Selected Java Collections

Modifier and Type	Method and Description
`boolean`	`add(E e)` Appends the specified element to the end of this list. (Always returns `true`.)
`void`	`add(int index, E element)` Inserts the specified element at the specified position in this list.
`void`	`clear()` Removes all of the elements from this list.
`Object`	`clone()` Returns a shallow copy of this `ArrayList` instance.
`boolean`	`contains(E o)` Returns `true` if this list contains the specified element.
`E`	`get(int index)` Returns the element at the specified position in this list.
`int`	`indexOf(E o)` Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element.
`boolean`	`isEmpty()` Returns `true` if this list contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this list in proper sequence.
`int`	`lastIndexOf(Object o)` Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element.
`E`	`remove(int index)` Removes the element at the specified position in this list.
`boolean`	`remove(E o)` Removes the first occurrence of the specified element from this list, if it is present. Returns `true` if the element was present.
`protected void`	`removeRange(int fromIndex, int toIndex)` Removes from this list all of the elements whose index is between `fromIndex`, inclusive, and `toIndex`, exclusive.
`E`	`set(int index, E element)` Replaces the element at the specified position in this list with the specified element.
`int`	`size()` Returns the number of elements in this list.
`Object[]`	`toArray()` Returns an array containing all of the elements in this list in proper sequence (from first to last element).
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array.
`void`	`trimToSize()` Trims the capacity of this `ArrayList` instance to be the list's current size.

Modifier and Type	Method and Description
`boolean`	`add(E e)` Adds the specified element to this set if it is not already present.
`void`	`clear()` Removes all of the elements from this set.
`Object`	`clone()` Returns a shallow copy of this `HashSet` instance: the elements themselves are not cloned.
`boolean`	`contains(Object o)` Returns `true` if this set contains the specified element.
`boolean`	`isEmpty()` Returns `true` if this set contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this set.
`boolean`	`remove(Object o)` Removes the specified element from this set if it is present.
`int`	`size()` Returns the number of elements in this set (its cardinality).

Modifier and Type	Method and Description
`void`	`clear()` Removes all of the mappings from this map.
`Object`	`clone()` Returns a shallow copy of this `HashMap` instance: the keys and values themselves are not cloned.
`boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`V`	`get(Object key)` Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
`boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`V`	`put(K key, V value)` Associates the specified value with the specified key in this map.
`V`	`remove(Object key)` Removes the mapping for the specified key from this map if present.
`int`	`size()` Returns the number of key-value mappings in this map.

Modifier and Type	Method and Description
`boolean`	`hasNext()` Returns `true` if the iteration has more elements.
`E`	`next()` Returns the next element in the iteration.
`void`	`remove()` Removes from the underlying collection the last element returned by this iterator. This optional operation should throw `UnsupportedOperationException` if not supported.