    }

    protected static String randomName() {
        Random randomGenerator = new Random();
        return "Name"+randomGenerator.nextInt(15);
    }

    public int getAge(){
        return age;
    }

    public String getName() {
        return name;
    }

    public String toString() {
        return "\nName: " + name + "\nAge: " + age + " yrs old\n";
    }

    public static void main (String[] args) {

        ArrayList<Human> randomHumans = new ArrayList<Human>();
        for (int j=0;j<2;j++) {
            Human randomPerson = new Human();
            randomHumans.add(randomPerson);
        }
        System.out.println(insert.something.here);
    }

}

Answer 1

So using three if-statements does not work, which is why I could have written an if/else if/else statement instead, but another simpler solution would have been to write this in the compareTo(Human o)-method;

return this.age - o.age

Then for the print in the end;

int b = randomHumans.get(0).compareTo(randomHumans.get(1));

if (b>0) {
    System.out.println(randomHumans.get(0).getName()+", "+randomHumans.get(0).getAge()+" yrs old, is older than "+randomHumans.get(1).getName()+", "+randomHumans.get(1).getAge()+" yrs old.");
}
else if (b<0) {
    System.out.println(randomHumans.get(0).getName()+", "+randomHumans.get(0).getAge()+" yrs old, is younger than "+randomHumans.get(1).getName()+", "+randomHumans.get(1).getAge()+" yrs old.");
}
else {
    System.out.println(randomHumans.get(0).getName()+", "+randomHumans.get(0).getAge()+" yrs old, is just as old as "+randomHumans.get(1).getName()+", "+randomHumans.get(1).getAge()+" yrs old.");
}