Analysis of HashMap source code in JDK7 and JDK8

1.jdk1.7 HashMap

The data structure of hashMap: array + linked list

Initial value of ArrayList: 10

Why not use the add method of list in hashMap. HashMap is a key-value structure. When getting (key), you need to traverse the entire array, which is not as efficient as list.get (subscript).

put operation:

1. Use the hashcode to find the length of the remainder array to get the subscript value of the index array

2. Put the new Entry element into table[index]

1.1 Construction method

Construction method, the default initial memory is 16, and the default parameter DEFAULT_LOAD_FACTOR=0.75f. I don't know what's the use, continue to analyze

Default initial capacity: static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

Default maximum capacity: static final int MAXIMUM_CAPACITY = 1 << 30;

Default parameters: static final float DEFAULT_LOAD_FACTOR = 0.75f;

/**
     * Constructs an empty <tt>HashMap</tt> with the default initial capacity
     * (16) and the default load factor (0.75).
     */
    public HashMap() {
        this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
    }

1.2 put method (TODO)

The put operation is roughly divided into the following steps.

1. The initial capacity is the power of 2

2. Put it in the first position as empty

3. Hash operation, & operation to traverse the key value to get the value value

4. Judging the conditions to expand the capacity, create a new entry<key,value> and put it into the entry[] array.

^=XOR operation, the same is 1, not the same is 0

|= The bitwise AND operation is different to 1, and the same is 0 or 1

&=Bitwise and post-assignment is different to 0, the same is the same value

public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);//初始化容量为2的冥次方 
        }
        if (key == null)
            return putForNullKey(value);//将null值放入第一个位置
        int hash = hash(key); //hash运算
        int i = indexFor(hash, table.length);//&运算
        for (Entry<K,V> e = table[i]; e != null; e = e.next) {//遍历返回值
            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);//将key,value放入entry
        return null;
    }