Collections Utility Class

1. What is Collections?

❗ Not same as Collection (interface vs utility class)
❗ All methods are static
❗ Provides:
- Algorithms (sort, search)
- Wrappers (synchronized, unmodifiable)
- Utility operations (reverse, shuffle, etc.)

Method Signature	Description	Behavior / Edge Case
`void sort(List<T> list)`	Sorts list (natural order)	Elements must implement `Comparable`, else `ClassCastException`
`void sort(List<T> list, Comparator<? super T> c)`	Custom sorting	Comparator decides order

Method Signature	Description	Behavior / Edge Case
`int binarySearch(List<? extends Comparable> list, T key)`	Searches element	List must be sorted, else unpredictable result
`int binarySearch(List<? extends T> list, T key, Comparator<? super T> c)`	Custom search	Uses comparator

List<Integer> list = List.of(3,1,2);
Collections.binarySearch(list, 2);

👉 Result = unpredictable (WRONG usage)

Method Signature	Description	Behavior / Edge Case
`void reverse(List<?> list)`	Reverses list	In-place operation
`void shuffle(List<?> list)`	Random order	Different order each time
`void rotate(List<?> list, int distance)`	Rotates elements	Positive → right shift, negative → left

Method Signature	Description	Behavior / Edge Case
`T min(Collection<? extends T> c)`	Finds minimum	Needs `Comparable`
`T max(Collection<? extends T> c)`	Finds maximum	Needs `Comparable`
`T min(Collection<? extends T> c, Comparator<? super T> comp)`	Custom min	Uses comparator
`T max(Collection<? extends T> c, Comparator<? super T> comp)`	Custom max	Uses comparator

Method Signature	Description	Behavior / Edge Case
`void fill(List<? super T> list, T obj)`	Replaces all elements	Overwrites entire list
`void copy(List<? super T> dest, List<? extends T> src)`	Copies elements	`dest.size()` must be ≥ `src.size()` else `IndexOutOfBoundsException`
`boolean replaceAll(List<T> list, T oldVal, T newVal)`	Replaces occurrences	Returns true if any replaced

List<Integer> src = List.of(1,2,3);
List<Integer> dest = new ArrayList<>();

Collections.copy(dest, src);

✔ Correct:

List<Integer> dest = new ArrayList<>(Arrays.asList(0,0,0));
Collections.copy(dest, src);

Method Signature	Description	Behavior / Edge Case
`int frequency(Collection<?> c, Object o)`	Count occurrences	Uses `equals()`
`boolean disjoint(Collection<?> c1, Collection<?> c2)`	No common elements	Returns true if no overlap

Method Signature	Description	Behavior / Edge Case
`List<T> emptyList()`	Returns empty list	Immutable
`List<T> singletonList(T o)`	Single element list	Immutable
`List<T> unmodifiableList(List<? extends T> list)`	Read-only list	Any modification → `UnsupportedOperationException`

List<Integer> list = Collections.unmodifiableList(List.of(1,2,3));
list.add(4);

👉 ❌ Throws UnsupportedOperationException

Method Signature	Description	Behavior / Edge Case
`List<T> synchronizedList(List<T> list)`	Thread-safe wrapper	Needs manual sync during iteration
`Set<T> synchronizedSet(Set<T> s)`	Thread-safe set	Same limitation
`Map<K,V> synchronizedMap(Map<K,V> m)`	Thread-safe map	Legacy-style sync