[英](Android) findViewById Fails to locate the view and returns null?
所以我刚开始学习 Android,我正在阅读 commonsware android 开发指南作为起点。 所以基本上我尝试使用 xml 创建一个视图,这个 xml 位于
res/layout/edittext.xml
xml 看起来像:
<?xml version="1.0" encoding="utf-8" ?>
<EditText xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/field"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:singleLine="false"/>
和我的代码
public class HelloWorld extends Activity implements View.OnClickListener {
Button btn;
/**
* Called when the activity is first created.
*/
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
System.out.println("HEY");
btn = new Button(this);
EditText editText = (EditText) findViewById(R.id.field);
editText.setText("Hey there how are you doing");
updateTime();
setContentView(R.layout.main);
}
public void onClick(View view) {
updateTime();
}
public void updateTime() {
btn.setText(new Date().toString());
}
}
并且editText返回null ...如果重要的话,这是R class。
/* AUTO-GENERATED FILE. DO NOT MODIFY.
*
* This class was automatically generated by the
* aapt tool from the resource data it found. It
* should not be modified by hand.
*/
package com.dennis;
public final class R {
public static final class attr {
}
public static final class drawable {
public static final int icon=0x7f020000;
}
public static final class id {
public static final int field=0x7f050000;
public static final int helloText=0x7f050001;
}
public static final class layout {
public static final int edittext=0x7f030000;
public static final int main=0x7f030001;
}
public static final class string {
public static final int app_name=0x7f040000;
}
}
堆栈跟踪
07-22 11:30:46.670: ERROR/AndroidRuntime(674): FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dennis/com.dennis.HelloWorld}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
at android.app.ActivityThread.access$2300(ActivityThread.java:125)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4627)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.dennis.HelloWorld.onCreate(HelloWorld.java:25)
在您的onCreate()
方法中,确保在任何其他与 UI 相关的任务之前调用setContentView()
。 即大多数人在super.onCreate()
之后立即调用它
此外,您似乎对布局有一些误解。 您通常应该有一个包含“布局” object ,例如放置View
对象的LinearLayout
,例如EditText
。 传递给setContentView()
的变量将是 R.layout。 您的 xml 文件的名称。
您需要更改setContentView(R.layout.main);
设置setContentView(R.layout.edittext);
,并且您需要在尝试访问 EditText 之前调用它
在尝试编辑editText 中的文本之前,您必须先设置ContentView。
setContentView(R.layout.main);
EditText editText = (EditText) findViewById(R.id.field);
editText.setText("Hey there how are you doing");
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