title: Why set is not allowed duplicate value, which kind of mechanism used behind them?
body: <p>I am new to java, i know <strong><em>set is not allowed duplicate value</em></strong> but i don't know <strong><em>why set is not allowed duplicate value</em></strong>, Actually i am doing practically, </p>
  
  <p>Declared one set and add duplicate value but no kind of error is occurring, no compile time error, no run time. <strong>why?</strong></p>