Java中Collections

本文主要是介绍Java中Collections，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

Collection是集合类的一个顶级接口，其直接继承接口有List与Set

而Collections则是集合类的一个工具类/帮助类，其中提供了一系列静态方法，用于对集合中元素进行排序、搜索以及线程安全等各种操作。此类不能实例化。Collections中有一些工具函数，比如说sort、reverse、fill等等。

看到了之后比较的好奇呗，在hashtable的时候进行同步，为了防止引用返回后别修改，使用了Collections.unmodifiableCollection 这个东西。
这个的主要用途是。可以得到一个集合的镜像。防止返回的东西被修改。只能查看当前集合的值，而不能去修改这个值。

看看源码就比较清楚这个过程了。

static class UnmodifiableCollection<E> implements Collection<E>, Serializable {private static final long serialVersionUID = 1820017752578914078L;final Collection<? extends E> c;UnmodifiableCollection(Collection<? extends E> c) {if (c==null)throw new NullPointerException();this.c = c;}public int size()                   {return c.size();}public boolean isEmpty()            {return c.isEmpty();}public boolean contains(Object o)   {return c.contains(o);}public Object[] toArray()           {return c.toArray();}public <T> T[] toArray(T[] a)       {return c.toArray(a);}public String toString()            {return c.toString();}public Iterator<E> iterator() {return new Iterator<E>() {private final Iterator<? extends E> i = c.iterator();public boolean hasNext() {return i.hasNext();}public E next()          {return i.next();}public void remove() {throw new UnsupportedOperationException();}@Overridepublic void forEachRemaining(Consumer<? super E> action) {// Use backing collection versioni.forEachRemaining(action);}};}public boolean add(E e) {throw new UnsupportedOperationException();}public boolean remove(Object o) {throw new UnsupportedOperationException();}public boolean containsAll(Collection<?> coll) {return c.containsAll(coll);}public boolean addAll(Collection<? extends E> coll) {throw new UnsupportedOperationException();}public boolean removeAll(Collection<?> coll) {throw new UnsupportedOperationException();}public boolean retainAll(Collection<?> coll) {throw new UnsupportedOperationException();}public void clear() {throw new UnsupportedOperationException();}// Override default methods in Collection@Overridepublic void forEach(Consumer<? super E> action) {c.forEach(action);}@Overridepublic boolean removeIf(Predicate<? super E> filter) {throw new UnsupportedOperationException();}@SuppressWarnings("unchecked")@Overridepublic Spliterator<E> spliterator() {return (Spliterator<E>)c.spliterator();}@SuppressWarnings("unchecked")@Overridepublic Stream<E> stream() {return (Stream<E>)c.stream();}@SuppressWarnings("unchecked")@Overridepublic Stream<E> parallelStream() {return (Stream<E>)c.parallelStream();}}

来个小例子。可以对这个集合修改后，由于引用了，所以可以得到这个引用的值。同事呢，在修改增加的时候是不允许这样处理的。非常棒的采用代理的模式，让我们集合的某一些属性被隐藏不能让别人进行处理。对于多线程中，这样保持，属性的封闭性起到了非常棒的效果。

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;public class Test1 {public static void main(String[] args) {// TODO Auto-generated method stubCollection<String> c = new ArrayList<String>();  Collection<String> s = Collections.unmodifiableCollection(c);  c.add("str");//s.add("ddd");/* Exception in thread "main" java.lang.UnsupportedOperationExceptionat java.util.Collections$UnmodifiableCollection.add(Collections.java:1075)at test.Test1.main(Test1.java:16)*/          System.out.println(s); //str}}

除了这些之外，还有一些简单粗暴的封装一些通用的集合，进行多线程的一致性的处理！
这些处理的时候，都采用了，很单一的方式维护为唯一性。Synchronized
synchronized关键字的作用域有二种：
1）是某个对象实例内，

synchronized aMethod(){}可以防止多个线程同时访问这个对象的synchronized方法（如果一个对象有多个synchronized方法，只要一个线程访问了其中的一个synchronized方法，其它线程不能同时访问这个对象中任何一个synchronized方法）

这时，不同的对象实例的synchronized方法是不相干扰的。也就是说，其它线程照样可以同时访问相同类的另一个对象实例中的synchronized方法；
2）是某个类的范围，synchronized static aStaticMethod{}防止多个线程同时访问这个类中的synchronized static 方法。它可以对类的所有对象实例起作用。

    static class SynchronizedCollection<E> implements Collection<E>, Serializable {private static final long serialVersionUID = 3053995032091335093L;final Collection<E> c;  // Backing Collectionfinal Object mutex;     // Object on which to synchronizeSynchronizedCollection(Collection<E> c) {this.c = Objects.requireNonNull(c);mutex = this;}SynchronizedCollection(Collection<E> c, Object mutex) {this.c = Objects.requireNonNull(c);this.mutex = Objects.requireNonNull(mutex);}public int size() {synchronized (mutex) {return c.size();}}public boolean isEmpty() {synchronized (mutex) {return c.isEmpty();}}public boolean contains(Object o) {synchronized (mutex) {return c.contains(o);}}public Object[] toArray() {synchronized (mutex) {return c.toArray();}}public <T> T[] toArray(T[] a) {synchronized (mutex) {return c.toArray(a);}}public Iterator<E> iterator() {return c.iterator(); // Must be manually synched by user!}public boolean add(E e) {synchronized (mutex) {return c.add(e);}}public boolean remove(Object o) {synchronized (mutex) {return c.remove(o);}}public boolean containsAll(Collection<?> coll) {synchronized (mutex) {return c.containsAll(coll);}}public boolean addAll(Collection<? extends E> coll) {synchronized (mutex) {return c.addAll(coll);}}public boolean removeAll(Collection<?> coll) {synchronized (mutex) {return c.removeAll(coll);}}public boolean retainAll(Collection<?> coll) {synchronized (mutex) {return c.retainAll(coll);}}public void clear() {synchronized (mutex) {c.clear();}}public String toString() {synchronized (mutex) {return c.toString();}}// Override default methods in Collection@Overridepublic void forEach(Consumer<? super E> consumer) {synchronized (mutex) {c.forEach(consumer);}}@Overridepublic boolean removeIf(Predicate<? super E> filter) {synchronized (mutex) {return c.removeIf(filter);}}@Overridepublic Spliterator<E> spliterator() {return c.spliterator(); // Must be manually synched by user!}@Overridepublic Stream<E> stream() {return c.stream(); // Must be manually synched by user!}@Overridepublic Stream<E> parallelStream() {return c.parallelStream(); // Must be manually synched by user!}private void writeObject(ObjectOutputStream s) throws IOException {synchronized (mutex) {s.defaultWriteObject();}}}