NullPointerException setOnClickListener

Question

我正在使用Android Studio中的应用程序，但是在运行它时，出现以下错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

我该如何解决？

Answer 1

检查以下各项是否正确完成：

1. 您的按钮显示在Java代码随附的适当XML布局中。像这样：

   :root {
       --anchorsFg: #0DAFA4;
   }
   a, a:visited, a:focus, a:active {
       text-decoration: none;
       color: var(--anchorsFg);
       outline: 0;
       font-style: italic;
   
       -webkit-transition: color 250ms ease-in-out;
       -moz-transition: color 250ms ease-in-out;
       -ms-transition: color 250ms ease-in-out;
       -o-transition: color 250ms ease-in-out;
       transition: color 250ms ease-in-out;
   }
   .sliding-middle-out {
       display: inline-block;
       position: relative;
       padding-bottom: 1px;
   }
   .sliding-middle-out:after {
       content: '';
       display: block;
       margin: auto;
       height: 1px;
       width: 0px;
       background-color: transparent;
   
       -webkit-transition: width 250ms ease-in-out, background-color 250ms ease-in-out;
       -moz-transition: width 250ms ease-in-out, background-color 250ms ease-in-out;
       -ms-transition: width 250ms ease-in-out, background-color 250ms ease-in-out;
       -o-transition: width 250ms ease-in-out, background-color 250ms ease-in-out;
       transition: width 250ms ease-in-out, background-color 250ms ease-in-out;
   }
   .sliding-middle-out:hover:after {
       width: 100%;
       background-color: var(--anchorsFg);
       outline: 0;
   }
   #reference {
     margin-top: 20px;
   }
   .sliding-middle-out:before {
     content: attr(data-content);
     display: attr(data-display);
   }
   

2. 检查此按钮是否已在Java代码中声明和初始化。像这样：

   var anchorsFg = randomColor();
   $( ".sliding-middle-out" ).hover(function(){
       $( ":root" ).css({"--anchorsFg" : anchorsFg});
   });
   
   $( "#reference" ).hover(
    function(){
       $(this).attr("data-content", "Hello World!").attr("data-display", "block").html("");
    },
    function(){
       $(this).attr("data-content", "Reference").attr("data-display", "inline").html("");
    }
   );
   

3. 检查onClickListner是否已正确设置为此按钮。像这样：

   <Button
       android:id="@+id/btn"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:text="@string/button_text" />
   

Answer 2

请确保在设置侦听器之前，已初始化您的按钮视图（即findViewById或通过依赖项注入）。