Legacy Collections Core Java

In this section, we discuss the collection classes that existed in the Java programming language since the beginning: the Hashtable class and its useful Properties subclass, the Stack subclass of Vector, and the BitSet class.

The Hashtable Class

The classic Hashtable class serves the same purpose as the HashMap and has essentially the same interface. Just like methods of the Vector class, the Hashtable methods are synchronized. If you do not require synchronization or compatibility with legacy code, you should use the HashMap instead.
NOTE: The name of the class is Hashtable, with a lowercase t. Under Windows, you’ll get strange error messages if you use HashTable, because the Windows file system is not case-sensitive but the Java compiler is.

Enumerations

The legacy collections use the Enumeration interface for traversing sequences of elements. The Enumeration interface has two methods, hasMoreElements and nextElement. These are entirely analogous to the hasNext and next methods of the Iterator interface.

For example, the elements method of the Hashtable class yields an object for enumerating the values in the table:

You will occasionally encounte r a legacy method that expects an enumeration parameter. The static method Collections.enumeration yields an enumeration object that enumerates the elements in the collection. For example:

NOTE: In C++, it is quite common to use iterators as parameters. Fortunately, in programming for the Java platform, very few programmers use this idiom. It is much smarter to pass around the collection than to pass an iterator. The collection object is more useful. The recipients can always obtain the iterator from the collection when they need to do so, plus they have all the collection methods at their disposal. However, you will find enumerations in some legacy code because they were the only available mechanism for generic collections until the collections framework appeared in Java SE 1.2.

java.util.Enumeration<E> 1.0

  • boolean hasMoreElements()
    returns true if there are more elements yet to be inspected.
  • E nextElement()
    returns the next element to be inspected. Do not call this method if hasMoreElements() returned false.

java.util.Hashtable<K, V> 1.0

  • Enumeration<K> keys()
    returns an enumeration object that traverses the keys of the hash table.
  • Enumeration<V> elements()
    returns an enumeration object that traverses the elements of the hash table.

java.util.Vector<E> 1.0

  • Enumeration<E> elements()
    returns an enumeration object that traverses the elements of the vector.

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Core Java Topics