Android Studio 中的初始屏幕问题：错误 "Unfortunately MyApplication has stopped"、java.lang.NullPointerException

Question

我正在运行启动画面 5 秒钟。一旦这段时间结束，我的应用程序将终止，而不是转到 MainActivity.java，并给我一个错误，指出"不幸的是，我的应用程序已停止"。

请查看下面的日志猫。

我也在想我可能在AndroidManifest.xml中错误地编写了动作android：name和类别android：name。

我在编程方面非常陌生，正在从Eclipse教程中学习Android Studio。我将非常感谢详细的指导。

下面是我的代码。

AndroidMenifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.myapplication">
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".Splash"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="com.example.android.myapplication.MainActivity" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>
</manifest>

主活动.java

package com.example.android.myapplication;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {

    int counter = 0;
    Button add = (Button) findViewById(R.id.bAdd);
    Button sub = (Button) findViewById(R.id.bSub);
    TextView display = (TextView) findViewById(R.id.tView);

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });


        add.setOnClickListener(new View.OnClickListener(){
            public void onClick(View v){
                counter++;
                display.setText("Your total is " + counter);
            }
        });
        sub.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                counter = counter - 1;
                display.setText("Your total is " + counter);
            }
        });


    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

飞溅.xml

package com.example.android.myapplication;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
/**
 * Created by Psp on 2016-02-02.
 */
public class Splash extends Activity {

    @Override
    protected void onCreate(Bundle PLovesMishti) {
        super.onCreate(PLovesMishti);
        setContentView(R.layout.splash);
        Thread timer = new Thread() {
            public void run() {
                try {
                    sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    Intent openStartingPoint = new Intent("com.example.android.myapplication.MainActivity");
                    startActivity(openStartingPoint);
                }
            }
        };
        timer.start();
    }
}

日志猫

02-03 23:53:45.920: E/AndroidRuntime(4264): ***FATAL EXCEPTION: main***
02-03 23:53:45.920: E/AndroidRuntime(4264): Process: com.example.android.myapplication, PID: 4264
02-03 23:53:45.920: E/AndroidRuntime(4264): ***java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.myapplication/com.example.android.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference***
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2209)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread.access$800(ActivityThread.java:144)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.os.Handler.dispatchMessage(Handler.java:102)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.os.Looper.loop(Looper.java:135)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread.main(ActivityThread.java:5221)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at java.lang.reflect.Method.invoke(Native Method)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at java.lang.reflect.Method.invoke(Method.java:372)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
02-03 23:53:45.920: E/AndroidRuntime(4264): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.Activity.findViewById(Activity.java:2071)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at com.example.android.myapplication.MainActivity.<init>(MainActivity.java:18)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at java.lang.reflect.Constructor.newInstance(Native Method)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at java.lang.Class.newInstance(Class.java:1572)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
02-03 23:53:45.920: E/AndroidRuntime(4264):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2199)

Answer 1

更改行

Intent openStartingPoint = new Intent("com.example.android.myapplication.MainActivity");
startActivity(openStartingPoint);

自

Intent openStartingPoint = new Intent(Splash.this, MainActivity.class);
startActivity(openStartingPoint);

您在活动实例化之前获得视图

Button add = (Button) findViewById(R.id.bAdd);
Button sub = (Button) findViewById(R.id.bSub);
TextView display = (TextView) findViewById(R.id.tView);

从全局声明中删除这些行，并将其添加到setContentView(R.layout.activity_main)后的MainActivity onCreate()

public void onCreate() {
    ....
    setContentView(R.layout.activity_main);
    ....
    Button add = (Button) findViewById(R.id.bAdd);
    Button sub = (Button) findViewById(R.id.bSub);
    TextView display = (TextView) findViewById(R.id.tView);
}

Answer 2

试试这个。问题是你的AndroidMenifest.xml代码只删除一些代码。

<intent-filter>
    <action android:name="com.example.android.myapplication.MainActivity" />
    <category android:name="android.intent.category.DEFAULT" />
</intent-filter>

在 MainActivity.java 文件中的以下更改 -

Intent openStartingPoint = new Intent("com.example.android.myapplication.MainActivity");
startActivity(openStartingPoint);

到下面给出的代码 -

Intent openStartingPoints = new Intent(SplashActivity.this,MainActivity.class);
startActivity(openStartingPoints);

Answer 3

更改行：

 Intent openStartingPoint = new Intent("com.example.android.myapplication.MainActivity");
    startActivity(openStartingPoint);

自

Intent openStartingPoint = new Intent(SplashActivity.this,MainActivity.class);
startActivity(openStartingPoint);

它可能会对你有所帮助。

Answer 4

使用此行并粘贴您的主活动对于空指针例外，因为初始化您的主活动视图似乎会出错。

Intent i= new Intent（SplashActivity.this，MainActivity.class）;启动活动（i）;

Answer 5

日志猫说你指向一个空对象基本上意味着计算机看到该对象，但它的值为 0 女巫意味着 ethier 它不存在或它没有数据可能想要检查您试图拉 spash 的位置.xml您的问题出在主要活动和飞溅上.xml你有时间.start（）; 没有停止点，女巫可能会崩溃，因为你在一个永无止境的循环中

Answer 6

替换 finally 块中的行

Intent openStartingPoint = new Intent("com.example.android.myapplication.MainActivity");
    startActivity(openStartingPoint);
to 

 Intent openStartingPoint = new Intent(Splash.this, MainActivity.java);
    startActivity(openStartingPoint);

主要活动应该是

public class MainActivity extends AppCompatActivity {

        int counter = 0;
        Button add; // Declare here 
        Button sub ;
        TextView display;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    // Initialize here after setContentView
         add = (Button) findViewById(R.id.bAdd);
         sub = (Button) findViewById(R.id.bSub);
         display = (TextView) findViewById(R.id.tView);
            Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
            setSupportActionBar(toolbar);
            FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
            fab.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                            .setAction("Action", null).show();
                }
            });


            add.setOnClickListener(new View.OnClickListener(){
                public void onClick(View v){
                    counter++;
                    display.setText("Your total is " + counter);
                }
            });
            sub.setOnClickListener(new View.OnClickListener() {
                public void onClick(View v) {
                    counter = counter - 1;
                    display.setText("Your total is " + counter);
                }
            });


        }
        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_main, menu);
            return true;
        }
        @Override
        public boolean onOptionsItemSelected(MenuItem item) {
            // Handle action bar item clicks here. The action bar will
            // automatically handle clicks on the Home/Up button, so long
            // as you specify a parent activity in AndroidManifest.xml.
            int id = item.getItemId();
            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }
            return super.onOptionsItemSelected(item);
        }
    }