title: it doesn't work by using MutableData.value in Android
body: <pre><code>class CounterViewModel: ViewModel() {
  
    val count = MutableLiveData&lt;Integer&gt;()
  
    fun increase() {
        count.value = count.value + 1
    }
  
    fun decrease() { 
  
    }
  }
  </code></pre>
  <p>and the 'count.value = count.value + 1' doesn't work.
  why can't i use plus in that code?</p>