Map

Map通过键值对存储

键不能重复,值可以重复

HashMap

public class HashMapDemo {
    
    
    /**
     * HashMap 键值对
     * 键是无序的,如果有重读的键后面的值会覆盖上一个键的值
     * 可以存储一个为null的键
     */
    public static void main(String[] args) {
    
    
        HashMap<String,String> hashMap = new HashMap<>();
        hashMap.put("1","a");
        hashMap.put("2","b");
        hashMap.put("1","c");
        hashMap.put("3","d");
        hashMap.put(null,"d");
        System.out.println(hashMap);

        hashMap.remove("2");//移除key为"2"的元素
        hashMap.remove("c");
        System.out.println(hashMap);

        //hashMap.clear();
        System.out.println(hashMap.containsKey("2"));//是否包含Key值"2"
        System.out.println(hashMap.containsValue("d"));//是否包含值"d"
        System.out.println(hashMap);

        System.out.println(hashMap.isEmpty());//是否为空
        System.out.println(hashMap.size());//获取长度
    }
}

TreeMap

 public static void main(String[] args) {
    
    
        /**
         * TreeMap 键值对
         * 键不重复
         * 可根据键的自然顺序排序,指定的键的类型的类必须事先Comparede接口,在排序时使用
         */
        TreeMap<String,String> treeMap = new TreeMap<>();
        treeMap.put("1","a");
        treeMap.put("2","b");
        treeMap.put("1","c");
        treeMap.put("4","c");
        treeMap.put("3","d");
        System.out.println(treeMap);
    }

运行结果:
{
    
    1=c, 2=b, 3=d, 4=c}

HashTable

public static void main(String[] args) {
    
    
        /**
         * Hashtable 无序的
         * 初始容量为11
         * 线程安全,不允许有键为null的键
         */
        Hashtable<String,String> hashtable = new Hashtable<>();
        hashtable.put("a","1");
        hashtable.put("b","2");
        hashtable.put("c","3");
        hashtable.put("d","4");
        //hashtable.put(null,"4");
        System.out.println(hashtable);
    }

运行结果:
{
    
    b=2, a=1, d=4, c=3}

Map集合的遍历

    public static void main(String[] args) {
    
    
        HashMap<String,String> hashMap = new HashMap<>();
        hashMap.put("1","a");
        hashMap.put("2","b");
        hashMap.put("1","c");
        hashMap.put("3","d");

        /*
        Map集合的遍历方法:
        1.foreach   2.keySet();     3.entrySet()
         */
        //hashMap.forEach((k,v)-> System.out.println(k+":"+v));

        /*
        keySet();取hashMap中所有的键,存储到Set集合中
        在对Set集合进行遍历,通过每次得到的Key去获取相对应的值
         */
        /*Set<String> set = hashMap.keySet();
        for (String key:set) {
            System.out.println(key+":"+hashMap.get(key));
        }*/

        /*
        values();获取hashMap中所有的值,存储到list集合中
         */
        /*Collection list = hashMap.values();
        System.out.println(list);*/


        /*
        entrySet();将hashMap中的键值对封装到一个entry对象中 
        可以通过entry获取key或者value
        */
        Set<Map.Entry<String,String>> entrySet = hashMap.entrySet();
        /*for (Map.Entry<String,String> entry: entrySet) {
            System.out.println(entry.getKey()+":"+entry.getValue());
        }*/
        Iterator<Map.Entry<String,String>> it = entrySet.iterator();
        while (it.hasNext()){
    
    
            /*Map.Entry<String,String> entry=it.next();
                System.out.println(entry.getKey()+":"+entry.getValue());*/
            System.out.println(it.next());
        }
    }

Collection类

public static void main(String[] args) {
    
    
        /*ArrayList arrayList = new ArrayList();
        arrayList.add("x");
        Collections.addAll(arrayList,"a","g","d");//多个元素进行添加
        System.out.println(arrayList);

        Collections.sort(arrayList);//对集合进行排序
        System.out.println(arrayList);
        System.out.println(Collections.binarySearch(arrayList,"d"));//二分查找
        */
        ArrayList arrayList1 = new ArrayList();
        Collections.addAll(arrayList1,"a","g","d","c");
        ArrayList arrayList2 = new ArrayList();
        Collections.addAll(arrayList2,"x","y","z");
        Collections.copy(arrayList1,arrayList2);//要求目标集合的size大于sec源集合的size
        System.out.println(arrayList1);

        List list = Collections.emptyList();//返回一个空集合,无法进行add操作
        //list.add("a");  java.lang.UnsupportedOperationException
        System.out.println(list);

        Collections.fill(arrayList2,"w");//用指定元素对目标集合进行覆盖
        System.out.println(arrayList2);

        System.out.println(Collections.max(arrayList1));//返回集合中最大元素
        System.out.println(Collections.replaceAll(arrayList1,"x","X"));//对集合中的指定元素进行替换,替换成功返回true
        System.out.println(arrayList1);

        Collections.reverse(arrayList1);//对集合进行翻转
        System.out.println(arrayList1);

        Collections.swap(arrayList1,0,3);//对集合中的两个元素进行互换
        System.out.println(arrayList1);
    }