import javax.swing.JOptionPane;


class UpperCase {

public static void main(String[] args){
  String name;
  int count, lengt;
   int converted =0;
  char letter;
  
  name = JOptionPane.showInputDialog(null, "Please Enter a Sentence.");
    


  count = name.length();

      for (int i = 0; i < name.length(); i++) {
      letter = name.charAt(i);
                      char ch = name.charAt(i);

                      if (Character.isUpperCase(ch))
                          ch = Character.toLowerCase(ch);
                      else if (Character.isLowerCase(ch))
                          ch = Character.toUpperCase(ch); {

                      converted++;
                  }
}
JOptionPane.showMessageDialog(null,  " Your sentence swithed converted " +  "" +
converted);
}
}

boolean isUpperCase(char c)
boolean isLowerCase(char c)
char toUpperCase(char c)
char toLowerCase(char c)

import javax.swing.JOptionPane;

//Warning - if you don't declare this as "public class UpperCase" (without quotes) it will have package scope.
class UpperCase //Is this the best and most descriptive name? Perhaps something like ChangeCase would be better.
{
   //It might be a good idea to use methods - if you're not sure how try googling "java methods", you'll find a whole host of resources.
   public static void main(String[] args)
   {
      StringBuffer temp = new StringBuffer(); //this is a mutable string class
      String name;
      //int count, lengt; //these variables aren't doing anything
      //int converted = 0;//Why do you need a count of how many characters it is looping through? You're not using it.
      //char letter; //this variable isn't doing anything
      char ch; //moved the declaration to here
      
      try //If the user presses cancel it bombs out with a null pointer exception. At the very least you should catch this. There are better ways of writing this, but this is quick and simple.
      {
         name = JOptionPane.showInputDialog(null, "Please enter a sentence.");
      }
      catch(NullPointerException npe)
      {
         System.exit(0); //exit the program if the user presses cancel
      }

      
      //count = name.length(); //this isn't doing anything
      
      for (int i = 0; i < name.length(); i++)
      {
         //letter = name.charAt(i); //redundant - ch is doing al the work
         ch = name.charAt(i); //moved the declaration from here
        
         if (Character.isUpperCase(ch))
            ch = Character.toLowerCase(ch);
         else if (Character.isLowerCase(ch))
            ch = Character.toUpperCase(ch);
         //{ //these braces are unneccesary
        
         //you aren't actually changing the string, you are just picking a character out of it and changing the character
         temp.append(ch);//this appends a character onto the StringBuffer
        
         //converted++; //you're not using this count anywhere
         //} //these braces are unneccesary
      }
      
      //JOptionPane.showMessageDialog(null, " Your sentence swithed converted " + "" + converted);//you're not outputing the string - you're outputting a count of how many characters are in the string
      JOptionPane.showMessageDialog(null, " Your sentence converted is " + temp.toString()); //try this - changed the output text to "Your sentence converted is", took away empty quotes and changed variable to temp.toString (which is how to extract a String from a StringBuffer)
   }
}

import javax.swing.JOptionPane;

class ChangeCase
{
   public static void main(String[] args)
   {
      StringBuffer temp = new StringBuffer();
      String name;
      char ch;
      
      try
      {
         name = JOptionPane.showInputDialog(null, "Please enter a sentence.");
      }
      catch(NullPointerException npe)
      {
         System.exit(0);
      }
      
      for (int i = 0; i < name.length(); i++)
      {
         ch = name.charAt(i);
        
         if (Character.isUpperCase(ch))
            ch = Character.toLowerCase(ch);
         else if (Character.isLowerCase(ch))
            ch = Character.toUpperCase(ch);
        
         temp.append(ch);
      }
      
      JOptionPane.showMessageDialog(null, " Your sentence converted is " + temp.toString());
   }
}