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Trail: Learning the Java Language
Lesson: Object and Data Basics

## Accessor Methods

> An object's instance variables are encapsulated within the object, hidden inside, safe from inspection or manipulation by other objects. With certain well-defined exceptions, the object's methods are the only means by which other objects can inspect or alter an object's instance variables. Encapsulation of an object's data protects the object from corruption by other objects and conceals an object's implementation details from outsiders. This encapsulation of data behind an object's methods is one of the cornerstones of object-oriented programming.

> Methods used to obtain information about an object are known as accessor methods. The `reverseIt` method uses two of `String`'s accessor methods to obtain information about the `source` string.

> >     class ReverseString {
> >         public static String reverseIt(String source) {
> >             int i, len = source.length();
> >             StringBuffer dest = new StringBuffer(len);

> >             for (i = (len - 1); i >= 0; i--) {
> >                 dest.append(source.charAt(i));
> >             }
> >             return dest.toString();
> >         }
> >     }

> First, `reverseIt` uses `String`'s `length` accessor method to obtain the length of the `String` `source`.

> >     int len = source.length();

> Note that `reverseIt` doesn't care if `String` maintains its length attribute as an integer, as a floating point number, or even if `String` computes its length on the fly. `reverseIt` simply relies on the public interface of the `length` method, which returns the length of the `String` as an integer. That's all `reverseIt` needs to know.

> Second, `reverseIt` uses the `charAt` accessor, which returns the character at the position specified in the parameter.

> >     source.charAt(i)

> The character returned by `charAt` is then appended to the `StringBuffer` `dest`. Since the loop variable `i` begins at the end of `source` and proceeds backwards over the string, the characters are appended in reverse order to the `StringBuffer`, thereby reversing the string.

### More Accessor Methods

> In addition to `length` and `charAt`, `String` supports a number of other accessor methods that provide access to substrings and the indices of specific characters in the `String`. `StringBuffer` has its own set of similar accessor methods.

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