Question

我正在尝试创建一个应用程序，在单击按钮时拍摄图像，然后拍摄该图像并将其放在图像视图字段中。给定的代码很好没有错误，我已经检查了很多次。即使我已经清除了清单文件中的活动，我仍然会收到运行时异常的错误。

这里是xml代码：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:weightSum="10"
    tools:context="com.example.android.tryingcamera.MainActivity">
<ImageView
    android:id="@+id/imageView"
    android:layout_width="wrap_content"
    android:layout_height="fill_parent"
    android:layout_weight="9"/>
<Button
    android:id="@+id/btnCamera"
    android:layout_weight="1"
    android:text="Tap To Open Camera"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="btncameraClicked"/>

这是java代码：

public class MainActivity extends AppCompatActivity {

private ImageView imageview=(ImageView)findViewById(R.id.imageView);
private Button btncamera = (Button) findViewById(R.id.btnCamera);
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}



public void btncameraClicked(View view){
    Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
    startActivityForResult(intent,0);
}

@Override
protected void onActivityResult(int requestCode,int resultCode,Intent data ){
    super.onActivityResult(requestCode,resultCode,data);
    Bitmap bitmap=(Bitmap)data.getExtras().get("data");
    imageview.setImageBitmap(bitmap);
}

}

logcat显示：

Process: com.example.android.tryingcamera, PID: 27769
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.tryingcamera/com.example.android.tryingcamera.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6119)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
    at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:117)
    at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:149)
    at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:29)
    at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:54)
    at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:31)
    at android.support.v7.app.AppCompatDelegateImplN.<init>(AppCompatDelegateImplN.java:31)
    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:198)
    at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:183)
    at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
    at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
    at com.example.android.tryingcamera.MainActivity.<init>(MainActivity.java:14)
    at java.lang.Class.newInstance(Native Method)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at 

com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Answer 1

检查您的代码您需要在findViewById()之后的onCreate()方法中 setContentView()

尝试此更改代码，如下面的代码

 public class MainActivity extends AppCompatActivity {

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

        imageview=(ImageView)findViewById(R.id.imageView);
        btncamera = (Button) findViewById(R.id.btnCamera);
    }



    public void btncameraClicked(View view){
        Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
        startActivityForResult(intent,0);
    }

    @Override
    protected void onActivityResult(int requestCode,int resultCode,Intent data ){
        super.onActivityResult(requestCode,resultCode,data);
        Bitmap bitmap=(Bitmap)data.getExtras().get("data");
        imageview.setImageBitmap(bitmap);
    }

}

Answer 2

当您将imageview和btnamera声明为类变量并使用findViewById将其全部分配时，findViewById方法在创建MainActivity类的实例时调用，但观点没有准备好。

您可以在已设置视图后分配变量。

private ImageView imageview;
private Button btncamera;

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

    imageview=(ImageView)findViewById(R.id.imageView);
    btncamera = (Button) findViewById(R.id.btnCamera);
}

没有错误，但仍在Android工作室关闭app力量

2 个答案: