totalcross.util

Class Hashtable

java.lang.Object

totalcross.util.Hashtable

Direct Known Subclasses:

AttributeList

public class Hashtable
extends java.lang.Object

This class implements a hash table, which maps keys to values. Any non-null object can be used as a key or as a value.

An instance of Hashtable has two parameters that affect its efficiency: its capacity and its load factor. The load factor should be between 0.0 and 1.0. When the number of entries in the hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method. Larger load factors use memory more efficiently, at the expense of larger expected time per lookup.

If many entries are to be made into a Hashtable, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table.

This example creates a hashtable of numbers. It uses the names of the numbers as keys:

     Hashtable numbers = new Hashtable(3);
     numbers.put("one", Convert.toString(1));
     numbers.put("two", Convert.toString(2));
     numbers.put("three", Convert.toString(3));
 

To retrieve a number, use the following code:

     String n = (String)numbers.get("two");
     if (n != null) 
        // "two = " + Convert.toInt(n);
 

This Hashtable class does not support Generics; use the HashMap class instead.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
protected static class	Hashtable.Entry Hashtable collision list.

Field Summary

Fields

Modifier and Type	Field and Description
boolean	allowDuplicateKeys Setting this to true will allow the hashtable to have more than one key with the same value.
int	collisions Computes the number of collisions for a set of inserts.
protected Hashtable.Entry[]	table The hash table data.

Constructor Summary

Constructors

Constructor and Description
Hashtable(int initialCapacity) Constructs a new, empty hashtable with the specified initial capacity and default load factor of 0.75f.
Hashtable(int initialCapacity, double loadFactor) Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
Hashtable(java.lang.Object[] keys, java.lang.Object values) Creates a Hashtable with the given keys and values.
Hashtable(java.lang.String res) Constructs a new hashtable, parsing the elements from the given String.

Method Summary

Modifier and Type	Method and Description
void	clear() Clears this hashtable so that it contains no keys.
void	copyInto(Hashtable target) Copies the keys and values of this Hashtable into the given Hashtable.
java.lang.StringBuffer	dumpKeysValues(java.lang.StringBuffer sb, java.lang.String keyvalueSeparator, java.lang.String lineSeparator) Dumps the keys and values into the given StringBuffer.
boolean	exists(java.lang.Object key) Checks if the value with the specified key is mapped in this hashtable.
java.lang.Object	get(int hash) Returns the value to which the specified hash is mapped in this hashtable.
java.lang.Object	get(int hash, java.lang.Object defaultValue) Returns the value to which the specified key is mapped in this hashtable.
java.lang.Object	get(java.lang.Object key) Returns the value to which the specified key is mapped in this hashtable.
java.lang.Object	get(java.lang.Object key, java.lang.Object defaultValue) Returns the value to which the specified key is mapped in this hashtable.
Vector	getKeys() Return a Vector of the keys in the Hashtable.
Vector	getKeyValuePairs(java.lang.String separator) Return a Vector with pairs in the form key=value from the Hashtable.
java.lang.String	getString(java.lang.Object key) Returns the value to which the specified key is mapped in this hashtable as a String.
java.lang.String	getString(java.lang.Object key, java.lang.String defaultValue) Returns the value to which the specified key is mapped in this hashtable as a String.
Vector	getValues() Return a Vector of the values in the Hashtable.
java.lang.Object	put(int hash, java.lang.Object value) Maps the specified key to the specified value in this hashtable.
java.lang.Object	put(java.lang.Object key, java.lang.Object value) Maps the specified key to the specified value in this hashtable.
protected void	rehash() Rehashes the contents of the hashtable into a hashtable with a larger capacity.
java.lang.Object	remove(int hash) Removes the key (and its corresponding value) from this hashtable.
java.lang.Object	remove(java.lang.Object key) Removes the key (and its corresponding value) from this hashtable.
int	size() Returns the number of keys in this hashtable.

Methods inherited from class java.lang.Object

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

Field Detail

table

protected Hashtable.Entry[] table

The hash table data.

collisions

public int collisions

Computes the number of collisions for a set of inserts. You must zero this each time you want to compute it. Here's a sample of how to determine the best values. Keep in mind that the lower collisions is better, but don't waste too much memory if its too high.

 int max = 0xFFFFFFF;
 for (int h = 5; ; h++)
 {
    IntHashtable ht = new IntHashtable(h);
    ht.put("nbsp".hashCode(),' ');
    ht.put("shy".hashCode(),'­');
    ht.put("quot".hashCode(),'"');
    ...
    if (ht.collisions < max)
    {
       Vm.debug("h: "+h+" colli: "+ht.collisions);
       max = ht.collisions;
       if (max == 0)
          break;
    }
 }
 

Since:

SuperWaba 5.71.

allowDuplicateKeys

public boolean allowDuplicateKeys

Setting this to true will allow the hashtable to have more than one key with the same value. In this case, the methods will always return the first matching key.

Since:

TotalCross 1.24

Constructor Detail

Hashtable

public Hashtable(int initialCapacity)

Constructs a new, empty hashtable with the specified initial capacity and default load factor of 0.75f.

Parameters:

initialCapacity - The number of elements you think the hashtable will end with. The hashtable will grow if necessary, but using a number near or above the final size can improve performance.

Hashtable

public Hashtable(int initialCapacity,
                 double loadFactor)

Constructs a new, empty hashtable with the specified initial capacity and the specified load factor. If initialCapacity is zero, it is changed to 5.

Parameters:

initialCapacity - The number of elements you think the hashtable will end with. The hashtable will grow if necessary, but using a number near or above the final size can improve performance.

loadFactor - a number between 0.0 and 1.0.

Hashtable

public Hashtable(java.lang.String res)

Constructs a new hashtable, parsing the elements from the given String. Each string must be in the form: key = value, splitted in lines. This aids the task of creating resource bundles to add localization to your application.
You can include txt files in your application's pdb file using /t, where each txt will hold the strings for a language. For example:

 // save these two lines in a file named EN.txt:
 Message = Message
 TestMsg = This is a test
 Exit = Exit
 // save these other two in a file named PT.txt:
 Message = Mensagem
 TestMsg = Isso é um teste
 Exit = Sair
 

The TotalCross deployer will include the two files referenced below in the tcz file.
Now, when your program starts, you can do:

 String txt = idiom == EN ? "EN.txt" : "PT.txt";
 byte[] b = Vm.getFile(txt);
 Hashtable res = new Hashtable(new String(b,0,b.length));
 new MessageBox(res.get("Message"), res.get("TestMsg"), new String[]{res.get("Exit")}).popupNonBlocking();
 

Note that the keys are case sensitive, and that all strings are trimmed.

Since:

SuperWaba 5.72

Hashtable

public Hashtable(java.lang.Object[] keys,
                 java.lang.Object values)

Creates a Hashtable with the given keys and values. The values can be two things:

1. An Object array (Object[]). In this case, the number of keys and values must match.
2. A single Object. This object is set as value to all keys.

The values parameter cannot be null.

Since:

TotalCross 1.5

Method Detail

clear

public void clear()

Clears this hashtable so that it contains no keys.

get

public java.lang.Object get(java.lang.Object key)

Returns the value to which the specified key is mapped in this hashtable.

Parameters:

key - a key in the hashtable.

Returns:

the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.

getString

public java.lang.String getString(java.lang.Object key)

Returns the value to which the specified key is mapped in this hashtable as a String. If the item is a String, a cast is made, otherwise, the toString method is called.

Parameters:

key - a key in the hashtable.

Returns:

the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.

Since:

TotalCross 1.24

getString

public java.lang.String getString(java.lang.Object key,
                                  java.lang.String defaultValue)

Returns the value to which the specified key is mapped in this hashtable as a String. If the item is a String, a cast is made, otherwise, the toString method is called.

Parameters:

key - a key in the hashtable.

Returns:

the value to which the key is mapped in this hashtable; defaultValue if the key is not mapped to any value in this hashtable.

Since:

TotalCross 1.24

get

public java.lang.Object get(java.lang.Object key,
                            java.lang.Object defaultValue)

Returns the value to which the specified key is mapped in this hashtable.

Parameters:

key - a key in the hashtable.

defaultValue - The default value to be returned if none is found.

Returns:

the value to which the key is mapped in this hashtable; defaultValue if the key is not mapped to any value in this hashtable.

Since:

TotalCross 1.15

get

public java.lang.Object get(int hash,
                            java.lang.Object defaultValue)

Returns the value to which the specified key is mapped in this hashtable.

Parameters:

hash - The key hash in the hashtable.

defaultValue - The default value to be returned if none is found.

Returns:

the value to which the key is mapped in this hashtable; defaultValue if the key is not mapped to any value in this hashtable.

Since:

TotalCross 1.15

get

public java.lang.Object get(int hash)

Returns the value to which the specified hash is mapped in this hashtable.

Caution: since you're passing an integer instead of an object, if there are two objects that map to the same key, this method will always return the first one only.

Parameters:

hash - The key hash in the hashtable.

Returns:

the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.

exists

public boolean exists(java.lang.Object key)

Checks if the value with the specified key is mapped in this hashtable.

Parameters:

key - a key in the hashtable.

Returns:

True if the key exists, false otherwise.

Since:

SuperWaba 5.8

getKeys

public Vector getKeys()

Return a Vector of the keys in the Hashtable. The order is the same of the getValues method.

See Also:

getValues(), getKeyValuePairs(java.lang.String)

getValues

public Vector getValues()

Return a Vector of the values in the Hashtable. The order is the same of the getKeys method.

Since:

SuperWaba 5.1

See Also:

getKeys(), getKeyValuePairs(java.lang.String)

getKeyValuePairs

public Vector getKeyValuePairs(java.lang.String separator)

Return a Vector with pairs in the form key=value from the Hashtable. Each vector's element can safely be casted to a String.

Parameters:

separator - the separator between the key and the value. Should be ": ","=", etc.

Since:

SuperWaba 5.1

copyInto

public void copyInto(Hashtable target)

Copies the keys and values of this Hashtable into the given Hashtable. Note that the target Hashtable is not cleared; you should do that by yourself.

Since:

TotalCross 1.15

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.Object value)

Maps the specified key to the specified value in this hashtable. Neither the key nor the value can be null.

The value can be retrieved by calling the get method with a key that is equal to the original key.

Parameters:

key - the hashtable key.

value - the value.

Returns:

the previous value of the specified key in this hashtable, or null if it did not have one.

See Also:

Object.equals(java.lang.Object)

put

public java.lang.Object put(int hash,
                            java.lang.Object value)

Maps the specified key to the specified value in this hashtable. Neither the key nor the value can be null.

The value can be retrieved by calling the get method with a key that is equal to the original key.

This method receives a hashcode instead of the object. You MUST use the get(int) method to retrieve the value, otherwise you will get a NullPointerException, because no key is stored using this method.

Parameters:

hash - the hashtable key's hash.

value - the value.

Returns:

the previous value of the specified key in this hashtable, or null if it did not have one.

See Also:

Object.equals(java.lang.Object)

rehash

protected void rehash()

Rehashes the contents of the hashtable into a hashtable with a larger capacity. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.

remove

public java.lang.Object remove(java.lang.Object key)

Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.

Parameters:

key - the key that needs to be removed.

Returns:

the value to which the key had been mapped in this hashtable, or null if the key did not have a mapping.

remove

public java.lang.Object remove(int hash)

Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.

Parameters:

hash - the hash code of the key that needs to be removed.

Returns:

the value to which the key had been mapped in this hashtable, or null if the key did not have a mapping.

size

public int size()

Returns the number of keys in this hashtable.

dumpKeysValues

public java.lang.StringBuffer dumpKeysValues(java.lang.StringBuffer sb,
                                             java.lang.String keyvalueSeparator,
                                             java.lang.String lineSeparator)

Dumps the keys and values into the given StringBuffer.

Parameters:

sb - The StringBuffer where the data will be dumped to

keyvalueSeparator - The separator between the key and the value (E.G.: ": ")

lineSeparator - The separator placed after each key+value pair (E.G.: "\r\n"). The last separator is cut from the StringBuffer.

Since:

TotalCross 1.23