What is legacy java classes

Legacy Classes — Java Collections

Early version of java did not include the Collections framework. It only defined several classes and interfaces that provide methods for storing objects. When Collections framework were added in J2SE 1.2, the original classes were reengineered to support the collection interface. These classes are also known as Legacy classes. All legacy classes and interface were redesign by JDK 5 to support Generics. In general, the legacy classes are supported because there is still some code that uses them.

The following are the legacy classes defined by java.util package

There is only one legacy interface called Enumeration

NOTE: All the legacy classes are synchronized

Enumeration interface

1. Enumeration interface defines method to enumerate(obtain one at a time) through collection of objects.
2. This interface is superseded(replaced) by Iterator interface.
3. However, some legacy classes such as Vector and Properties defines several method in which Enumeration interface is used.
4. It specifies the following two methods

boolean hasMoreElements() //It returns true while there are still more elements to extract, and returns false when all the elements have been enumerated. Object nextElement() //It returns the next object in the enumeration i.e. each call to nextElement() method obtains the next object in the enumeration. It throws NoSuchElementException when the enumeration is complete.

Vector class

1. Vector is similar to ArrayList which represents a dynamic array.
2. There are two differences between Vector and ArrayList. First, Vector is synchronized while ArrayList is not, and Second, it contains many legacy methods that are not part of the Collections Framework.
3. With the release of JDK 5, Vector also implements Iterable. This means that Vector is fully compatible with collections, and a Vector can have its contents iterated by the for-each loop.
4. Vector class has following four constructor

Vector() //This creates a default vector, which has an initial size of 10. Vector(int size) //This creates a vector whose initial capacity is specified by size. Vector(int size, int incr) //This creates a vector whose initial capacity is specified by size and whose increment is specified by incr. The increment specifies the number of elements to allocate each time when a vector is resized for addition of objects. Vector(Collection c) //This creates a vector that contains the elements of collection c.

Vector defines several legacy methods. Lets see some important legacy methods defined by Vector class.

Method	Description
void addElement(E element)	adds element to the Vector
E elementAt(int index)	returns the element at specified index
Enumeration elements()	returns an enumeration of element in vector
E firstElement()	returns first element in the Vector
E lastElement()	returns last element in the Vector
void removeAllElements()	removes all elements of the Vector

Example of Vector

import java.util.*; public class Test < public static void main(String[] args) < Vectorve = new Vector(); ve.add(10); ve.add(20); ve.add(30); ve.add(40); ve.add(50); ve.add(60); Enumeration en = ve.elements(); while(en.hasMoreElements()) < System.out.println(en.nextElement()); >> >

Hashtable class

1. Like HashMap, Hashtable also stores key/value pair. However neither keys nor values can be null.
2. There is one more difference between HashMap and Hashtable that is Hashtable is synchronized while HashMap is not.
3. Hashtable has following four constructor

Hashtable() //This is the default constructor. The default size is 11. Hashtable(int size) //This creates a hash table that has an initial size specified by size. Hashtable(int size, float fillratio) //This creates a hash table that has an initial size specified by size and a fill ratio specified by fillRatio. This ratio must be between 0.0 and 1.0, and it determines how full the hash table can be before it is resized upward. Specifically, when the number of elements is greater than the capacity of the hash table multiplied by its fill ratio, the hash table is expanded. If you do not specify a fill ratio, then 0.75 is used. Hashtable(Map < ? extends K, ? extends V>m) //This creates a hash table that is initialized with the elements in m. The capacity of the hash table is set to twice the number of elements in m. The default load factor of 0.75 is used.

Example of Hashtable

import java.util.*; class HashTableDemo < public static void main(String args[]) < Hashtableht = new Hashtable(); ht.put("a",new Integer(100)); ht.put("b",new Integer(200)); ht.put("c",new Integer(300)); ht.put("d",new Integer(400)); Set st = ht.entrySet(); Iterator itr=st.iterator(); while(itr.hasNext()) < Map.Entry m=(Map.Entry)itr.next(); System.out.println(itr.getKey()+" "+itr.getValue()); >> >

Difference between HashMap and Hashtable

Hashtable	HashMap
Hashtable class is synchronized.	HashMap is not synchronized.
Because of Thread-safe, Hashtable is slower than HashMap	HashMap works faster.
Neither key nor values can be null	Both key and values can be null
Order of table remain constant over time.	does not guarantee that order of map will remain constant over time.

Properties class

1. Properties class extends Hashtable class.
2. It is used to maintain list of value in which both key and value are String
3. Properties class define two constructor

Properties() //This creates a Properties object that has no default values Properties(Properties propdefault) //This creates an object that uses propdefault for its default values.

Note: In both cases, the property list is empty

Example of Properties class

import java.util.*; public class Test < public static void main(String[] args) < Properties pr = new Properties(); pr.put("Java", "James Ghosling"); pr.put("C++", "Bjarne Stroustrup"); pr.put("C", "Dennis Ritchie"); pr.put("C#", "Microsoft Inc."); Set< ?>creator = pr.keySet(); for(Object ob: creator) < System.out.println(ob+" was created by "+ pr.getProperty((String)ob) ); >> >

Java was created by James Ghosling C++ was created by Bjarne Stroustrup C was created by Dennis Ritchie C# was created by Microsoft Inc

Stack class

1. Stack class extends Vector.
2. It follows last-in, first-out principle for the stack elements.
3. It defines only one default constructor

Stack() //This creates an empty stack

You can use peek() method to return, but not remove, the top object. The empty() method returns true if nothing is on the stack. The search() method determines whether an object exists on the stack and returns the number of pops that are required to bring it to the top of the stack.

Example of Stack

import java.util.*; class StackDemo < public static void main(String args[]) < Stack st = new Stack(); st.push(11); st.push(22); st.push(33); st.push(44); st.push(55); Enumeration e1 = st.elements(); while(e1.hasMoreElements()) System.out.print(e1.nextElement()+" "); st.pop(); st.pop(); System.out.println("\nAfter popping out two elements"); Enumeration e2 = st.elements(); while(e2.hasMoreElements()) System.out.print(e2.nextElement()+" "); >>

11 22 33 44 55 After popping out two elements 11 22 33

Dictionary class

1. Dictionary is an abstract class.
2. It represents a key/value pair and operates much like Map.
3. Although it is not currently deprecated, Dictionary is classified as obsolete, because it is fully superseded by Map class.

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What is legacy java classes

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What is legacy java classes

Early version of java did not include the Collection framework. It only defined several classes and interface that provide method for storing objects. When Collection framework were added in J2SE 1.2, the original classes were reengineered to support the collection interface. These classes are also known as Legacy classes. All legacy claases and interface were redesign by JDK 5 to support Generics.

The following are the legacy classes defined by java.util package

There is only one legacy interface called Enumeration

NOTE: All the legacy classes are syncronized

Enumeration interface

1. Enumeration interface defines method to enumerate through collection of object.
2. This interface is suspended by Iterator interface.
3. However some legacy classes such as Vector and Properties defines several method in which Enumeration interface is used.
4. It specifies the following two methods

boolean hasMoreElements() Object nextElement()

Vector class

1. Vector is similar to ArrayList which represents a dynamic array.
2. The only difference between Vector and ArrayList is that Vector is synchronised while Array is not.
3. Vector class has following four constructor

Vector() Vector(int size) Vector(int size, int incr) Vector(Collection < ? extends E>c)

Vector defines several legacy method. Lets see some important legacy method define by Vector class.

Method	Description
addElement()	add element to the Vector
elementAt()	return the element at specified index
elements	return an enumeration of element in vector
firstElement()	return first element in the Vector
lastElement()	return last element in the Vector
removeAllElement()	remove all element of the Vector

Example of Vector

import java.util.*; public class Test < public static void main(String[] args) < Vectorve = new Vector(); ve.add(10); ve.add(20); ve.add(30); ve.add(40); ve.add(50); ve.add(60); Enumeration en = ve.elements(); while(en.hasMoreElements()) < System.out.println(en.nextElement()); >> >

Hashtable class

1. Like HashMap, Hashtable also stores key/value pair in hashtable. However neither keys nor values can be null.
2. There is one more difference between HashMap and Hashtable that is Hashtable is synchronized while HashMap is not.
3. Hashtable has following four constructor

Hashtable() Hashtable(int size) Hashtable(int size, float fillratio) Hashtable(Map < ? extends K, ? extends V>m)

Example of Hashtable

import java.util.*; class HashTableDemo < public static void main(String args[]) < Hashtable < String,Integer>ht = new Hashtable< String,Integer>(); ht.put("a",new Integer(100)); ht.put("b",new Integer(200)); ht.put("c",new Integer(300)); ht.put("d",new Integer(400)); Set st = ht.entrySet(); Iterator itr=st.iterator(); while(itr.hasNext()) < Map.Entry m=(Map.Entry)itr.next(); System.out.println(itr.getKey()+" "+itr.getValue()); >> >

Difference between HashMap and Hashtable

Hashtable	HashMap
Hashtable class is synchronized.	HastMap is not synchronize.
Because of Thread-safe, Hashtable is slower than HashMap	HashMap works faster.
Neither key nor values can be null	Both key and values can be null
Order of table remain constant over time.	does not guarantee that order of map remain constant over time.

Properties class

1. Properties class extends Hashtable class.
2. It is used to maintain list of value in which both key and value are String
3. Properties class define two constructor

Properties() Properties(Properties default)

Example of Properties class

import java.util.*; public class Test < public static void main(String[] args) < Properties pr = new Properties(); pr.put("Java", "James Ghosling"); pr.put("C++", "Bjarne Stroustrup"); pr.put("C", "Dennis Ritchie"); pr.put("C#", "Microsoft Inc."); Set< ?>creator = pr.keySet(); for(Object ob: creator) < System.out.println(ob+" was created by "+ pr.getProperty((String)ob) ); >> >

Java was created by James Ghosling C++ was created by Bjarne Stroustrup C was created by Dennis Ritchie C# was created by Microsoft Inc

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