HashMap和HahSet的用法

HashMap的工作原理 ：HashMap是基于散列法（又称哈希法）的原理，使用put(key, value)存储对象到HashMap中，使用get(key)从HashMap中获取对象。当我们给put()方法传递键和值时，我们先对键调用hashCode()方法，返回的hashCode用于找到bucket（桶）位置来储存Entry对象。HashMap是在bucket中储存键对象和值对象，作为Map.Entry，并不是仅仅只在bucket中存储值。

- HashMap

// "static void main" must be defined in a public class.
public class Main {
   
    public static void main(String[] args) {
   
        // 1. initialize a hash map
        Map<Integer, Integer> hashmap = new HashMap<>();
        // 2. insert a new (key, value) pair
        hashmap.putIfAbsent(0, 0);
        hashmap.putIfAbsent(2, 3);
        // 3. insert a new (key, value) pair or update the value of existed key
        hashmap.put(1, 1);
        hashmap.put(1, 2);
        // 4. get the value of specific key
        System.out.println("The value of key 1 is: " + hashmap.get(1));
        // 5. delete a key
        hashmap.remove(2);
        // 6. check if a key is in the hash map
        if (!hashmap.containsKey(2)) {
   
            System.out.println("Key 2 is not in the hash map.");
        }
        // 7. get the size of the hash map
        System.out.println("The size of hash map is: " + hashmap.size()); 
        // 8. iterate the hash map
        for (Map.Entry<Integer, Integer> entry : hashmap.entrySet()) {
   
            System.out.print("(" + entry.getKey() + "," + entry.getValue() + ") ");
        }
        System.out.println("are in the hash map.");
        // 9. clear the hash map
        hashmap.clear();
        // 10. check if the hash map is empty
        if (hashmap.isEmpty()) {
   
            System.out.println("hash map is empty now!");
        }
    }
}

HashSet是限制了功能的HashMap，所以了解HashMap的实现原理，这个HashSet自然就通
对于HashSet中保存的对象，主要要正确重写equals方法和hashCode方法，以保证放入Set对象的唯一性
虽说时Set是对于重复的元素不放入，倒不如直接说是底层的Map直接把原值替代了（这个Set的put方法的返回值真有意思）
HashSet没有提供get()方法，愿意是同HashMap一样，Set内部是无序的，只能通过迭代的方式获得

- HashSet

// "static void main" must be defined in a public class.
public class Main {
   
    public static void main(String[] args) {
   
        // 1. initialize the hash set
        Set<Integer> hashSet = new HashSet<>();     
        // 2. add a new key
        hashSet.add(3);
        hashSet.add(2);
        hashSet.add(1);
        // 3. remove the key
        hashSet.remove(2);        
        // 4. check if the key is in the hash set
        if (!hashSet.contains(2)) {
   
            System.out.println("Key 2 is not in the hash set.");
        }
        // 5. get the size of the hash set
        System.out.println("The size of has set is: " + hashSet.size());     
        // 6. iterate the hash set
        for (Integer i : hashSet) {
   
            System.out.print(i + " ");
        }
        System.out.println("are in the hash set.");
        // 7. clear the hash set
        hashSet.clear();
        // 8. check if the hash set is empty
        if (hashSet.isEmpty()) {
   
            System.out.println("hash set is empty now!");
        }
    }
}