Click here to watch in Youtube :
https://www.youtube.com/watch?v=aMOlLxjzPtI&list=UUhwKlOVR041tngjerWxVccw

StringBuilderDemo.java

/*
 * public int length()
 * 
 * Parameters: 
 * ----------
 * the length of the sequence of characters currently
 * represented by this object
 */
public class StringBuilderDemo
{

    public static void main(String[] args)
    {
        StringBuilder sb = new StringBuilder();
        System.out.println("sb = " + sb);
        System.out.println("length = " + sb.length());

        sb.append("Welcome");
        System.out.println("sb = " + sb);
        System.out.println("length = " + sb.length());

    }
}