hashMap中的get方法

HashMap的get方法的实现，其判断标准是通过hashCode和equals方法。

/**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * 
More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * 
A return value of {@code null} does not necessarily
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     *
     * @see #put(Object, Object)
     */
    public V get(Object key) {
        if (key == null)
            return getForNullKey();
        Entry entry = getEntry(key);

        return null == entry ? null : entry.getValue();
    }

    /**
     * Offloaded version of get() to look up null keys.  Null keys map
     * to index 0.  This null case is split out into separate methods
     * for the sake of performance in the two most commonly used
     * operations (get and put), but incorporated with conditionals in
     * others.
     */
    private V getForNullKey() {
        if (size == 0) {
            return null;
        }
        for (Entry e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

/**
     * Returns the entry associated with the specified key in the
     * HashMap.  Returns null if the HashMap contains no mapping
     * for the key.
     */
    final Entry getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : hash(key);
        for (Entry e = table[indexFor(hash, table.length)];
             e != null;
             e = e.next) {
            Object k;
            if (e.hash == hash &&
                ((k = e.key) == key || (key != null && key.equals(k))))
                return e;
        }
        return null;
    }

 

hashMap(key,value)的key和value都可以是null。

 

解析：

1、get的时候，如果key==null，判断Map的长度也为空的话就返回null，如果Map长度不为空，则e也不空，遍历table[0]，返回e.value.

2、getEntry的时候，首先要获取hash(key)的值，通过hash&table.length获取到的hashCode值得到entry在桶中存放的位置，判断如果传入的key与要获得key的hash相等的话并且key.equals（e.key）也相等，则返回entry，如果返回的jentry不为空的话，则getValue值。