當我單擊一個按鈕以錯誤打開新活動時，我的應用程序崩潰：NullPointerException

Question

所以我使我的應用程序正常運行並在我的設備上運行（即使它什么也不做），然后我創建了第二個活動和一個切換到該第二個活動的按鈕）這是我的代碼：

AndroidManifest.xml`

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".menu_state"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".activity_game_state"
        android:configChanges="orientation|keyboardHidden|screenSize"
        android:label="@string/title_activity_activity_game_state"
        android:theme="@style/FullscreenTheme" >
        <intent-filter>
            <action android:name="dk.carlemil.upanddown.activity_game_state" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
</application>

和我的主要活動名為menu_state

import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.ImageButton;


public class menu_state extends ActionBarActivity {

    private static ImageButton image_button_sbm;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_menu_state);
        onClickImageButtonListener();
    }
    public  void onClickImageButtonListener(){
        image_button_sbm = (ImageButton)findViewById(R.id.imageButton2);
        image_button_sbm.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        Intent intent = new Intent("dk.carlemil.upanddown.activity_game_state");
                        startActivity(intent);
                    }
                }
        );

    }
    @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_menu_state, 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);
    }
    // i don't know where this should be placed but I'm trying to make it switch to the other screen when the gamemenu button gets clicked




}

我的第二項活動名為activity_game_state

  package dk.carlemil.upanddown;

import dk.carlemil.upanddown.util.SystemUiHider;

import android.annotation.TargetApi;
import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;


/**
 * An example full-screen activity that shows and hides the system UI (i.e.
 * status bar and navigation/system bar) with user interaction.
 *
 * @see SystemUiHider
 */
public class activity_game_state extends Activity {
    /**
     * Whether or not the system UI should be auto-hidden after
     * {@link #AUTO_HIDE_DELAY_MILLIS} milliseconds.
     */
    private static final boolean AUTO_HIDE = true;

    /**
     * If {@link #AUTO_HIDE} is set, the number of milliseconds to wait after
     * user interaction before hiding the system UI.
     */
    private static final int AUTO_HIDE_DELAY_MILLIS = 3000;

    /**
     * If set, will toggle the system UI visibility upon interaction. Otherwise,
     * will show the system UI visibility upon interaction.
     */
    private static final boolean TOGGLE_ON_CLICK = true;

    /**
     * The flags to pass to {@link SystemUiHider#getInstance}.
     */
    private static final int HIDER_FLAGS = SystemUiHider.FLAG_HIDE_NAVIGATION;

    /**
     * The instance of the {@link SystemUiHider} for this activity.
     */
    private SystemUiHider mSystemUiHider;

    /*@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_activity_game_state);

        final View controlsView = findViewById(R.id.fullscreen_content_controls);
        final View contentView = findViewById(R.id.fullscreen_content);

        // Set up an instance of SystemUiHider to control the system UI for
        // this activity.
        mSystemUiHider = SystemUiHider.getInstance(this, contentView, HIDER_FLAGS);
        mSystemUiHider.setup();
        mSystemUiHider
                .setOnVisibilityChangeListener(new SystemUiHider.OnVisibilityChangeListener() {
                    // Cached values.
                    int mControlsHeight;
                    int mShortAnimTime;

                    @Override
                    @TargetApi(Build.VERSION_CODES.HONEYCOMB_MR2)
                    public void onVisibilityChange(boolean visible) {
                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR2) {
                            // If the ViewPropertyAnimator API is available
                            // (Honeycomb MR2 and later), use it to animate the
                            // in-layout UI controls at the bottom of the
                            // screen.
                            if (mControlsHeight == 0) {
                                mControlsHeight = controlsView.getHeight();
                            }
                            if (mShortAnimTime == 0) {
                                mShortAnimTime = getResources().getInteger(
                                        android.R.integer.config_shortAnimTime);
                            }
                            controlsView.animate()
                                    .translationY(visible ? 0 : mControlsHeight)
                                    .setDuration(mShortAnimTime);
                        } else {
                            // If the ViewPropertyAnimator APIs aren't
                            // available, simply show or hide the in-layout UI
                            // controls.
                            controlsView.setVisibility(visible ? View.VISIBLE : View.GONE);
                        }

                        if (visible && AUTO_HIDE) {
                            // Schedule a hide().
                            delayedHide(AUTO_HIDE_DELAY_MILLIS);
                        }
                    }
                });

        // Set up the user interaction to manually show or hide the system UI.
        contentView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (TOGGLE_ON_CLICK) {
                    mSystemUiHider.toggle();
                } else {
                    mSystemUiHider.show();
                }
            }
        });


    }
*/
    @Override
    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);

        // Trigger the initial hide() shortly after the activity has been
        // created, to briefly hint to the user that UI controls
        // are available.
        delayedHide(100);
    }


    /**
     * Touch listener to use for in-layout UI controls to delay hiding the
     * system UI. This is to prevent the jarring behavior of controls going away
     * while interacting with activity UI.
     */
    View.OnTouchListener mDelayHideTouchListener = new View.OnTouchListener() {
        @Override
        public boolean onTouch(View view, MotionEvent motionEvent) {
            if (AUTO_HIDE) {
                delayedHide(AUTO_HIDE_DELAY_MILLIS);
            }
            return false;
        }
    };

    Handler mHideHandler = new Handler();
    Runnable mHideRunnable = new Runnable() {
        @Override
        public void run() {
            mSystemUiHider.hide();
        }
    };

    /**
     * Schedules a call to hide() in [delay] milliseconds, canceling any
     * previously scheduled calls.
     */
    private void delayedHide(int delayMillis) {
        mHideHandler.removeCallbacks(mHideRunnable);
        mHideHandler.postDelayed(mHideRunnable, delayMillis);
    }
}

和我的主要活動（menu_sate）.xml

 <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:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".menu_state"
    android:screenOrientation="portrait">

    <ImageButton
        android:layout_width="150sp"
        android:layout_height="150sp"
        android:id="@+id/imageButton"
        android:layout_above="@+id/imageButton2"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="50dp"
        android:background="@drawable/logo_image"/>

    <ImageButton
        android:layout_width="wrap_content"
        android:layout_height="120sp"
        android:id="@+id/imageButton2"
        android:background="@drawable/start_image1"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />

    <ImageButton
        android:layout_width="wrap_content"
        android:layout_height="120sp"
        android:id="@+id/imageButton3"
        android:background="@drawable/start_image2"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="30sp"/>


</RelativeLayout>
and my second activity(activity_game_state) .xml file
    <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:background="#ffffffff"
    tools:context="dk.carlemil.upanddown.activity_game_state"
    android:orientation="horizontal">


    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="1"
        android:id="@+id/button1"
        android:background="@drawable/cirkle_button"
        android:layout_above="@+id/button4"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />


    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="2"
        android:id="@+id/button2"
        android:background="@drawable/cirkle_button"
        android:layout_above="@+id/button5"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="3"
        android:id="@+id/button3"
        android:background="@drawable/cirkle_button"
        android:layout_above="@+id/button6"
        android:layout_alignLeft="@+id/button6"
        android:layout_alignStart="@+id/button6" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="4"
        android:id="@+id/button4"
        android:background="@drawable/cirkle_button"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="5"
        android:id="@+id/button5"
        android:background="@drawable/cirkle_button"
        android:layout_alignTop="@+id/button6"
        android:layout_centerHorizontal="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="6"
        android:id="@+id/button6"
        android:background="@drawable/cirkle_button"
        android:layout_alignParentBottom="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />
</RelativeLayout>

最后是我認為問題所在的logcat

    rocess: dk.carlemil.upanddown, PID: 9017
    java.lang.NullPointerException: Attempt to invoke virtual method 'void dk.carlemil.upanddown.util.SystemUiHider.hide()' on a null object reference
            at dk.carlemil.upanddown.activity_game_state$2.run(activity_game_state.java:145)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5942)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)
07-20 22:14:43.974     871-1210/? E/CSLegacyTypeTracker﹕ add() : Adding agent NetworkAgentInfo{ ni{[type: WIFI[] - WIFI, state: CONNECTED/CONNECTED, reason: (unspecified), extra: "fam_bager_net", roaming: false, failover: false, isAvailable: true, isConnectedToProvisioningNetwork: false]}  network{545}  lp{{InterfaceName: wlan0 LinkAddresses: [192.168.1.12/24,]  Routes: [192.168.1.0/24 -> 0.0.0.0 wlan0,0.0.0.0/0 -> 192.168.1.1 wlan0,] DnsAddresses: [192.168.1.1,] Domains: null MTU: 0 TcpBufferSizes: 524288,1048576,4525824,524288,1048576,4525824}}  nc{[ Transports: WIFI Capabilities: INTERNET&NOT_RESTRICTED&TRUSTED&NOT_VPN LinkUpBandwidth>=1048576Kbps LinkDnBandwidth>=1048576Kbps]}  Score{60} validated{false} created{true} explicitlySelected{false} } for legacy network type 1
07-20 22:14:44.884    8415-5428/? E/SPPClientService﹕ [e] Push Channel Exception : java.net.SocketException: recvfrom failed: ETIMEDOUT (Connection timed out)
07-20 22:14:44.924    8415-5428/? E/SPPClientService﹕ [e] exceptionCaught(). NET_TIMEOUT
07-20 22:14:48.784    9179-9179/? E/SysUtils﹕ ApplicationContext is null in ApplicationStatus

Answer 1

您的Intent不正確。

Intent intent = new Intent(menu_state.this, activity_game_state.class);
startActivity(intent);

此外，您第二個活動中的onCreate方法也會被注釋掉。 這需要不加注釋。

Answer 2

這樣寫你的意圖，它應該可以工作：

Intent intent = new Intent(menu_state.this, activity_game_state.class);
startActivity(intent);