Java 集合体系与 Collection 遍历方法

集合体系

Collection 单列集合，Map 双列集合。

集合特点：

• List 集合：添加的元素是有序、可重复、有索引
  • ArrayList、LinkedList：有序、可重复、有索引
• Set 集合：添加的元素是无序、不重复、无索引
  • HashSet：无序、不重复、无索引
  • LinkedHashSet：有序、不重复、无索引
  • TreeSet：按照大小默认升序排序、不重复、无索引

Collections1.java

package com.example.collections;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;

public class Collections1 {
    public static void main(String[] args) {
        Collection<String> c = new ArrayList<>();
        c.add("山");
        c.add("重");
        c.add("水");
        c.add("复");
        System.out.println(c);
        c.clear();
        System.out.println(c);
        System.out.println(c.isEmpty());
        System.out.println(c.size());
        c.add("山");
        c.add("重");
        c.add("水");
        c.add("复");
        System.out.println(c.contains("山"));
        System.out.println(c.contains(null));
        System.out.println(c.remove("山"));
        System.out.println(c);
        Object[] arr = c.toArray();
        System.out.println(Arrays.toString(arr));
        String[] arr2 = c.toArray(new String[c.size()]);
        System.out.println(Arrays.toString(arr2));
        
        Collection<String> c1 =  <>();
        c1.add();
        c1.add();
        Collection<String> c2 =  <>();
        c2.add();
        c2.add();
        c1.addAll(c2);
        System.out.println(c1);
        System.out.println(c2);
    }
}