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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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