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org.apache.derby.iapi.store.access
Class KeyHasher  

java.lang.Object
  org.apache.derby.iapi.store.access.KeyHasher

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public class KeyHasher

extends java.lang.Object

Provides the ability to hash on multiple objects.

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Field Summary
private  java.lang.Object[]	objects

 

Constructor Summary

KeyHasher(int size)

 

Method Summary
static java.lang.Object	buildHashKey(java.lang.Object[] objects, int[] indexes)           Static method to return the object to hash on.
boolean	equals(java.lang.Object obj)           Determine whether this Object is semantically equal to another Object.
java.lang.Object	getObject(int index)           Get the object stored at the specified index.
int	hashCode()           Get a value that represents this Object, as uniquely as possible within the confines of an int.
void	setObject(int index, java.lang.Object object)           Set array element at the specified index to the specified object.

 

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait

 

Field Detail

objects

private final java.lang.Object[] objects

Constructor Detail

KeyHasher

public KeyHasher(int size)

Method Detail

setObject

public void setObject(int index,
                      java.lang.Object object)

Set array element at the specified index to the specified object.

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getObject

public java.lang.Object getObject(int index)

Get the object stored at the specified index.

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buildHashKey

public static java.lang.Object buildHashKey(java.lang.Object[] objects,
                                            int[] indexes)

Static method to return the object to hash on. (Object stored in specifed array, if only a single object, otherwise a KeyHasher wrapping the objects to hash on. (NOTE: We optimize for in-memory hash tables, hence we only create a wrapper when needed.)

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hashCode

public int hashCode()

Description copied from class: java.lang.Object

Get a value that represents this Object, as uniquely as possible within the confines of an int.

There are some requirements on this method which subclasses must follow:

• Semantic equality implies identical hashcodes. In other words, if a.equals(b) is true, then a.hashCode() == b.hashCode() must be as well. However, the reverse is not necessarily true, and two objects may have the same hashcode without being equal.
• It must be consistent. Whichever value o.hashCode() returns on the first invocation must be the value returned on all later invocations as long as the object exists. Notice, however, that the result of hashCode may change between separate executions of a Virtual Machine, because it is not invoked on the same object.

Notice that since hashCode is used in java.util.Hashtable and other hashing classes, a poor implementation will degrade the performance of hashing (so don't blindly implement it as returning a constant!). Also, if calculating the hash is time-consuming, a class may consider caching the results.

The default implementation returns System.identityHashCode(this)

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equals

public boolean equals(java.lang.Object obj)

Description copied from class: java.lang.Object

Determine whether this Object is semantically equal to another Object.

There are some fairly strict requirements on this method which subclasses must follow:

• It must be transitive. If a.equals(b) and b.equals(c), then a.equals(c) must be true as well.
• It must be symmetric. a.equals(b) and b.equals(a) must have the same value.
• It must be reflexive. a.equals(a) must always be true.
• It must be consistent. Whichever value a.equals(b) returns on the first invocation must be the value returned on all later invocations.
• a.equals(null) must be false.
• It must be consistent with hashCode(). That is, a.equals(b) must imply a.hashCode() == b.hashCode(). The reverse is not true; two objects that are not equal may have the same hashcode, but that has the potential to harm hashing performance.

This is typically overridden to throw a java.lang.ClassCastException if the argument is not comparable to the class performing the comparison, but that is not a requirement. It is legal for a.equals(b) to be true even though a.getClass() != b.getClass(). Also, it is typical to never cause a java.lang.NullPointerException.

In general, the Collections API (java.util) use the equals method rather than the == operator to compare objects. However, java.util.IdentityHashMap is an exception to this rule, for its own good reasons.

The default implementation returns this == o.

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