JAVA集合框架之Collection

JAVA集合框架

前言

想研究框架源码，也不知道从那个框架开始，网上有篇博客说
JAVA集合框架->IOStream->struts->《how tomcat work》
不管了 就按他说的来吧

Collection

Collection是所有集合类的总接口，它在java.util包下

我们看到，常见的List Queue Set都是接口，它们继承了Collection接口

而Collection本身又继承了Iterable接口，这就使Collection一定是可迭代的，所以实现了Collection接口的具体类也必然要实现Iterable接口的方法

Collection接口都抽象出了那些通用方法？

那么，List Queue Set 一类的子接口都有那些共同“行为”可以提取到父接口呢？

从名字可以很容易的判断这些方法的含义
如果面试官问起了 我们就从增删改查4个方面来回忆
增：add , addAll
删：remove removeAll removeIf
改：
查：iterator contains containsAll
再来几个加分项：retainAll（取交集） stream(jdk8新特性 用于支持map（）等函数编程)
这些方法看似简单，其实来看一下源码，这些抽象函数都定义了许多规范，希望继承Collection接口的时候遵守
如add（E e）方法

/**
     * Ensures that this collection contains the specified element (optional
     * operation).  Returns <tt>true</tt> if this collection changed as a
     * result of the call.  (Returns <tt>false</tt> if this collection does
     * not permit duplicates and already contains the specified element.)<p>
     *
    * 不同实现类对于e的限制不同，建议写文档中
     * Collections that support this operation may place limitations on what
     * elements may be added to this collection.  In particular, some
     * collections will refuse to add <tt>null</tt> elements, and others will
     * impose restrictions on the type of elements that may be added.
     * Collection classes should clearly specify in their documentation any
     * restrictions on what elements may be added.<p>
     * 如果有特殊的原因（不是因为对象已存在于集合中）不同意添加接收到的e，应抛出异常而不是返回false
     * If a collection refuses to add a particular element for any reason
     * other than that it already contains the element, it <i>must</i> throw
     * an exception (rather than returning <tt>false</tt>).  This preserves
     * the invariant that a collection always contains the specified element
     * after this call returns.
     *
     * @param e element whose presence in this collection is to be ensured
     * @return <tt>true</tt> if this collection changed as a result of the
     *         call
    * 在什么情况下应该抛出什么异常
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this collection
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this collection
     * @throws NullPointerException if the specified element is null and this
     *         collection does not permit null elements
     * @throws IllegalArgumentException if some property of the element
     *         prevents it from being added to this collection
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to insertion restrictions
     */

optional operation的含义：

有些实现类可以不支持某些方法，比如一个不可更改的集合，不想支持add方法，则可以在overrride时直接抛出UnsupportedOperationException 异常

toArray() 和 toArray(T[])

前者返回一个Object[]
后者通过传递一个具体类型的数组，通过反射，返回一个具体类型的数组：
String str[] = strList.toArray(new String[0]);