Question

我一直收到这个错误，我不确定导致它的原因。我声明并初始化了变量，但错误并没有消失。我一直在看同样的错误，但我无法抓住这个错误。

05-01 20:57:43.162  32675-32675/com.ammar.customlistview1.app W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x40be21f8)
05-01 20:57:43.172  32675-32675/com.ammar.customlistview1.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.ammar.customlistview1.app/com.ammar.customlistview1.app.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1891)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992)
            at android.app.ActivityThread.access$600(ActivityThread.java:127)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4511)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:976)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:743)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:1810)
            at com.ammar.customlistview1.app.MainActivity.<init>(MainActivity.java:16)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1319)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1026)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1882)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992)
            at android.app.ActivityThread.access$600(ActivityThread.java:127)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4511)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:976)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:743)
            at dalvik.system.NativeStart.main(Native Method)

这是我的班级代码：

package com.ammar.customlistview1.app;

/**
 * Created by Ammar on 5/1/2014.
 */
public class person {

    private String name = "Ammar";
    private int age = 21;
    private int picture = R.drawable.ic_launcher;

    public person(String name, int age, int picture) {
        this.name = name;
        this.age = age;
        this.picture = picture;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getPicture() {
        return picture;
    }

    public void setPicture(int picture) {
        this.picture = picture;
    }
}

这是我的主要活动代码：

package com.ammar.customlistview1.app;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ListView;

import java.util.ArrayList;


public class MainActivity extends Activity {

    personadaptor adapter ;
    ArrayList <person> theperson = new ArrayList<person>();
    ListView lv = (ListView) findViewById(R.id.lvmain);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        person p1 = new person("Ammar", 21 , R.drawable.ic_launcher);
        person p2 = new person("Ali", 25 ,  R.drawable.ic_launcher);
        person p3 = new person("Saber", 23 ,  R.drawable.ic_launcher);

        theperson.add(p1);
        theperson.add(p2);
        theperson.add(p3);

        adapter = new personadaptor(theperson, this);
        lv.setAdapter(adapter);
    }


    @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;
    }

    @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();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

Answer 1

在您拨打findViewById()之前，您无法致电setContentView()。

Answer 2

您不能在Activity（或类似）类的初始值设定项中使用Android UI方法，例如findViewById（），因为这些方法仅在调用onCreate（）期间和之后有效。

将初始化代码移至onCreate（）

（当你这样做时，一定要把setContentView（）之后的findViewById（）放到Matiash中，这样才能正确指出 - 否则你只会得到一个不同的空指针异常）

Answer 3

1.ListView lv; Listview初始化应该在

之后

setContentView(R.layout.activity_main);
lv = (ListView) findViewById(R.id.lvmain);

2.无需亲自初始化此名称，年龄和图片

    private String name;
    private int age ;
    private int picture ;

3.in adapterclass你需要布局

adapter = new personadaptor(theperson, this);//where is your row layout

由java.lang.NullPointerException和java.lang.RuntimeException引起

3 个答案: