在springcloud微服务调用返回数据时，出现获取数据为null的情况：
一开始我是用R继承hashmap然后定义一个私有属性data，没想到出错了，因为序列化过程中会忽略掉类的私有属性（继承hashmap的类），因此在反序列过程中会吧data的值丢失。

public class R<T> extends HashMap<String,T> {
    private T data;
    public void setData(T data){
        this.data = (T) data;
    };

    public T getData(){
        return data;
    }
}
class t{
    public static void main(String[] args) {
        R r = new R();
        r.setData(new Integer(1));
        System.out.println(r.getData());
        //序列化
        String s = JSON.toJSONString(r);
        System.out.println(s);
        //反序列化
        R r1 = JSON.parseObject(s, R.class);
        System.out.println(r1.getData());
        /**结果为
         * 1
         * {}
         * null
         */
    }
}

因此在放入数据时，需要利用put的方法放入map中，而又因为springcloud在调用其他服务后返回结果会转换为LinkedHashMap（前提是，如果map中的value为复杂对象，例如为List。这里不是很清楚，懂得人可以解答下），所以先将LinkedHashMap转换为json字符串，然后再转换为自己想要的类型对象

public class R extends HashMap<String, Object> {
	private static final long serialVersionUID = 1L;

	public <T> T getData(TypeReference<T> typeReference){
		Object data = get("data");
		String s = JSON.toJSONString(data);
		T t = JSON.parseObject(s, typeReference);
		return t;
	}

	public R setData(Object data){
		put("data",data);
		return this;
	}

反射获取hashMap

反射是基于hashmap内部实现原理，Hashmap的核心在putval方法和内部类Node的组成.

核心方法putVal

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
                   boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                        ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

可以看到hashmap由Node对象构成基础，那么我们反射的时候就要想法设法的把它的node给搞出来

Node构成

static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;

        Node(int hash, K key, V value, Node<K,V> next) {
            this.hash = hash;
            this.key = key;
            this.value = value;
            this.next = next;
        }

        public final K getKey()        { return key; }
        public final V getValue()      { return value; }
        public final String toString() { return key + "=" + value; }

        public final int hashCode() {
            return Objects.hashCode(key) ^ Objects.hashCode(value);
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public final boolean equals(Object o) {
            if (o == this)
                return true;
            if (o instanceof Map.Entry) {
                Map.Entry<?,?> e = (Map.Entry<?,?>)o;
                if (Objects.equals(key, e.getKey()) &&
                    Objects.equals(value, e.getValue()))
                    return true;
            }
            return false;
        }
    }

node由hash，key，value，next这几个构成，他们都为属性名，那么就是可以通过反射获取到的。

反射的实现

//o 就是一个hash数组 里面有{code，msg，data}
private Object  GetHashTableValue(Object o){
        Object resObj = new Object();
    //定义hashmap
        Class clsHashMap = null;
    //定义hashmap里的Node
        Class clsHashMap$Node = null;
    
        Field[] f = null;
        Field t = null, fNode = null;

        try {
            clsHashMap = Class.forName("java.util.HashMap");
            clsHashMap$Node = Class.forName("java.util.HashMap$Node");

            // 获取hashmap里的全部属性
            //里面有serialVersionUID，DEFAULT_INITIAL_CAPACITY，MAXIMUM_CAPACITY，DEFAULT_LOAD_FACTOR，TREEIFY_THRESHOLD，UNTREEIFY_THRESHOLD，MIN_TREEIFY_CAPACITY，
            f = clsHashMap.getDeclaredFields();
            
            AccessibleObject.setAccessible(f, true);
            for (Field field : f) {
                // System.out.println(field.getName());
                //获取属性名为table的，然后赋值给 filed
                if (field.getName() == "table")
                    t = field;
            }
            //这这里的t为tree  transient Node<K,V>[] table; 由此可见这个是一个数组，所以要Object[] 来转换下
             //注：如果是继承hashmap那么继承的属性也会在这里面。比如有个getmes类，其中有code，msg属性，那么o1也会有
            Object[] o1 = ((Object[]) t.get(o));
            //这里面遍历的便是node
            for (Object o2 : o1) {
                if (o2!=null){
                    Object next = o2;//这个next就相当于是node一个类
                    while (next!=null){
                        //反射value属性
                        Field value = clsHashMap$Node.getDeclaredField("value");
                        //反射next属性
                        Field o_next  = clsHashMap$Node.getDeclaredField("next");
                        //根据自己需要还可以反射key和hash，方法一样

                        o_next.setAccessible(true);
                        value.setAccessible(true);

                        //得到value值
                        resObj = value.get(next);
                        
                        //得到下个节点node 可以赋值给fNode
                        Object objNext = o_next.get(next);
                        next = objNext;
                    }
                }
            }
        } catch (ClassNotFoundException | IllegalAccessException | NoSuchFieldException e) {
            e.printStackTrace();
        }

        return resObj;
    }

可以看到hashmap有大量的属性

 transient Node<K,V>[] table;

private static final long serialVersionUID = 362498820763181265L;

   

    /**
     * The default initial capacity - MUST be a power of two.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16

    /**
     * The maximum capacity, used if a higher value is implicitly specified
     * by either of the constructors with arguments.
     * MUST be a power of two <= 1<<30.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * The load factor used when none specified in constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     * The bin count threshold for using a tree rather than list for a
     * bin.  Bins are converted to trees when adding an element to a
     * bin with at least this many nodes. The value must be greater
     * than 2 and should be at least 8 to mesh with assumptions in
     * tree removal about conversion back to plain bins upon
     * shrinkage.
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     * The bin count threshold for untreeifying a (split) bin during a
     * resize operation. Should be less than TREEIFY_THRESHOLD, and at
     * most 6 to mesh with shrinkage detection under removal.
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     * The smallest table capacity for which bins may be treeified.
     * (Otherwise the table is resized if too many nodes in a bin.)
     * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
     * between resizing and treeification thresholds.
     */
    static final int MIN_TREEIFY_CAPACITY = 64;
    /**
     * Holds cached entrySet(). Note that AbstractMap fields are used
     * for keySet() and values().
     */
    transient Set<Map.Entry<K,V>> entrySet;

    /**
     * The number of key-value mappings contained in this map.
     */
    transient int size;

    /**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient int modCount;

 
    int threshold;

    final float loadFactor;

​ 注：transient是短暂的意思。对于transient 修饰的成员变量，在类的实例对象的序列化处理过程中会被忽略。 因此，transient变量不会贯穿对象的序列化和反序列化，生命周期仅存于调用者的内存中而不会写到磁盘里进行持久化

总结：

​ 反射hashmap核心还是反射其中hashmap的内部类node类，只要拿的到它，就能反射相应的元素

https://blog.csdn.net/u010575093/article/details/50731281

抽象类中可以定义成员变量，而接口中定义的成员变量实际上都是常量
接口不可继承抽象类，抽象类可实现多个接口。接口与接口之间是继承，用extends，可继承多个接口

mport java.util.HashMap;
import java.util.Map;

interface Test{
        public void mm();
}

interface Test1{
        public void kk();
}
//接口可以继承多个接口
interface Test2 extends Test, Test1{
        public void mk();
}
//抽象类成员都是public，
//抽象类中可以定义成员变量，而接口中定义的成员变量实际上都是常量。
//抽象类可以继承接口,并实现接口方法
abstract class ATest1 implements Test1{
        public void kk(){  System.out.println("ATtest mm");  }
}

abstract class ATest2 implements Test1, Test{
        public void mm(){       System.out.println("ATest2 mm");        }
        public void kk(){}
}
// 抽象类可以继承抽象类 也可以继承实体类
abstract class ATest3 extends ATest1{

}

class Solution{
	//时间复杂度n
        public void tosum(int[] nums, int target){
                Map<Integer, Integer> map = new HashMap<>();
                for(int i = 0; i < nums.length; i++)
                        map.put(nums[i],i);
                for(int i = 0; i < nums.length; i++){
                        int comp = target - nums[i];
                        System.out.println("comp="+comp+" containsKey "+map.containsKey(comp));
                        System.out.println("map get "+map.get(comp));
                        if(map.containsKey(comp) && map.get(comp)!=i){
                                System.out.println("match:"+i+" "+map.get(comp));
                        }
                }
                System.out.println("tosum");
        }
        //时间复杂度1
        public int[] tosum1(int[] nums, int target){
                Map<Integer, Integer> map = new HashMap<>();
                for (int i = 0; i < nums.length; i++) {
                        int complement = target - nums[i];
                        if (map.containsKey(complement)) {
                                return new int[] { map.get(complement), i };
                        }
                        map.put(nums[i], i);
                }
                throw new IllegalArgumentException("No two sum solution");
        }
}
class HashSum{

        public static void main(String argv[]){
                System.out.println("111");
                new Solution().tosum();
        }
}