android开发中Caused by: java.lang.NullPointerException

Question

main.xml文
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >
    <RatingBar
        android:id="@+id/mybar1"
        android:numStars="4"
        android:stepSize="0.5"
        android:isIndicator="false"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
     <RatingBar
        android:id="@+id/mybar2"
        android:numStars="4"
        android:isIndicator="true"
        android:rating="2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:id="@+id/mytext"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" /></LinearLayout>mainActivity.java
package com.example.ratingbarproject1;import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.widget.RatingBar;
import android.widget.RatingBar.OnRatingBarChangeListener;
import android.widget.TextView;public class MainActivity extends Activity {
private RatingBar ratingbar=null;
private TextView textview=null;
protected void onCreate(Bundle savedInstanceState) {
this.ratingbar=(RatingBar)super.findViewById(R.id.mybar1);//获取组件
this.textview=(TextView)super.findViewById(R.id.mytext);
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
this.ratingbar.setOnRatingBarChangeListener(new OnRatingBarChangeListener1());//设置监听
}
private class OnRatingBarChangeListener1 implements OnRatingBarChangeListener{
//@Override
public void onRatingChanged(RatingBar ratingBar, float rating,
boolean fromUser) {
// 在显示文本中显示星星
MainActivity.this.textview.append("***当前的星评分："
+ratingBar.getNumStars()+"***当前的步长："+ratingBar.getStepSize()+"\n");

}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}}
错误。
android开发

Answer 1

        this.ratingbar=(RatingBar)super.findViewById(R.id.mybar1);//获取组件         this.textview=(TextView)super.findViewById(R.id.mytext);         super.onCreate(savedInstanceState);         setContentView(R.layout.activity_main);
你不能在setContentView去获取组件,放到下面来

Answer 2

像这样
    setContentView(R.layout.activity_main);   
    this.ratingbar=(RatingBar)super.findViewById(R.id.mybar1);//获取组件
    this.textview=(TextView)super.findViewById(R.id.mytext);

Answer 3

super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
this.ratingbar=(RatingBar)findViewById(R.id.mybar1);//获取组件
        this.textview=(TextView)findViewById(R.id.mytext);
        

Answer 4

顺序oncreate-setContentView-findView-setView

Answer 5

没有加载布局，
缺少 setContentView(R.layout.activity_main); 这句话
就不能找到里面的控件

Answer 6

setContentView后才会建立当前窗口的主view，之前获取view肯定是错的、

Answer 7

setContentView(R.layout.activity_main);  是加载布局文件，获得当前窗口的view组件是定义在布局文件中的，只有先加载布局文件，才能找到控件，所以，获取组件要在之后。另外，加载控件格式不对：有两种方式：
1、先定义，后获取组件。
    private RatingBar ratingbar;
    private TextView textview;
    ratingbar=(RatingBar)findViewById(R.id.mybar1);
    textview=(TextView)findViewById(R.id.mytext);
    注：不用null,this,super
2、组件定义和获取同时进行
   RatingBar ratingbar=(RatingBar)findViewById(R.id.mybar1);
   TextView textview=(TextView)findViewById(R.id.mytext);