Activity 的 onCreate() 方法中的空指针异常

Question

我在 2 天前开始使用 Java 和 AS。我已经按照说明完成了所有操作，但是当模拟器中的运行应用程序他退出并显示错误

`java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.project1/com.example.user.project1.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference`

我尝试用三个 TextView 创建 ListView。这是代码：

package com.example.user.project1;

import android.app.Activity;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.app.ListActivity;
import android.widget.Toast;
import android.view.View;
import android.widget.SimpleAdapter;
import java.util.ArrayList;
import java.util.HashMap;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ListView lv = (ListView) findViewById(R.id.list);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.rowlayout);
        ArrayList<Contact> list = new ArrayList<Contact>();
        list.add(new Contact("Surdialx", "Status","Available Works"));
        list.add(new Contact("Surdial", "Status","Available Works"));
        list.add(new Contact("Surdialx", "Status","Available Works"));
        list.add(new Contact("Surdial", "Status","Available Works"));
        list.add(new Contact("Surdial", "Status","Available Works"));
        list.add(new Contact("Surdialxx", "Status","Available Works"));
        SimpleAdapter adapter = new SimpleAdapter(this,list, R.layout.content_scrolling, new String[]{Contact.Serial,Contact.Status,Contact.WorkSt}, new int[]{R.id.tvSerial,R.id.tvStatus,R.id.tvWorkSt});

//据我了解问题出现在这里，但我无法解决它。

        lv.setAdapter(adapter);
    }
    public class Contact extends HashMap<String, String> {

        public static final String Serial = "Serial";
        public static final String Status = "Status";
        public static final String WorkSt = "WorkSt";

        public Contact(String Ser, String Stat, String Work) {
            super();
            super.put(Serial, Ser);
            super.put(Status, Stat);
            super.put(WorkSt, Work);
        }
    }
}

Answer 1

您正在尝试 findViewById，然后再将内容设置为 activity。 ListView内容设置完成后需要初始化

应该是这样的：

super.onCreate(savedInstanceState);
setContentView(R.layout.rowlayout);
ListView lv = (ListView) findViewById(R.id.list);

Activity 的 onCreate() 方法中的空指针异常

Null Pointer Exception in an Activity's onCreate() method

android

listview

adapter