Collection接口实现类的特点:
public interface Collection<E> extends Iterable<E>
- collection实现子类可以存放多个元素,每个元素可以是object
- 有些Collection的实现类,可以存放重复的元素,有些不可以
- 有些Collection的实现类,有些是有序的List,有些是无序的Set
- Collection接口没有直接的实现子类,是通过他的子接口Set和List来实现的
Collection接口和常用方法:
ArrayList为例:
public class CollectionMethod {
public static void main(String[] args) {
//add添加元素
ArrayList arrayList = new ArrayList();
arrayList.add("jack");
arrayList.add(0);
arrayList.add(true);
System.out.println(arrayList);
//remove删除元素
arrayList.remove(true);
System.out.println(arrayList);
//contains查找元素是否存在,返回值为boolean
System.out.println(arrayList.contains("jack"));
//size获取元素个数
System.out.println(arrayList.size());
//isEmpty判断是否为空
System.out.println(arrayList.isEmpty());
//clear清空
arrayList.clear();
System.out.println(arrayList.isEmpty());
//addAll添加多个元素
ArrayList arrayList1 = new ArrayList();
arrayList1.add("三国演义");
arrayList1.add("红楼梦");
arrayList.addAll(arrayList1);
System.out.println(arrayList);
//containsAll 查找多个元素是否都存在
System.out.println(arrayList.containsAll(arrayList1));
//removeAll 删除多个元素
arrayList.removeAll(arrayList1);
System.out.println(arrayList);
}
}
Collection接口遍历元素的方式1-使用迭代器Iterator
-
- Iterator对象称为迭代器,主要用于遍历Collection集合中的元素
- 所有实现了Collection接口的集合类都有一个iterator()方法,用于返回一个实现了Iterator接口的对象,即可以返回一个迭代器
- Iterator的结构
- Iterator仅用于遍历集合,Iterator本身不存放对象
- 迭代器执行原理:
- Iterator iterator = coll.iterator();//得到一个集合的迭代器
- hasNext():判断是否还有下一个元素
- while(iterator.hasNext())
- next()作用1.指针下移2.将下移以后集合位置傻姑娘的元素返回
- 迭代器方法:
- hasNext();
- next();
- remove();
- 在调用iterator.next()方法之前必须要调用iterator.hasNext()进行检测,若不调用,且下一条记录无效,直接调用it.next()会抛出NoSuchElementException异常
-
@SuppressWarnings({"all"}) public class CollectionIterator { @SuppressWarnings({"all"}) public static void main(String[] args) { Collection colletion = new ArrayList(); colletion.add(new Book("三国演义", "罗贯中", 10.1)); colletion.add(new Book("小李飞刀", "古龙", 5.1)); colletion.add(new Book("红楼梦", "曹雪芹", 34.6)); Iterator iterator = colletion.iterator(); //itit 快捷键 所有快捷键ctrl + j while (iterator.hasNext()) { Object next = iterator.next(); } iterator = colletion.iterator();//重置迭代器 while (iterator.hasNext()) { Object obj = iterator.next();//向上转型 System.out.println(obj); } //iterator.next(); //当退出while循环后,这时iterator指向最后的元素 //如果需要再次遍历,需要重置迭代器 } } @SuppressWarnings({"all"}) class Book { private String name; @Override public String toString() { return "Book{" + "name='" + name + '\'' + ", author='" + author + '\'' + ", price=" + price + '}'; } private String author; private double price; public Book(String name, String author, double price) { this.name = name; this.author = author; this.price = price; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getAuthor() { return author; } public void setAuthor(String author) { this.author = author; } public double getPrice() { return price; } public void setPrice(double price) { this.price = price; } }
Collection接口遍历对象方式2-for循环增强
增强for循环,可以代替iterator迭代器,特点:增强for就是简化版的iterator,本质一样,只能用于遍历集合和数组
基本语法:
for(元素类型 元素名:集合名/数组名){ 访问元素; }
public class CollectionFor { public static void main(String[] args) { Collection colletion = new ArrayList(); colletion.add(new Book("三国演义", "罗贯中", 10.1)); colletion.add(new Book("小李飞刀", "古龙", 5.1)); colletion.add(new Book("红楼梦", "曹雪芹", 34.6)); //底层也是使用迭代器 for(Object book:colletion){ System.out.println(book); } //快捷键 I for (Object o :colletion) { } } }