cd to the folder that contains .class file, type the command below (without .class extension). The class will be de-compiled to java code

C:\Path-to-Class-File\classes\mypackage>javap -c Hello

You’ll see the code of .java file that look like this in the terminal.

Compiled from "Hello.java"
public final class mypackage.Hello extends javax.servlet.http.HttpServlet{
public mypackage.Hello();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method javax/servlet/http/HttpServlet."<init>":()V

   4:   return

public void doGet(javax.servlet.http.HttpServletRequest, javax.servlet.http.Http
ServletResponse)   throws java.io.IOException, javax.servlet.ServletException;
...