我是应用程序开发的新手，想要制作一个简单的手电筒应用程序以获得乐趣。但是，由于某些原因，我的手电筒应用程序崩溃了所有其他发射。我想我可能需要onDestroy()方法，但不确定是否存在问题。我也总是得到一个NullPointerException，问题似乎在onCreate()方法中。谢谢你的帮助！
package com.monstartech.superuser.flashlight;
import android.annotation.SuppressLint;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.os.Handler;
import android.view.MotionEvent;
import android.view.View;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.pm.PackageManager;
import android.hardware.Camera;
import android.hardware.Camera.Parameters;
import android.widget.ImageButton;
/**
* An example full-screen activity that shows and hides the system UI (i.e.
* status bar and navigation/system bar) with user interaction.
*/
public class FullscreenActivity extends AppCompatActivity {
//Camera variables:
private Camera camera;
ImageButton flashlightSwitchImg;
private boolean isFlashLightOn;
Parameters params;
/**
* 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;
/**
* Some older devices needs a small delay between UI widget updates
* and a change of the status and navigation bar.
*/
private static final int UI_ANIMATION_DELAY = 300;
private final Handler mHideHandler = new Handler();
private View mContentView;
private final Runnable mHidePart2Runnable = new Runnable() {
@SuppressLint("InlinedApi")
@Override
public void run() {
// Delayed removal of status and navigation bar
// Note that some of these constants are new as of API 16 (Jelly Bean)
// and API 19 (KitKat). It is safe to use them, as they are inlined
// at compile-time and do nothing on earlier devices.
mContentView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LOW_PROFILE
| View.SYSTEM_UI_FLAG_FULLSCREEN
| View.SYSTEM_UI_FLAG_LAYOUT_STABLE
| View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
| View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
| View.SYSTEM_UI_FLAG_HIDE_NAVIGATION);
}
};
private View mControlsView;
private final Runnable mShowPart2Runnable = new Runnable() {
@Override
public void run() {
// Delayed display of UI elements
ActionBar actionBar = getSupportActionBar();
if (actionBar != null) {
actionBar.show();
}
mControlsView.setVisibility(View.VISIBLE);
}
};
private boolean mVisible;
private final Runnable mHideRunnable = new Runnable() {
@Override
public void run() {
hide();
}
};
/**
* 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.
*/
private final View.OnTouchListener mDelayHideTouchListener = new View.OnTouchListener() {
@Override
public boolean onTouch(View view, MotionEvent motionEvent) {
if (AUTO_HIDE) {
delayedHide(AUTO_HIDE_DELAY_MILLIS);
}
return false;
}
};
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_fullscreen);
mVisible = true;
mControlsView = findViewById(R.id.fullscreen_content_controls);
mContentView = findViewById(R.id.fullscreen_content);
// Set up the user interaction to manually show or hide the system UI.
mContentView.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
toggle();
}
});
// Upon interacting with UI controls, delay any scheduled hide()
// operations to prevent the jarring behavior of controls going away
// while interacting with the UI.
//TODO: uncoment? findViewById(R.id.dummy_button).setOnTouchListener(mDelayHideTouchListener);
//Camera Stuff's old spot
//Camera Stuff:
flashlightSwitchImg = (ImageButton) findViewById(R.id.flashlightSwitch);
boolean isCameraFlash = getApplicationContext().getPackageManager().
hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);
if(!isCameraFlash) {
showNoCameraAlert();
} else {
camera = Camera.open();
params = camera.getParameters();
}
flashlightSwitchImg.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
if(isFlashLightOn) {
setFlashLightOff();
} else {
setFlashLightOn();
}
}
});
}
private void showNoCameraAlert() {
new AlertDialog.Builder(this).setTitle("Error: No Camera Flash Found!!").
setMessage("Camera flashlight not available in this Android device!").
setPositiveButton(android.R.string.yes,new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
finish();
}
})
.setIcon(android.R.drawable.ic_dialog_alert)
.show();
return;
}
//setFlashLightOn
private void setFlashLightOn() {
params = camera.getParameters();
params.setFlashMode(Parameters.FLASH_MODE_TORCH);
camera.setParameters(params);
camera.startPreview();
isFlashLightOn = true;
flashlightSwitchImg.setImageResource(R.drawable.light_on);
}
//setFlashLightOff
private void setFlashLightOff() {
params.setFlashMode(Parameters.FLASH_MODE_OFF);
camera.setParameters(params);
camera.stopPreview();
isFlashLightOn = false;
flashlightSwitchImg.setImageResource(R.drawable.light_off);
}
@Override
protected void onStop() {
super.onStop();
if(camera != null) {
camera.release();
camera = null;
}
}
//end Camera stuff
@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);
}
private void toggle() {
if (mVisible) {
hide();
} else {
show();
}
}
private void hide() {
// Hide UI first
ActionBar actionBar = getSupportActionBar();
if (actionBar != null) {
actionBar.hide();
}
mControlsView.setVisibility(View.GONE);
mVisible = false;
// Schedule a runnable to remove the status and navigation bar after a delay
mHideHandler.removeCallbacks(mShowPart2Runnable);
mHideHandler.postDelayed(mHidePart2Runnable, UI_ANIMATION_DELAY);
}
@SuppressLint("InlinedApi")
private void show() {
// Show the system bar
mContentView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
| View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION);
mVisible = true;
// Schedule a runnable to display UI elements after a delay
mHideHandler.removeCallbacks(mHidePart2Runnable);
mHideHandler.postDelayed(mShowPart2Runnable, UI_ANIMATION_DELAY);
}
/**
* 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);
}
@Override
protected void onDestroy(){
super.onDestroy();
//Your code
}
}
logcat的
05-11 15:11:39.793 18729-18729/com.monstartech.superuser.flashlight D/Activity: #1 setTransGradationModeColor false
05-11 15:11:40.583 18729-18729/com.monstartech.superuser.flashlight D/ViewRootImpl: ViewPostImeInputStage ACTION_DOWN
05-11 15:11:40.653 18729-18729/com.monstartech.superuser.flashlight D/AndroidRuntime: Shutting down VM
05-11 15:11:40.653 18729-18729/com.monstartech.superuser.flashlight W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x41d01da0)
05-11 15:11:40.653 18729-18729/com.monstartech.superuser.flashlight E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.monstartech.superuser.flashlight, PID: 18729
java.lang.NullPointerException
at com.monstartech.superuser.flashlight.FullscreenActivity.setFlashLightOn(FullscreenActivity.java:173)
at com.monstartech.superuser.flashlight.FullscreenActivity.access$700(FullscreenActivity.java:24)
at com.monstartech.superuser.flashlight.FullscreenActivity$6.onClick(FullscreenActivity.java:150)
at android.view.View.performClick(View.java:4754)
at android.view.View$PerformClick.run(View.java:19591)
at android.os.Handler.handleCallback(Handler.java:733)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:146)
at android.app.ActivityThread.main(ActivityThread.java:5731)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1291)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1107)
at dalvik.system.NativeStart.main(Native Method)
05-11 15:11:41.773 18729-18729/com.monstartech.superuser.flashlight
I/Process: Sending signal. PID: 18729 SIG: 9
答案 0 :(得分：3)
您正在清空相机onStop，但没有重新创建相机对象onStart。我会取消相机，但不能将其设为空onStop。
onStop并不代表您的应用被销毁。它将在您离开时重新启动，在这种情况下使用null相机对象。
答案 1 :(得分：1)
根据您的Logcat，应用程序在第173行崩溃，看起来就像这一行：
params = camera.getParameters();
所以你的相机变量可能是空的。所以检查一个空相机。您可以使用第137行中的相同逻辑来初始化相机。
private void setFlashLightOn() {
if (camera == null) {
// make isCameraFlash a class level variable
if(!isCameraFlash) {
showNoCameraAlert();
} else {
camera = Camera.open();
params = camera.getParameters();
}
}
params = camera.getParameters();
// etc