这里需要注意的是，List 接口的实现类
插入的值允许为空，也允许重复。ArrayList 1. ArrayList 可以插入空值，也可以插入重复值 2. ArrayList 是基于数组的时候，所以很多数组的特性也直接应用到了 ArrayList。 3. ArrayList 的性能消耗主要来源于扩容和固定位置的增删。 4. ArrayList 创建的时候 需要考虑是否要初始化最小容量，以此避免扩容带来的消耗。
上述的 ArrayList 不是线程安全的。那么 Vector 就可以看作是 ArrayList 的一个线程安全版本,实现同步的方式 是通过 synchronized。由于也是实现了 List 接口，所以也是 可以插入空值，可以插入重复的值。 它和 HashTable 一样，是属于一种同步容器，而不是一种并发容器。（参考《Java并发编程实战》，类似CopyOnWriteArrayList，ConcurrentHashMap这种就属于并发容器）
Iterator接口中的remove会删除上一次调用next返回的值。 删除第一个元素：
Iterator< String> it = c.iterator(); it.next(); it.remove();删除两个相邻的元素：
it.remove(); it.next(); it.remove();Iterator接口中没有add方法（只有对自然有序的集合list使用迭代器添加元素才有意义），而子接口ListIterator中包含add方法（在迭代器位置添加元素）,为迭代器之前的元素set新值。
package java.util; /** * An iterator for lists that allows the programmer * to traverse the list in either direction, modify * the list during iteration, and obtain the iterator's * current position in the list. A {@code ListIterator} * has no current element; its <I>cursor position</I> always * lies between the element that would be returned by a call * to {@code previous()} and the element that would be * returned by a call to {@code next()}. * An iterator for a list of length {@code n} has {@code n+1} possible * cursor positions, as illustrated by the carets ({@code ^}) below: * <PRE> * Element(0) Element(1) Element(2) ... Element(n-1) * cursor positions: ^ ^ ^ ^ ^ * </PRE> * Note that the {@link #remove} and {@link #set(Object)} methods are * <i>not</i> defined in terms of the cursor position; they are defined to * operate on the last element returned by a call to {@link #next} or * {@link #previous()}. * * <p>This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @author Josh Bloch * @see Collection * @see List * @see Iterator * @see Enumeration * @see List#listIterator() * @since 1.2 */ public interface ListIterator<E> extends Iterator<E> { // Query Operations boolean hasNext(); /** * Returns the next element in the list and advances the cursor position. * This method may be called repeatedly to iterate through the list, * or intermixed with calls to {@link #previous} to go back and forth. * (Note that alternating calls to {@code next} and {@code previous} * will return the same element repeatedly.) * * @return the next element in the list * @throws NoSuchElementException if the iteration has no next element */ E next(); /** * Returns {@code true} if this list iterator has more elements when * traversing the list in the reverse direction. (In other words, * returns {@code true} if {@link #previous} would return an element * rather than throwing an exception.) * * @return {@code true} if the list iterator has more elements when * traversing the list in the reverse direction */ boolean hasPrevious(); /** * Returns the previous element in the list and moves the cursor * position backwards. This method may be called repeatedly to * iterate through the list backwards, or intermixed with calls to * {@link #next} to go back and forth. (Note that alternating calls * to {@code next} and {@code previous} will return the same * element repeatedly.) * * @return the previous element in the list * @throws NoSuchElementException if the iteration has no previous * element */ E previous(); /** * Returns the index of the element that would be returned by a * subsequent call to {@link #next}. (Returns list size if the list * iterator is at the end of the list.) * * @return the index of the element that would be returned by a * subsequent call to {@code next}, or list size if the list * iterator is at the end of the list */ int nextIndex(); /** * Returns the index of the element that would be returned by a * subsequent call to {@link #previous}. (Returns -1 if the list * iterator is at the beginning of the list.) * * @return the index of the element that would be returned by a * subsequent call to {@code previous}, or -1 if the list * iterator is at the beginning of the list */ int previousIndex(); // Modification Operations /** * Removes from the list the last element that was returned by {@link * #next} or {@link #previous} (optional operation). This call can * only be made once per call to {@code next} or {@code previous}. * It can be made only if {@link #add} has not been * called after the last call to {@code next} or {@code previous}. * * @throws UnsupportedOperationException if the {@code remove} * operation is not supported by this list iterator * @throws IllegalStateException if neither {@code next} nor * {@code previous} have been called, or {@code remove} or * {@code add} have been called after the last call to * {@code next} or {@code previous} */ void remove(); /** * Replaces the last element returned by {@link #next} or * {@link #previous} with the specified element (optional operation). * This call can be made only if neither {@link #remove} nor {@link * #add} have been called after the last call to {@code next} or * {@code previous}. * * @param e the element with which to replace the last element returned by * {@code next} or {@code previous} * @throws UnsupportedOperationException if the {@code set} operation * is not supported by this list iterator * @throws ClassCastException if the class of the specified element * prevents it from being added to this list * @throws IllegalArgumentException if some aspect of the specified * element prevents it from being added to this list * @throws IllegalStateException if neither {@code next} nor * {@code previous} have been called, or {@code remove} or * {@code add} have been called after the last call to * {@code next} or {@code previous} */ void set(E e); /** * Inserts the specified element into the list (optional operation). * The element is inserted immediately before the element that * would be returned by {@link #next}, if any, and after the element * that would be returned by {@link #previous}, if any. (If the * list contains no elements, the new element becomes the sole element * on the list.) The new element is inserted before the implicit * cursor: a subsequent call to {@code next} would be unaffected, and a * subsequent call to {@code previous} would return the new element. * (This call increases by one the value that would be returned by a * call to {@code nextIndex} or {@code previousIndex}.) * * @param e the element to insert * @throws UnsupportedOperationException if the {@code add} method is * not supported by this list iterator * @throws ClassCastException if the class of the specified element * prevents it from being added to this list * @throws IllegalArgumentException if some aspect of this element * prevents it from being added to this list */ void add(E e); }set方法用一个新元素取代上一次调用next或previous返回的值 用新值取代第一个元素：
ListIterator<String> iter = list.listIterator(); String oldValue = iter.next(); iter.set(newValue);对象的比较： 1. 对象实现Comparable接口，重写public int compareTo(T other)方法。 2. 将Compatator对象传入TreeSet构造器
package treeSet; import java.util.*; /** * An item with a description and a part number. */ public class Item implements Comparable<Item> { private String description; private int partNumber; /** * Constructs an item. * * @param aDescription * the item's description * @param aPartNumber * the item's part number */ public Item(String aDescription, int aPartNumber) { description = aDescription; partNumber = aPartNumber; } /** * Gets the description of this item. * * @return the description */ public String getDescription() { return description; } public String toString() { return "[descripion=" + description + ", partNumber=" + partNumber + "]"; } public boolean equals(Object otherObject) { if (this == otherObject) return true; if (otherObject == null) return false; if (getClass() != otherObject.getClass()) return false; Item other = (Item) otherObject; return Objects.equals(description, other.description) && partNumber == other.partNumber; } public int hashCode() { return Objects.hash(description, partNumber); } public int compareTo(Item other) { return Integer.compare(partNumber, other.partNumber); } } package treeSet; /** @version 1.12 2012-01-26 @author Cay Horstmann */ import java.util.*; /** This program sorts a set of item by comparing their descriptions. */ public class TreeSetTest { public static void main(String[] args) { SortedSet<Item> parts = new TreeSet<>(); //默认比较器，实现Comparable接口 parts.add(new Item("Toaster", 1234)); parts.add(new Item("Widget", 4562)); parts.add(new Item("Modem", 9912)); System.out.println(parts); SortedSet<Item> sortByDescription = new TreeSet<>(new //匿名内部类，定制的比较器 Comparator<Item>() { public int compare(Item a, Item b) { String descrA = a.getDescription(); String descrB = b.getDescription(); return descrA.compareTo(descrB); } }); sortByDescription.addAll(parts); System.out.println(sortByDescription); } }PriorityQueue: 堆（heap）
package priorityQueue; import java.util.*; /** * This program demonstrates the use of a priority queue. * @version 1.01 2012-01-26 * @author Cay Horstmann */ public class PriorityQueueTest { public static void main(String[] args) { PriorityQueue<GregorianCalendar> pq = new PriorityQueue<>(); pq.add(new GregorianCalendar(1906, Calendar.DECEMBER, 9)); // G. Hopper pq.add(new GregorianCalendar(1815, Calendar.DECEMBER, 10)); // A. Lovelace pq.add(new GregorianCalendar(1903, Calendar.DECEMBER, 3)); // J. von Neumann pq.add(new GregorianCalendar(1910, Calendar.JUNE, 22)); // K. Zuse System.out.println("Iterating over elements..."); for (GregorianCalendar date : pq) System.out.println(date.get(Calendar.YEAR)); System.out.println("Removing elements..."); while (!pq.isEmpty()) System.out.println(pq.remove().get(Calendar.YEAR)); } }Collection的功能方法：
