易错

每次comparator都不记得写object的leixing

 Arrays.sort ( intervals, new Comparator<Interval> (){
            public int compare ( Interval in1, Interval in2 ){
                return in1.start - in2.start;
            }
        });

HashMap 

void	clear() Removes all of the mappings from this map.
Object	clone() Returns a shallow copy of this  HashMap instance: the keys and values themselves are not cloned.
boolean	containsKey(Object key) Returns  true if this map contains a mapping for the specified key.
boolean	containsValue(Object value) Returns  true if this map maps one or more keys to the specified value.
Set<Map.Entry<K,V>>	entrySet() Returns a  Set view of the mappings contained in this map.
V	get(Object key) Returns the value to which the specified key is mapped, or  null if this map contains no mapping for the key.
boolean	isEmpty() Returns  true if this map contains no key-value mappings.
Set<K>	keySet() Returns a  Set view of the keys contained in this map.
V	put(K key, V value) Associates the specified value with the specified key in this map.
void	putAll(Map<? extends K,? extends V> m) Copies all of the mappings from the specified map to this map.
V	remove(Object key) Removes the mapping for the specified key from this map if present.
int	size() Returns the number of key-value mappings in this map.
Collection<V>	values() Returns a  Collection view of the values contained in this map.