125 lines
3.6 KiB
Markdown
125 lines
3.6 KiB
Markdown
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---
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title: Collections
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localeTitle: 集合
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---
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# 集合
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Java中的Collection是一组可以排序（LinkedList）或无序（Set）的对象。 Collection接口位于层次结构的顶部，所有其他类和接口都从此接口扩展。它位于java.util包中。
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Collection接口还扩展了Iterable接口，这意味着java中的每个集合都必须是可迭代的。这反过来意味着for-each循环可用于按顺序获取它们。
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```java
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public interface Collection<E> extends Iterable<E>
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```
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此接口提供的一些最常用的方法是：
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```java
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boolean add(E e) // Adds the specified element to the collection if not present and returns true if this collection changed.
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void clear() // Removes all the elements from the collection.
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boolean contains(Object o) // Returns true if the specified element is in the collection else false
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boolean isEmpty() // Returns true if the collection is empty else false
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boolean remove(Object o) // Removes the specifies element and return true on successful removal else false.
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int size() // Returns number of items in the collection.
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```
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这些和其他各种方法必须由实现Collection接口的任何类实现。
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## 接口扩展Collection接口
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扩展集合接口的其他重要接口是：
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组： 仅包含唯一元素的集合。
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队列： 实现队列行为，其中元素仅在开头添加并从末尾删除。
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列表： 此集合处理对象的列表/序列。
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这四个接口（Collection，Set，Queue，List）以及SortedSet，Deque和NavigableSet构成了集合Collection层次结构。
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# LinkedList类
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LinkedList是最重要的Collection类之一，它提供了双向链表实现。它实现了List，Deque，Cloneable和Serializable接口。
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**创建LinkedList**
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```java
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LinkedList<Integer> intList = new LinkedList<Integer>(); // Creates a new list of Integer objects.
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```
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您还可以创建任何其他对象类型的列表。例如。
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```java
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LinkedList<String> stringList = new LinkedList();
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LinkedList<LinkedList<Integer>> listOfList = new LinkedList();
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```
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注意：自JDK 1.5以来，Java中的所有集合都已转换为泛型类型
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**将元素添加到列表中**
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```java
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intList.add(new Integer(1)); // Add 1 to the end.
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intList.add(2); // This works as Java provides autoboxing and unboxing of primitive datatypes and their respective wrapper classes
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intList.addFirst(3); // Add to the beginning of the list
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intList.addLast(2); // Add to the end of the list
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intList.add(2, 5); // Add element 5 at index 2
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```
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让我们打印清单
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```java
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System.out.println(intList); // toString() method is automatically called on the list
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```
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输出： \[3,1,5,2,2\]
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**从列表中检索元素**
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```java
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intList.get(3); // Returns element at index 3 ie 2
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intList.getFirst(); // Get the first element ie 3
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intList.getLast(); // Returns last element ie 2
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intList.indexOf(2); // Returns first occured index of 2 ie 3
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intList.lastIndexOf(2); // Returns last occured index of 2 ie 4
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```
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**LinkedList作为堆栈**
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由于Java没有提供单独的
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```java
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intList.push(5); // Add element to the end of list. Works same as addLast()
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intList.pop(); // Removes and returns the last element of the list.
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```
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**从列表中删除元素**
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```java
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intList.remove(3); // Removes the element at index 3 of the list
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intList.removeFirst(); // Removes first element of the list
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intList.removeLast(); // Removes last element of the list
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```
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注意：上述所有用于删除和获取元素的方法都会在空列表中返回NoSuchElementException。
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#### 更多信息：
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* 来源： [Java文档](https://docs.oracle.com/javase/9/docs/api/overview-summary.html)
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