安卓 监听器 报空指针错误

Posted 2023-03-21

public class Main extends Activity

EditText edittext;

RadioButton rb1, rb2;

double result_number = 0;

Button btn1;

Button.OnClickListener Listener = new Button.OnClickListener()

public void onClick(View v)

double result = 0;

TextView tv = (TextView) findViewById(R.id.tv1);

tv.setText(String.valueOf(result));



;

@Override

protected void onCreate(Bundle savedInstanceState)

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

edittext = (EditText) findViewById(R.id.editText1);

rb1 = (RadioButton) findViewById(R.id.lunch_rb);

rb2 = (RadioButton) findViewById(R.id.dinner_rb);

btn1 = (Button) findViewById(R.id.btn);

btn1.setOnClickListener(Listener); //为什么这句话执行后会报空指针？？



xml文件：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="vertical" >

<TextView

android:name="@+id/tv1"

android:layout_width="match_parent"

android:layout_height="wrap_content" />

<Button

android:name="@+id/btn"

android:layout_width="match_parent"

android:layout_height="wrap_content" >

</Button>

<EditText

android:id="@+id/editText1"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:hint="PRINT NUMBER:"

android:inputType="number" >

</EditText>

<RadioGroup

android:id="@+id/time_period"

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:gravity="center"

android:orientation="horizontal" >

<RadioButton

android:id="@+id/lunch_rb"

android:checked="true"

android:tag="lunch"

android:text="Lunch" />

<RadioButton

android:id="@+id/dinner_rb"

android:tag="dinner"

android:text="Dinner" />

</RadioGroup>

<SeekBar

android:id="@+id/seekBar1"

android:layout_width="match_parent"

android:layout_height="wrap_content" />

</LinearLayout>

参考技术A package com.example.klsahkl;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class MainActivity extends Activity
private Button b1 ;

@Override
protected void onCreate(Bundle savedInstanceState)
super.onCreate(savedInstanceState);

b1 = (Button)findViewById(R.id.bt);
//只要设置按键监听就会提示“很抱歉，XXX已停止运行。”
b1.setOnClickListener(new OnClickListener()
@Override
public void onClick(View v)
// TODO Auto-generated method stub


);

setContentView(R.layout.activity_main);


@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;



提示错误：
java.lang.RuntimeException: Unable to start activity ComponentInfocom.example.klsahkl/com.example.klsahkl.MainActivity: java.lang.NullPointerException

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
掂庆煌乎

jsoup解析本地html文件，老是报空指针错误，求大神指导，小菜鸟一枚，代码如下：

package jsouptest;
import java.io.File;
import java.io.IOException;
//import java.lang.*;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;
public class test0
public static void main(String args[]) throws IOException
File input=new File("D:/maopu.html");
//File input=new File("maopu.html");
Document doc=Jsoup.parse(input, "UTF-8", "");
Element content=doc.getElementById("content");
Elements links=content.getElementsByTag("a");
for(Element link:links)
String linkHref=link.attr("href");
System.out.println("\n"+linkHref);

参考技术A 猜想你的16行是
Elements links=content.getElementsByTag("a");

这一句
你的content没有被正确取出，所以content是空。
推荐你使用doc.select("#content").first();本回答被提问者和网友采纳 参考技术B File input=new File("D:/maopu.html");这个应该是错误的吧？能找到该文件？》？
还有就是你的每一个节点是否都有且没有拼写错误
还有每个对象在使用前判断不是null后，再对它引用就更加规范 参考技术C 确定你的Html里面有id=content这个东西吗