尝试运行android应用时,我不断收到错误消息,告诉我有null
对象引用。我仍然是android和java的初学者,我将不胜感激我能得到的任何帮助。
这是logcat
:
01-17 17:11:49.156 27778-27778/com.example.owner.sketchy D/AndroidRuntime﹕ Shutting down VM
--------- beginning of crash
01-17 17:11:49.157 27778-27778/com.example.owner.sketchy E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.owner.sketchy, PID: 27778
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.owner.sketchy/com.example.owner.sketchy.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.getChildAt(int)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.getChildAt(int)' on a null object reference
at com.example.owner.sketchy.MainActivity.onCreate(MainActivity.java:33)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
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:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
这是我的MainActivity.java
:
package com.example.owner.sketchy;
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.ImageButton;
import android.widget.LinearLayout;
public class MainActivity extends ActionBarActivity {
private ImageButton currPaint;
private DrawingView drawView;
public void paintClicked(View view) {
if (view != currPaint) {
ImageButton imgView = (ImageButton) view;
String color = view.getTag().toString();
imgView.setImageDrawable(getResources().getDrawable(R.drawable.paint_pressed));
currPaint.setImageDrawable(getResources().getDrawable(R.drawable.paint));
currPaint = (ImageButton) view;
drawView.setColor(color);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
drawView = (DrawingView)findViewById(R.id.drawing);
LinearLayout paintLayout = (LinearLayout)findViewById(R.id.paint_colors);
currPaint = (ImageButton)paintLayout.getChildAt(0);
}
@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);
}
}
您paintLayout
为空,而您的问题为onCreate()
。从超类覆盖方法时,应始终(大多数时候)使用调用该类中的方法super.method()
,以允许进行常规处理。
另外,您也缺少对的呼叫setContentView(R.layout.your_layout)
。因此,请不要忘记这两行:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.your_layout);
drawView = (DrawingView)findViewById(R.id.drawing);
LinearLayout paintLayout = (LinearLayout)findViewById(R.id.paint_colors);
currPaint = (ImageButton)paintLayout.getChildAt(0);
}
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