Java Collections Framework

Collections

A data structure is a collection of data organized in some fashion. The structure not only stores data but also supports operations for accessing and manipulating the data. The java.util package contains one of Java’s most powerful subsystems: The Collections Framework. The Collections Framework is a sophisticated hierarchy of interfaces and classes that provide state-of-the-art technology for managing groups of objects.

So What Do You Do with a Collection?

There are a few basic operations you'll normally use with collections

• Add objects to the collection.
• Remove objects from the collection.
• Find out if an object (or group of objects) is in the collection.
• Retrieve an object from the collection (without removing it).
• Iterate through the collection, looking at each element (object) one after another.
• Key Interfaces and Classes of the Collections Framework 

The collections API begins with a group of interfaces, but also gives you a truckload of concrete classes.

Maps	Sets	Lists	Queues	Utilities
HashMap	HashSet	ArrayList	PriorityQueue	Collections
Hashtable	LinkedHashSet	Vector		Arrays
TreeMap	TreeSet	LinkedList
LinkedHashMap

The interface and class hierarchy for collections

Collections come in four basic flavors:

• Lists Lists of things (classes that implement List). 
• Sets Unique things (classes that implement Set). 
• Maps Things with a unique ID (classes that implement Map). 
• Queues Things arranged by the order in which they are to be processed. 

Ordered When a collection is ordered, it means you can iterate through the collection in a specific (not-random) order.

Sorted A sorted collection means that the order in the collection is determined according to some rule or rules, known as the sort order.

List Interface
A List cares about the index. The one thing that List has that non-lists don't have is a set of methods related to the index. Those key methods include things like get(int index), indexOf(Object o), add(int index, Object obj), and so on. All three List implementations are ordered by index position—a position that you determine either by setting an object at a specific index or by adding it without specifying position, in which case the object is added to the end. The three List implementations are described in the following sections.

1 . ArrayList Think of this as a growable array. It gives you fast iteration and fast random access. To state the obvious: it is an ordered collection (by index), but not sorted. You might want to know that as of version 1.4, ArrayList now implements the new RandomAccess interface—a marker interface (meaning it has no methods) that says, "this list supports fast (generally constant time) random access." Choose this over a LinkedList when you need fast iteration but aren't as likely to be doing a lot of insertion and deletion.

2 . Vector Vector is a holdover from the earliest days of Java; Vector and Hashtable were the two original collections, the rest were added with Java 2 versions 1.2 and 1.4. A Vector is basically the same as an ArrayList, but Vector methods are synchronized for thread safety. You'll normally want to use ArrayList instead of Vector because the synchronized methods add a performance hit you might not need. And if you do need thread safety, there are utility methods in class Collections that can help. Vector is the only class other than ArrayList to implement RandomAccess.

3. LinkedList A LinkedList is ordered by index position, like ArrayList, except that the elements are doubly-linked to one another. This linkage gives you new methods (beyond what you get from the List interface) for adding and removing from the beginning or end, which makes it an easy choice for implementing a stack or queue. Keep in mind that a LinkedList may iterate more slowly than an ArrayList, but it's a good choice when you need fast insertion and deletion. As of Java 5, the LinkedList class has been enhanced to implement the java.util.Queue interface. As such, it now supports the common queue methods: peek(), poll(), and offer().

Set Interface
A Set cares about uniqueness—it doesn't allow duplicates. Your good friend the equals() method determines whether two objects are identical (in which case only one can be in the set). The three Set implementations are described in the following sections.

1. HashSet A HashSet is an unsorted, unordered Set. It uses the hashcode of the object being inserted, so the more efficient your hashCode() implementation the better access performance you'll get. Use this class when you want a collection with no duplicates and you don't care about order when you iterate through it.

2. LinkedHashSet A LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Use this class instead of HashSet when you care about the iteration order. When you iterate through a HashSet the order is unpredictable, while a LinkedHashSet lets you iterate through the elements in the order in which they were inserted.

3. TreeSet The TreeSet is one of two sorted collections (the other being TreeMap). It uses a Red-Black tree structure (but you knew that), and guarantees that the elements will be in ascending order, according to natural order. Optionally, you can construct a TreeSet with a constructor that lets you give the collection your own rules for what the order should be (rather than relying on the ordering defined by the elements' class) by using a Comparable or Comparator.

Map Interface
A Map cares about unique identifiers. You map a unique key (the ID) to a specific value, where both the key and the value are, of course, objects. You're probably quite familiar with Maps since many languages support data structures that use a key/value or name/value pair. The Map implementations let you do things like search for a value based on the key, ask for a collection of just the values, or ask for a collection of just the keys. Like Sets, Maps rely on the equals() method to determine whether two keys are the same or different.

1. HashMap The HashMap gives you an unsorted, unordered Map. When you need a Map and you don't care about the order (when you iterate through it), then HashMap is the way to go; the other maps add a little more overhead. Where the keys land in the Map is based on the key's hashcode, so, like HashSet, the more efficient your hashCode() implementation, the better access performance you'll get.HashMap allows one null key and multiple null values in a collection.

2. Hashtable Like Vector, Hashtable has existed from prehistoric Java times. For fun, don't forget to note the naming inconsistency: HashMap vs. Hashtable. Where's the capitalization of t? Oh well, you won't be expected to spell it. Anyway, just as Vector is a synchronized counterpart to the sleeker, more modern ArrayList, Hashtable is the synchronized counterpart to HashMap. Remember that you don't synchronize a class, so when we say that Vector and Hashtable are synchronized, we just mean that the key methods of the class are synchronized. Another difference, though, is that while HashMap lets you have null values as well as one null key, a Hashtable doesn't let you have anything that's null.

3. LinkedHashMap Like its Set counterpart, LinkedHashSet, the LinkedHash-Map collection maintains insertion order (or, optionally, access order). Although it will be somewhat slower than HashMap for adding and removing elements, you can expect faster iteration with a LinkedHashMap.

4. TreeMap You can probably guess by now that a TreeMap is a sorted Map. And you already know that by default, this means "sorted by the natural order of the elements." Like TreeSet, TreeMap lets you define a custom sort order (via a Comparable or Comparator) when you construct a TreeMap, that specifies how the elements should be compared to one another when they're being ordered.

Queue Interface
A Queue is designed to hold a list of "to-dos," or things to be processed in some way. Although other orders are possible, queues are typically thought of as FIFO (first-in, first-out). Queues support all of the standard Collection methods and they also add methods to add and subtract elements and review queue elements.

1. PriorityQueue This class is new with Java 5. Since the LinkedList class has been enhanced to implement the Queue interface, basic queues can be handled with a LinkedList. The purpose of a PriorityQueue is to create a "priority-in, priority out" queue as opposed to a typical FIFO queue. A PriorityQueue's elements are ordered either by natural ordering (in which case the elements that are sorted first will be accessed first) or according to a Comparator. In either case, the elements' ordering represents their relative priority.

Basic Features of Main Interfaces

Core collection interfaces are the base to Java Collections framework. They include the interfaces Collection, Set, List, Queue and Map on which all the data structures are built.

1. Collection is the root interface for all the hierarchy (except Map).
2. Set interface unique feature is that it does not accept duplicate elements. That is, no two elements will be the same.
3. SortedSet interface is derived from Set interface and adds one more feature that the elements are arranged in sorted order by default.
4. List interface permits duplicate elements.
5. Queue interface holds elements and returns in FIFO order.
6. Map adds the elements in key/value pairs. Duplicate keys are not allowed, but duplicate values are allowed. One key can map one value only.
7. SortedMap interface is a particular case of Map. Keys are sorted by default.

How Clone method works in JAVA...???

What is clone of object in Java

An object which is returned by clone() method is known as clone of original instance. A clone object should follow basic characteristics e.g. a.clone() != a, which means original and clone are two separate object in Java heap, a.clone().getClass() == a.getClass() and clone.equals(a), which means clone is exact copy of original object. These characteristic is followed by a well behaved, correctly overridden clone() method in Java, but it's not enforced by cloning mechanism. Which means, an object returned by clone() method may violate any of these rules. By following convention of returning object by calling super.clone(), when overriding clone() method, you can ensure that it follows first two characteristics. In order to follow third characteristic, you must override equals method to enforce logical comparison, instead of physical comparison exists in java.lang.Object. For example, clone() method of Rectangle class in this method return object, which has these characteristics, but if you run same program by commenting equals(), you will see that third invariant i.e. clone.equals(a) will return false. By the way there are couple of good items on Effective Java regarding effective use of clone method, I highly recommend to read those items after going through this article.

How Clone method works in Java

java.lang.Object provides default implementation of clone() method in Java. It's declared as protected and native in Object class, so implemented in native code. Since it's convention to return clone() of object by calling super.clone() method, any cloning process eventually reaches to java.lang.Object clone() method. This method, first checks if corresponding object implements Cloneableinterface, which is a marker interface. If that instance doesn't implements Cloneable then it throws CloneNotSupported in Java, a checked exception, which is always required to be handled while cloning an object. If object pass this check, than java.lang.Object's clone() method creates a shallow copy of object and returned it to the caller. Since Object class' clone() method creates copy by creating new instance, and then copying field-by-field, similar to assignment operator, it's fine for primitives and Immutable object, but not suited if your class contains some mutable data-structure e.g. Collection classes like ArrayList or arrays. In that case, both original object and copy of object will point to the same object in heap. You can prevent this by using technique known as deep cloning, on which each mutable field is cloned separately. In short, here is how clone method works in Java :

1) Any class calls clone() method on instance, which implements Cloneable and overrides protected clone() method from Object class, to create a copy.

  Rectangle rec = new Rectangle(30, 60);

  logger.info(rec);

      

    try {

         logger.info("Creating Copy of this object using Clone method");

         Rectangle copy = rec.clone();

         logger.info("Copy " + copy);

          

    } catch (CloneNotSupportedException ex) {

         logger.debug("Cloning is not supported for this object");

    }

2) Call to clone() method on Rectangle is delegated to super.clone(), which can be a custom super class or by default java.lang.Object

    @Override

    protected Rectangle clone() throws CloneNotSupportedException {

        return (Rectangle) super.clone();

    }

3) Eventually call reaches to java.lang.Object's clone() method, which verify if corresponding instance implements Cloneable interface, if not then it throws CloneNotSupportedException, otherwise it creates a field-by-field copy of instance of that class and returned to caller.

So in order for clone() method to work properly, two things need to happen, a Class should implement Cloneable interface and should override clone() method of Object class. By the way this was the simplest example of overriding clone method and how it works, things gets more complicated with real object, which contains mutable fields, arrays, collections, Immutable object and primitives.

Key Points to remember...

1. Clone method is used to create a copy of object in Java. In order to use clone() method, class must implement java.lang.Cloneable interface and override protected clone() method from java.lang.Object. A call to clone() method will result in CloneNotSupportedException, if that class doesn't implement Cloneable interface.
2. No constructor is called during cloning of Object in Java.
3. Default implementation of clone() method in Java provides "shallow copy" of object, because it creates copy of Object by creating new instance and then copying content by assignment, which means if your Class contains a mutable field, then both original object and clone will refer to same internal object. This can be dangerous, because any change made on that mutable field will reflect in both original and copy object. In order to avoid this, override clone() method to provide deep copy of object.

Java - Exception Hierarchy

An exception is a problem that arises during the execution of a program. An exception can occur for many different reasons, including the following:

• A user has entered invalid data.
• A file that needs to be opened cannot be found.
• A network connection has been lost in the middle of communications or the JVM has run out of memory.

Some of these exceptions are caused by user error, others by programmer error, and others by physical resources that have failed in some manner.

All exception classes are subtypes of the java.lang.Exception class. The exception class is a subclass of the Throwable class. Other than the exception class there is another subclass called Error which is derived from the Throwable class.

Errors are not normally trapped form the Java programs. These conditions normally happen in case of severe failures, which are not handled by the java programs. Errors are generated to indicate errors generated by the runtime environment. Example : JVM is out of Memory. Normally programs cannot recover from errors.

Catching Exceptions:

A method catches an exception using a combination of the try and catch keywords. A try/catch block is placed around the code that might generate an exception. Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:

try
{
   //Protected code
}catch(ExceptionName e1)
{
   //Catch block
}

A catch statement involves declaring the type of exception you are trying to catch. If an exception occurs in protected code, the catch block (or blocks) that follows the try is checked. If the type of exception that occurred is listed in a catch block, the exception is passed to the catch block much as an argument is passed into a method parameter.

The throws/throw Keywords:

If a method does not handle a checked exception, the method must declare it using the throws keyword. The throws keyword appears at the end of a method's signature.
You can throw an exception, either a newly instantiated one or an exception that you just caught, by using thethrow keyword. Try to understand the different in throws and throw keywords.
The following method declares that it throws a RemoteException:

import java.io.*;
public class className
{
   public void deposit(double amount) throws RemoteException
   {
      // Method implementation
      throw new RemoteException();
   }
   //Remainder of class definition
}

For more info about throw click here

The finally Keyword:

The finally keyword is used to create a block of code that follows a try block. A finally block of code always executes, whether or not an exception has occurred.
Using a finally block allows you to run any cleanup-type statements that you want to execute, no matter what happens in the protected code.
A finally block appears at the end of the catch blocks and has the following syntax:

try
{
   //Protected code
}catch(ExceptionType1 e1)
{
   //Catch block
}catch(ExceptionType2 e2)
{
   //Catch block
}catch(ExceptionType3 e3)
{
   //Catch block
}finally
{
   //The finally block always executes.
}

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