java - 由于 NullPointerException，应用程序崩溃

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我正在尝试创建一个启动NewEventActivity的 Intent 不幸的是，当按下 float 操作按钮时，应用程序崩溃了。这是我的代码:

主要 Activity :

    public class MainActivity extends AppCompatActivity {
        @Override
        protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

            }
            public void newEventIntent(View v){
                Intent intent = new Intent(this, NewEventActivity.class);
                startActivity(intent);
        }
    }

activity_main.xml:

        <android.support.design.widget.FloatingActionButton
        android:onClick="newEventIntent"
        android:id="@+id/add_event_fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:layout_alignParentBottom="true"
        android:layout_marginRight="20dp"
        android:layout_marginBottom="20dp"
        android:src="@drawable/baseline_add_24"/>

LogCat:

java.lang.RuntimeException: Unable to start activity componentInfo{com.example.deadline/com.example.deadline.NewEventActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.design.widget.FloatingActionButton.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

我确保这两项 Activity 都在 list 中。

Answer 1

原因是您尝试为 FloatingActionButton 设置监听器，但它在布局中没有引用。

您应该通过 id 找到它:

添加private FloatingActionButton mFloatingActionButton;

和

floatingActionButton=findViewById(R.id.add_event_fab);

public class MainActivity extends AppCompatActivity {

    private FloatingActionButton mFloatingActionButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    floatingActionButton=findViewById(R.id.add_event_fab);

        }
        public void newEventIntent(View v){
            Intent intent = new Intent(this, NewEventActivity.class);
            startActivity(intent);
    }
}