public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);    //说明key和value值都是可以为null
        int hash = hash(key);
        int i = indexFor(hash, table.length);
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {
            Object k;
            if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
                V oldValue = e.value;
                e.value = value;
                e.recordAccess(this);
                return oldValue;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

public synchronized V put(K key, V value) {
        // Make sure the value is not null
        if (value == null) {    //当value==null的时候，会抛出异常
            throw new NullPointerException();
        }

        // Makes sure the key is not already in the hashtable.
        Entry tab[] = table;
        int hash = hash(key);
        int index = (hash & 0x7FFFFFFF) % tab.length;
        for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
            if ((e.hash == hash) && e.key.equals(key)) {
                V old = e.value;
                e.value = value;
                return old;
            }
        }

        modCount++;
        if (count >= threshold) {
            // Rehash the table if the threshold is exceeded
            rehash();

            tab = table;
            hash = hash(key);
            index = (hash & 0x7FFFFFFF) % tab.length;
        }

        // Creates the new entry.
        Entry<K,V> e = tab[index];
        tab[index] = new Entry<>(hash, key, value, e);
        count++;
        return null;
    }

 HashMap<String, Integer> map = new HashMap<String, Integer>(20);

Map<String, Integer> map = new HashMap<String, Integer>(20);
        for(Map.Entry<String, Integer> entry : map.entrySet()){    //直接遍历出Entry
            System.out.println("key-->"+entry.getKey()+",value-->"+m.get(entry.getValue()));
        }

Map<String, Integer> map = new HashMap<String, Integer>(20);
        Iterator<String> keySet = map.keySet().iterator();    //遍历Hash表中的key值集合，通过key获取value
        while(keySet .hasNext()){
            Object key = keySet .next();
            System.out.println("key-->"+key+",value-->"+m.get(key));
        }

异常信息：java.lang.OutOfMemoryError:PermGen space

方法区溢出也是一种常见的内存溢出异常，一个类如果要被垃圾收集器回收，判定条件是很苛刻的。在经常动态生成大量Class的应用中，要特别注意这点。

public class OutClass {
    private String mName = "lly";
    static int mAge = 12;

    class InnerClass{
        String name;
        int age;
        private void getName(){
            name = mName;
            age = mAge;
            System.out.println("name="+name+",age="+age);
        }
    }
 
    public static void main(String[] args) {
        //第一种初始化内部类方法
        OutClass.InnerClass innerClass = new OutClass().new InnerClass();
        innerClass.getName();
        //第二种初始化内部类方法
        OutClass out = new OutClass();
        InnerClass in = out.new InnerClass();
        in.getName();
    }
}

textView.setOnClickListener(new View.OnClickListener(){    //OnClickListener为一个接口interface
        public void onClick(View v){
            ...
        }
    });

public abstract class Animal {
    public abstract void getColor();
}
    
public class Dog{
    public static void main(String[] args) {
        Animal dog = new Animal() {
            @Override
            public void getColor() {
                System.out.println("黑色");
            }
        };
        dog.getColor();
    }
}