教程：设置值

import java.util.*;

class Aquarium {
    private ArrayList<Fish> fish;

    public static void main(String[] args) {
        var aquarium = new Aquarium();
        System.out.println(aquarium.getFish());
        // fish has already been initialized
        System.out.println(aquarium.getFish()); // line n1
    }

    private ArrayList<Fish> getFish() {
        if (fish == null) initFish();
        return fish;
    }

    private void initFish() {
        fish = new ArrayList<>(Arrays.asList(
                new Fish("Bubbles"),
                new Fish("Calypso"),
                new Fish("Dory")
        ));
    }
}

class Fish {
    private String name;

    Fish(String name) {
        this.name = name;
    }

    public String toString() {
        return name;
    }
}