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IMMUTABILITY in Java.

Summary: Immutable objects have a number of properties that make working with them easier, including relaxed synchronization requirements and the freedom to share and cache object references without concern for data corruption. While immutability may not necessarily make sense for all classes, most programs have at least a few classes that would benefit from being immutable.

An immutable object is one whose externally visible state cannot change after it is instantiated. The String, Integer, and BigDecimal classes in the Java class library are examples of immutable objects — they represent a single value that cannot change over the lifetime of the object.

Benefits of immutability.

Immutable classes, when used properly, can greatly simplify programming. They can only be in one state, so as long as they are properly constructed, they can never get into an inconsistent state. You can freely share and cache references to immutable objects without having to copy or clone them; you can cache their fields or the results of their methods without worrying about the values becoming stale or inconsistent with the rest of the object’s state. Immutable classes generally make the best map keys. And they are inherently thread-safe, so you don’t have to synchronize access to them across threads.

Freedom to cache.

Because there is no danger of immutable objects changing their value, you can freely cache references to them and be confident that the reference will refer to the same value later. Similarly, because their properties cannot change, you can cache their fields and the results of their methods.
If an object is mutable, you have to exercise some care when storing a reference to it. Consider the code in Listing 1, which queues two tasks for execution by a scheduler. The intent is that the first task would start now and the second task would start in one day.

When to use immutable classes.

Immutable classes are ideal for representing values of abstract data types, such as numbers, enumerated types, or colors. The basic numeric classes in the Java class library, such as Integer, Long, and Float, are immutable, as are other standard numeric types such as BigInteger and BigDecimal. Classes for representing complex numbers or arbitrary-precision rational numbers would be good candidates for immutability. Depending on your application, even abstract types that contain many discrete values — such as vectors or matrices — might be good candidates for implementing as immutable classes.
Another good example of immutability in the Java class library is java.awt.Color. While colors are generally represented as an ordered set of numeric values in some color representation (such as RGB, HSB, or CMYK), it makes more sense to think of a color as a distinguished value in a color space, rather than an ordered set of individually addressable values, and therefore it makes sense to implement Color as an immutable class.
Should we represent objects that are containers for multiple primitive values, such as points, vectors, matrices, or RGB colors, with mutable or immutable objects? The answer is. . . it depends. How are they going to be used? Are they being used primarily to represent multi-dimensional values (like the color of a pixel), or simply as containers for a collection of related properties of some other object (like the height and width of a window)? How often are these properties going to be changed? If they are changed, do the individual component values have meaning within the application on their own?
Events are another good example of candidates for implementation with immutable classes. Events are short-lived, and are often consumed in a different thread than they were created in, so making them immutable has more advantages than disadvantages. Most AWT event classes are not implemented as being strictly immutable, but could be with small modifications. Similarly, in a system that uses some form of messaging to communicate between components, making the message objects immutable is probably sensible.

Guidelines for writing immutable classes.

Writing immutable classes is easy. A class will be immutable if all of the following are true:
All of its fields are final
The class is declared final
The this reference is not allowed to escape during construction
Any fields that contain references to mutable objects, such as arrays, collections, or mutable classes like Date:
Are private
Are never returned or otherwise exposed to callers
Are the only reference to the objects that they reference
Do not change the state of the referenced objects after construction

Why does everything have to be in a class ?

Java is an object-oriented(00) language. It’s not Iike the old days
when you had steam driven compilers and wrote one monolithic source
file with a pile of procedures.
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What is method and instance variable ?

Things an object knows about itself are called instance variable.
While Things an object can do are called methods.
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What’s the difference between a class and an object?

A class is blueprint for an object . It tells JVM how to make an
object of that perticular type. Each object made from that class
can have its own values for the instance variables of that class.
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Identify who am I ?

I am compiled from a .java file. : class
My instance variable values can be different from my buddy’s values. : object
I behave like a template. : class
I like to do stuff. : object, method
I can have many methods. : class,object
I represent ‘state’. : instance variable
I have behaviors. : object,class
I am located in objects. : method, instance variable
I live on the heap. : object
I am used to create object instances. : class
My state can change.: object,instance variable
I declare methods. : class
I can change at runtime. : object, instance variable
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What happens if the argument you want to pass is an object instead of a primitive?

Java passes everything by value. Everything. But…value means bits
inside the variable. And remember, you don’t stuff objects into variables;
the variable is a remote control – a reference to an object. So, if you pass a
reference to an object into a method, you’re passing a copy of the remote control.
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Can a method declare multiple return values? Or Is there some way to return more than one value?

Sort of.A method can declare only one return value. BUT… If you want to return, say, three int values,
then the declared return type can be an Int array. Stuff those ints into the array,and passIt on back. It’s
a little more involved to return multiple values with different types.
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Some basics about java

1. Real-world objects contain state and behavior.
2. A software object’s state is stored in fields.
3. A software object’s behavior is exposed through methods.
4. Hiding internal data from the outside world, and accessing it only through publicly exposed methods is known as data encapsulation.
5. A blueprint for a software object is called a class.
6. Common behavior can be defined in a superclass and inherited into a subclass using the extends keyword.
7. A collection of methods with no implementation is called an interface.
8. A namespace that organizes classes and interfaces by functionality is called a package.
9. The term API stands for Application Programming Interface.

Simplest Example of making reverse string using java…