java中ArrayList中的ArrayList

Question

If I have the code likes this List<List<Maindish>> meal , the List<Maindish> is in other class like: public Maindish (salad, rice, drink) . So if I want the value of the second maindish (rice) in the first meal, (I may need to go through the loop later for all valuable) should I do something like:

for (i = 0; i < meal.length(); i++) {
        return meal[i][1];
    }

Answer 1

The data type you are using is using is Lists not Arrays. Try using List.get(int i) other than [].

Answer 2

If you want to do an operation on first elements of your sub-lists, you can try the code below:

 for (List<Maindish> list : meal) {
        System.out.println(list.get(0));    
    }

Answer 3

First you need to create setter / getter for your model class Maindish

than you can iterate like below to get first item in your sub list

     // Assuming your sublist have 1 Maindish
    for(List<Maindish> submeal : meal){
        String rice = submeal.get(0).getRice(); // Assuming getRice() is your getter and return type is String
    }

Answer 4

You don't have to iterate by each element to read or write elemts in Lists. You can utilize the following methods.

• add(int index, E element)
• addAll(Collection c)
• addAll(int index, Collection c)
• get(int index)
• set(int index, E element)

You can find the full list of methods available here .

you ask help with value of the second maindish (rice) in the first meal , Since this is a list of a list including maindishes what we can retrieve is the second maindish in the first meal of the first list of meals . that can archive by the following code.

meal.get(0).get(0).getRice();

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Assumption : That you have implemented all the setter and getters in Maindish class.

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Answer 5

If you are saying at least for this example that you want to always return the 2nd main dish for each meal, you would probably need to build another ArrayList of only the 2nd main dishes. By returning in your example, you will only get the first middle dish no matter how many items are in meal. I'm making a lot of assumptions because you didn't give enough info, but if we say that the actual dishes stored in a Maindish object are strings, then you could do the following:

ArrayList<String> secondMains = new ArrayList<>();
Iterator<MainDish> it = meal.iterator();

while(it.hasNext()) {
    Maindish mainDish = it.next();
    if (mainDish.size() >= 2) {
        secondMains.add(mainDish.get(1));
    }
}
return secondMains;