android开发中Caused by: java.lang.NullPointerException解决方法

android开发中Caused by: java.lang.NullPointerException
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。。。。。。。。。。。。。。。。

------解决方案--------------------
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);

------解决方案--------------------
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);
        
------解决方案--------------------
顺序oncreate-setContentView-findView-setView
------解决方案--------------------
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
------解决方案--------------------
setContentView后才会建立当前窗口的主view，之前获取view肯定是错的、
------解决方案--------------------