java - 我的应用程序在 android studio 中崩溃了，在查看日志时我无法弄清楚出了什么问题

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 7 年前。

所以我试图在 Android Studio 中构建一个登录应用程序，当我测试它时，该应用程序由于某种原因崩溃了。我只能通过查看日志来找出问题，但我无法针对此异常错误执行此操作。下面粘贴的是我的日志。

02-09 16:18:09.482 2467-2467/? I/art: Not late-enabling -Xcheck:jni         (already on)
02-09 16:18:09.585 2467-2467/edu.dtcc.bwharto9.loginform W/System:  ClassLoader referenced unknown         path: /data/app/edu.dtcc.bwharto9.loginform-2/lib/x86
02-09 16:18:09.590 2467-2467/edu.dtcc.bwharto9.loginform D/AndroidRuntime: Shutting down VM
02-09 16:18:09.590 2467-2467/edu.dtcc.bwharto9.loginform E/AndroidRuntime: FATAL EXCEPTION: main
Process: edu.dtcc.bwharto9.loginform, PID: 2467
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{edu.dtcc.bwharto9.loginform/edu.dtcc.bwharto9.loginform.MainActivity}:                
java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2327)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
at android.app.Activity.findViewById(Activity.java:2090)
at edu.dtcc.bwharto9.loginform.MainActivity.<init>(MainActivity.java:17)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1067)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2317)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
at android.app.ActivityThread.-wrap11(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5417) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) `

这也是我的 Java

    package edu.dtcc.bwharto9.loginform;

    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.EditText;
    import android.widget.Toast;

    public class MainActivity extends AppCompatActivity {'

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

    EditText userName = (EditText)findViewById(R.id.userName);
    EditText password = (EditText)findViewById(R.id.password);
    public void login() {
        if (userName.getText().toString().equals("student") && password.getText().toString().equals("password"))
        {
            Toast.makeText(getApplicationContext(), "You have sucessfully logged in!", Toast.LENGTH_LONG).show();
        }
        else
        {
            Toast.makeText(getApplicationContext(), "Login error! Please check your username or password!", Toast.LENGTH_LONG).show();
        }
    }

}

XML

 <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="edu.dtcc.bwharto9.loginform.MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Please enter your username and Password "
        android:id="@+id/instructionsText"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="37dp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:ems="10"
        android:id="@+id/userName"
        android:layout_below="@+id/instructionsText"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="55dp"
        android:hint="Username" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPassword"
        android:ems="10"
        android:id="@+id/password"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:hint="Password"
        android:password="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Log In"
        android:id="@+id/btnLogIn"
        android:layout_below="@+id/password"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="51dp"
        android:onClick="login" />
</RelativeLayout>

感谢所有帮助我的人，我已经完全康复了!

Answer 1

EditText userName = (EditText)findViewById(R.id.userName);
EditText password = (EditText)findViewById(R.id.password);

这行不通。您正在从字段初始值设定项调用 findViewById()。一般来说，您无法可靠地调用从 Activity 继承的方法。而且，对于 findViewById()，只有在调用 setContentView() 后才能获取结果。

将这些行替换为:

EditText userName;
EditText password;

然后，将 onCreate() 替换为:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    userName = (EditText)findViewById(R.id.userName);
    password = (EditText)findViewById(R.id.password);
}