String Buffer

 StringBuffer.java

A string buffer is like a {@link String}, but can be modified.At any 
  point in time it contains some particular sequence of characters, but 
  the length and content of the sequence can be changed through certain 
  method calls.

Exampl - ReverseString.java

public 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();
}
}

Exception in thread "main" java.lang.NoClassDefFoundError: echoStr ERROR

java -classpath . helloworld

 Run your program using this.