# Sets, Iterating & the Collections Class — Core Java

Source: https://www.geekswithgeeks.com/en/core-java/java-collections-utility

> Round out the Set family, learn the safe ways to iterate (and remove during iteration), and use java.util.Collections helpers for sorting, immutability and synchronization.

## HashSet, LinkedHashSet, TreeSet

The `Set` family mirrors the `Map` family exactly, because each is internally backed by a corresponding map: `HashSet`→`HashMap`, `LinkedHashSet`→`LinkedHashMap`, `TreeSet`→`TreeMap`. Same ordering trade-offs apply.

## Removing while iterating

Modifying a collection with `list.remove(...)` while a for-each loop walks it throws `ConcurrentModificationException`. Use the `Iterator`'s own `remove()`, or `removeIf`, instead.

```java
List<Integer> nums = new ArrayList<>(List.of(1, 2, 3, 4, 5));

// Safe:
nums.removeIf(n -> n % 2 == 0);

// Also safe:
Iterator<Integer> it = nums.iterator();
while (it.hasNext()) {
    if (it.next() > 3) it.remove();
}
```

## java.util.Collections helpers

The `Collections` utility class (not to be confused with the `Collection` interface) offers static helpers: sort a list in place, wrap one as unmodifiable, or make it thread-safe.

```java
List<Integer> nums = new ArrayList<>(List.of(3, 1, 2));
Collections.sort(nums);
Collections.reverse(nums);

List<Integer> readOnly = Collections.unmodifiableList(nums);
List<Integer> safe = Collections.synchronizedList(nums);
```