Object.hashCode() algorithm

Question

I\'m looking for the algorithm of Object.hashCode().

This code is native in Object.java.

Is this because

(a) the code is in assemb

Answer 1

Despite the Javadoc, the algo only may use the address as an input. This means that even though new objects use the same address in eden space they won't have the same hashCode.

There is a number of algos it might be using and not all use the address.

Note: the hashCode() is 31-bit.

BTW You can set it with Unsafe.putInt(object, 1, value)on Hotspot.

Set ints = new LinkedHashSet<>();
int negative = 0, nonneg = 0;
for (int i = 0; i < 100; i++) {
    System.gc();
    for (int j = 0; j < 100; j++) {
        int h = new Object().hashCode();
        ints.add(h);
        if (h < 0) negative++;
        else nonneg++;
    }
}
System.out.println("unique: " + ints.size() + " negative: " + negative + " non-neg: " + nonneg);

prints

unique: 10000 negative: 0 non-neg: 10000

Using Unsafe

Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
theUnsafe.setAccessible(true);
Unsafe unsafe = (Unsafe) theUnsafe.get(null);

Object o = new Object();
System.out.println("From header " + Integer.toHexString(unsafe.getInt(o, 1L)));
// sets the hashCode lazily
System.out.println("o.hashCode()  " + Integer.toHexString(o.hashCode()));
// it's here now.
System.out.println("after hashCode() From header " + Integer.toHexString(unsafe.getInt(o, 1L)));
unsafe.putInt(o, 1L, 0x12345678);
System.out.println("after change o.hashCode()  " + Integer.toHexString(o.hashCode()));

prints

From header 0
o.hashCode()  2260e277
after hashCode() From header 2260e277
after change o.hashCode()  12345678