”准备用HashMap存1w条数据，构造时传10000会触发扩容吗？“

小咪咪 2023-05-29 05:20 1阅读 0赞

构造10000

/**
         * Constructs an empty <tt>HashMap</tt> with the specified initial
         * capacity and the default load factor (0.75).
         *
         * @param  initialCapacity the initial capacity.
         * @throws IllegalArgumentException if the initial capacity is negative.
         */
        public HashMap(int initialCapacity) {
            this(initialCapacity, DEFAULT_LOAD_FACTOR);
        }

最后调用的是

public HashMap(int initialCapacity, float loadFactor) {
            if (initialCapacity < 0)
                throw new IllegalArgumentException("Illegal initial capacity: " +
                                                   initialCapacity);
            if (initialCapacity > MAXIMUM_CAPACITY)
                initialCapacity = MAXIMUM_CAPACITY;
            if (loadFactor <= 0 || Float.isNaN(loadFactor))
                throw new IllegalArgumentException("Illegal load factor: " +
                                                   loadFactor);
            this.loadFactor = loadFactor;
            this.threshold = tableSizeFor(initialCapacity);
        }

重点tableSizeFor

/**
         * Returns a power of two size for the given target capacity.
         */
        static final int tableSizeFor(int cap) {
            int n = cap - 1;
            n |= n >>> 1;
            n |= n >>> 2;
            n |= n >>> 4;
            n |= n >>> 8;
            n |= n >>> 16;
            return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
        }

此方法，10000进去，16384出来  
  
回到问题本身，装10000个会不会触发`resize()`,默认扩容因子是0.75  
  
16384\*0.75 = 12288>10000  
  
所以不会！  
  
类似问题：[java的hashmap，如果确定只装载100个元素，new HashMap(?)多少是最佳的，why？][java_hashmap_100_new HashMap_why]  
  
[读HashMap源码之tableSizeFor][HashMap_tableSizeFor]  
  
[HashMap面试常问的那些常量、数值][HashMap]

[java_hashmap_100_new HashMap_why]: https://blog.csdn.net/leisure_life/article/details/102898866
[HashMap_tableSizeFor]: https://blog.csdn.net/leisure_life/article/details/101674535
[HashMap]: https://blog.csdn.net/leisure_life/article/details/101776136