Iterable, Comparable, Collection

✅ Iterable, Iterator

• 처음부터 끝까지 순회하기
• array: iterate with index
• node: iterate with node reference(before and after node)
• tree: iterate with parent node, child node
• ➡️ Java provides iterator for all data architecture type

✅ Java Iterator

• hasNext(): 다음 값이 있니?
• next(): 다음으로 간다

 ArrayList<Integer> arrayList = new ArrayList<>();
arrayList.add(1);
arrayList.add(2);
arrayList.add(3);

Iterator<Integer> iterator = arrayList.iterator(); //get iterator
while(iterator.hasNext()){ //hasNext(): 다음이 있어?
    Integer next = iterator.next(); //next(): 있으면 다음꺼 반환해
}

✅ Comparable, Comparator

• 데이터를 정렬하기

• Arrays.sort()

Integer[] array = {3, 2, 1};
Arrays.sort(array); //정렬 {1, 2, 3};

• 내가 정렬 기준을 만들고 싶다면?
• create class that implement Comparator
• override compare()

  static class AscComparator implements Comparator<Integer>{ //implement Comparator
        @Override
        public int compare(Integer o1, Integer o2) {
            System.out.println("o1: " + o1 +" o2 = " + o2);
            return (o1 < o2) ? -1 : (o1 == o2) ? 0 : 1 ; //ascending
        }
    }

Arrays.sort(array, new AscComparator());  //array, my order class
Arrays.sort(array, new AscComparator().reversed()); //reverse 3->2->1

✅ Comparator to make my own compare()

• ✔️ I want to compare based on user age
• in MyUser class
• implement comparable

public class MyUser implements Comparable<MyUser> { //implement comparable

    @Override
    public int compareTo(MyUser o) { //age ascending order
        return age > o.getAge() ? 1 : (age == o.getAge()) ? 0 : -1;
    }
}

• ✔️ I want to create another sort, sort with Id
• create new class IdComparator
• implements Comparator

public class IdComparator implements Comparator<MyUser> { //implements Comparator
    @Override
    public int compare(MyUser o1, MyUser o2) {
        return o1.getId().compareTo(o2.getId()); //compare ascending Id
    }

}

• ✔️ Main class

//use compareTo in MyUser class
Arrays.sort(array); //order in age 10 -> 20 -> 30

//use IdComparator class
Arrays.sort(array, new IdComparator()); //order in iD A->B->c
Arrays.sort(array, new IdComparator().reversed()); //order in ID C->B->A

☑️ Comparator in linkedList

• ✔️ Main class

//use compareTo in MyUser class
linkedList.sort(null); //order in age 10 -> 20 -> 30

//use IdComparator class
linkedList.sort(new IdComparator()); //order in iD A->B->c

☑️ Comparator in tree

• in tree, order & input value is done at the same time
• 트리에서는 자료를 넣을 때부터 정렬해서 넣기 때문에 따로 정렬이 필요가 없음

TreeSet<MyUser> treeSet = new TreeSet<>();
//이미 treeSet은 myUser을 age순서로 정렬해 저장해 둠

//use IdComparator class
//I want to order user with Id
//input in tree constructor
TreeSet<MyUser> treeSet = new TreeSet<>(new IdComparator()); //use IdComparator compare()

✅ Collection

//max
Integer max = Collections.max(list);

//shuffle
Collections.shuffle(list);

//sort
Collections.sort(list);

//reverse
Collections.reverse(list);

//create immutable list
List<Integer> list = List.of(1, 2, 3);

//make list multi-thread safe
Collections.synchronizedList(arrayList);

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