HashMap中的tableSizeFor()方法 原

tableSizeFor(int cap)該方法用來返回大於等於該給定整數的最小2^次冪值；

用到的地方

public HashMap(int initialCapacity, float loadFactor);

final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict);

private void readObject(java.io.ObjectInputStream s);

tableSizeFor()方法

static final int MAXIMUM_CAPACITY = 1 << 30;

static final int tableSizeFor(int cap) {
        int n = cap - 1;
        //相當於 n = n | n >>> 1
        //n >>> 1 右移一位
        n |= n >>> 1;
        System.out.println("n >>> 1 = " + n);
        n |= n >>> 2;
        System.out.println("n >>> 2 = " + n);
        n |= n >>> 4;
        System.out.println("n >>> 4 = " + n);
        n |= n >>> 8;
        System.out.println("n >>> 8 = " + n);
        n |= n >>> 16;
        System.out.println("n >>> 16 = " + n);
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }

Test測試類

public class MapTest {

    public static final int MAXIMUM_CAPACITY = 1 << 30;

    public static void main(String[] args) {
       System.out.println(tableSizeFor(10));//16

    }

    static final int tableSizeFor(int cap) {
        int n = cap - 1;
        //相當於 n = n | n >>> 1
        n |= n >>> 1;
        System.out.println("n >>> 1 = " + n);//13
        n |= n >>> 2;
        System.out.println("n >>> 2 = " + n);//15
        n |= n >>> 4;
        System.out.println("n >>> 4 = " + n);//15
        n |= n >>> 8;
        System.out.println("n >>> 8 = " + n);//15
        n |= n >>> 16;
        System.out.println("n >>> 16 = " + n);//15
        return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
    }
}