how to set hover in bootstrap code example

Example 1: bootstrap 4 class for hover

<div class="view overlay">
    <img src="https://mdbootstrap.com/img/Photos/Horizontal/City/6-col/img%20(3).jpg" class="img-fluid " alt="">
     <div class="mask rgba-red-strong">

    </div>
</div>

Example 2: hover option bootstrap

<div class="dropdown">
  <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown" data-hover="dropdown">
   Dropdown <span class="caret"></span>
  </button>
  <ul class="dropdown-menu">
    <li><a href="#">Action</a></li>
    <li><a href="#">Another action</a></li>
    <li class="dropdown">
      <a href="#">One more dropdown</a>
      <ul class="dropdown-menu">
        <li><a href="#">Action</a></li>
        <li><a href="#">Another action</a></li>
        <li class="dropdown">
          <a href="#">One more dropdown</a>
          <ul class="dropdown-menu">
          ...
          </ul>
        </li>
        <li><a href="#">Something else here</a></li>
        <li><a href="#">Separated link</a></li>
       </ul>
    </li>
    <li><a href="#">Something else here</a></li>
    <li><a href="#">Separated link</a></li>
  </ul>
</div>

Example 3: bootstrap div hoover

.hoverDiv {background: #fff;}
.hoverDiv:hover {background: #f5f5f5;}