Java, in its earliest versions, didn’t come with the Collection framework. Instead, it relied on defining specific classes and interfaces to store objects and their assigned methods. It wasn’t until J2SE 1.2 that the collection interface was introduced, which is commonly known as legacy classes in Java. Since then, these classes have been engineered to work correctly with Generics, and given below are some of the widespread usage of legacy classes:

• Dictionary: An abstract class in nature assigned to hold values as the value pair.
• Properties: Thread-safe by nature, it doesn’t require any manual sync to hold values in the key pair, and is an extension to Hashtable class.
• HashTable: It is a large part of the java.util package that is an extension to the dictionary class. In-sync with the Java1.2 framework, it works on the execution of the map interface, being a package of the collection framework.
• Vector: Identical to the ArrayList, except for a few stark differences, this class is mainly used by programmers who aren’t sure about the array’s length. 
• Stack: Commonly referred to as LIFO, it is an extension to the vector class.

Apart from the legacy classes, there is only one legacy interface in Java called the Enumeration. 

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The legacy classes in Java are what was called as the ad hoc classes that used to contain values and object groups. Java 2 used to be missing a unified framework called the collections framework before it was introduced in Java 2. The objectives of such a framework are:

• It needs to be a high-performing interface with several features like linked lists, trees, dynamic arrays, and more to remain hugely efficient. 
• It would also enable different collection frameworks to work in sync with its superior interoperability. 

In general, a collections framework stands for combined infrastructure built for the representation and operating methods and variables. Developed around a predefined set of standard interfaces like LinkedList, HashSet, and more, these frameworks can be entirely used as per its presets or customized accordingly.

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Read more: A Guide to Iterator Implementation in Java

Collection Interfaces: Enumeration Interface and legacy classes in Java

The Enumeration: It is a legacy interface by nature; it can enumerate several values into a collection of objects and is followed by an Iterator. 

The Enumeration interface works similarly to that of its superseding Iterator as it obtains values one at a time. This method is also invoked by the legacy classes in Java-like that of the Vector and Properties for carrying out a few ways. Here are some characteristics of an Enumeration interface:

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• Any class that works on the execution of the Enumeration interface would gain values in a long series, yielding one value at a time.
• Superseded by the Iterator, the enumeration processes only get copied by the Iterator. It also includes a few more methods like remove() and more in newer enforcing of the enumeration interface.

The methods followed by the Enumeration interface are:

• boolean hasMoreElements(): This function returns 1, if more values are remaining for extraction, and returns 0 if all the values have been enumerated. 
• Object nextElement(): This function returns the upcoming value in the enumeration process, and sets off the NoSuchElementException in the case of no more objects. 

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Vector Class Constructors and Legacy Methods

• Vector(): By-default vector creation with a predefined size of 10.
• Vector(int size): Creation of a vector with the initial capacity based upon its size.
• Vector(int size, int incr): Making a vector with its initial capacity specified by size and increment specified by incr. Here, the increment function designates respective elements every time the vector gets resized and added by objects.
• Vector (Collection C): Creation of a vector that consists of elements from the collection C

The Legacy Methods include:

• void addElement(E element): It helps in adding up of element into a vector
• E elementAt(int index): It returns the value at predefined index
• Enumeration elements(): This returns an item, obtained one-by-one in Vector
• E firstElement(): It returns only the first element in Vector
• E last element (): It returns only the final component of the vector.
• void removeAllElements(): It can remove all the elements of a vector.

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HashTable and HashMap: Key Differences 

That was all about legacy classes in Java, we hope you found that useful!

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Conclusion

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