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

Question

I keep getting this error, I am not sure what's causing it. I declared and initialized the variables, but the error didn't disappear. I have been looking at the same error but I can't catch the mistake.

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)

Here is my class code:

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;
    }
}

Here is my main activity code:

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

您必须先调用setContentView()才能调用findViewById() setContentView() 。

Answer 2

You cannot use Android UI methods such as findViewById() in the initializers of an Activity (or similar) Class, as these are only valid during and after the call to onCreate().

Move your initialization code to onCreate()

(And when you do so, be sure to put the findViewById() after setContentView() as Matiash so correctly points out - otherwise you will only get a different null pointer exception)

Answer 3

1.ListView lv; Listview initialization shld be after

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

2.no need to initialize this name,age and picture in person just write

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

3.in adapterclass u need layout

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