org.opendaylight.yangtools.util.HashCodeBuilder Maven / Gradle / Ivy
/*
* Copyright (c) 2013 Cisco Systems, Inc. and others. All rights reserved.
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License v1.0 which accompanies this distribution,
* and is available at http://www.eclipse.org/legal/epl-v10.html
*/
package org.opendaylight.yangtools.util;
/**
* Utility class for incrementally building object hashCode by hashing together component objects, one by one.
*
* @param Component object type
*/
public final class HashCodeBuilder {
/**
* The value 31 was chosen because it is an odd prime. If it were even and the multiplication overflowed,
* information would be lost, as multiplication by 2 is equivalent to shifting. The advantage of using a prime is
* less clear, but it is traditional. A nice property of 31 is that the multiplication can be replaced by a shift
* and a subtraction for better performance: 31 * i == (i << 5) - i. Modern VMs do this sort of optimization
* automatically.
*
* (from Joshua Bloch's Effective Java, Chapter 3, Item 9: Always override hashcode when you override equals,
* page 48)
*/
private static final int PRIME = 31;
private int currentHash;
/**
* Create a new instance, with internal hash initialized to 1, equivalent of HashCodeBuilder(1).
*/
public HashCodeBuilder() {
this(1);
}
/**
* Create a new instance, with internal hash set to specified seed.
*
* @param seedHash Seed hash value
*/
public HashCodeBuilder(final int seedHash) {
this.currentHash = seedHash;
}
/**
* Determine the next hash code combining a base hash code and the hash code of an object.
*
* @param hashCode base hash code
* @param obj Object to be added
* @return Combined hash code
*/
public static int nextHashCode(final int hashCode, final Object obj) {
return PRIME * hashCode + obj.hashCode();
}
/**
* Update the internal hash code with the hash code of a component object.
*
* @param obj Component object
*/
public void addArgument(final T obj) {
currentHash = nextHashCode(currentHash, obj);
}
/**
* Return the currently-accumulated hash code.
*
* @return Current hash code
*/
public int build() {
return currentHash;
}
}