NullPointerException 去另一个活动，android studio

Question

更新 我发布了新的 LogCat 报告，并且我知道与我的第二个名为“Done”的类返回 null 有关，这导致了此错误：

致命异常：主要 java.lang.RuntimeException：无法启动活动 ComponentInfo{com.rocrider.foodmenue/com.rocrider.foodmenue.Done}：java.lang.NullPointerException

我认为问题与Done 类、按钮BDone 和second_activity XML 文件有关。

仅供参考，当我的应用启动时，我确实为 food1amount 和 food2amount 提供了超过 0 的值。按下“BDone”按钮后，它崩溃了。

这个应用程序应该是供服务员和女服务员在平板电脑上接受顾客的订单，然后程序应该列出订购量大于 0 的食物，“后来，不在此代码中”它是应该能够将其发送到餐厅厨房的打印机。

我的主类名为 MainActivity：

package com.rocrider.foodmenue;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button Bdone = (Button) findViewById(R.id.Bdone);
        Bdone.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(MainActivity.this, Done.class));
            }
        });
    }

    @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;
    }
}

我的第二个类名为 Done，据说与 second_activity.xml 相关：

package com.rocrider.foodmenue;

import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;

import java.util.ArrayList;


public class Done extends MainActivity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second_activity);
        Done();
    }

    public void BDone(){
        EditText Tfood1amount = (EditText) findViewById(R.id.TFood1Amount);
        EditText Tfood2amount = (EditText) findViewById(R.id.TFood2Amount);
        ListView lvlast = (ListView) findViewById(R.id.LVfinal);
        final ArrayList<String> OrderList = new ArrayList<String>();
        final ArrayAdapter<String> adapter;
        int food1amount = 0;
        int food2amount = 0;
        food1amount = Integer.parseInt(String.valueOf(Tfood1amount.getText()));
        food2amount = Integer.parseInt(String.valueOf(Tfood2amount.getText()));

        adapter = new ArrayAdapter<String>(this,R.id.LVfinal,OrderList);
        lvlast.setAdapter(adapter);

        if(food1amount != 0){
            class alaki{
                public void adder(View v){
                    OrderList.add("Food1: ");
                    adapter.notifyDataSetChanged();
                }
            }
        }

        if(food2amount != 0){
            class alaki{
                public void adder(View v){
                    OrderList.add("Food2: ");
                    adapter.notifyDataSetChanged();
                }
            }
        }
    }
}

我的 androidmanifest.xml 文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.rocrider.foodmenue"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="7"
        android:targetSdkVersion="16" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.rocrider.foodmenue.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".Done"/>
    </application>
</manifest>

最新的 LogCat 报告：

09-29 10:40:58.976  15228-15228/com.rocrider.foodmenue E/AndroidRuntime? FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.rocrider.foodmenue/com.rocrider.foodmenue.Done}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1667)
    at android.app.ActivityThread.access$1500(ActivityThread.java:117)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:935)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:3687)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:842)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.rocrider.foodmenue.Done.BDone(Done.java:37)
    at com.rocrider.foodmenue.Done.onCreate(Done.java:19)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
    ... 11 more
    09-29 10:40:58.980      118-136/? E/? Dumpstate > /data/log/dumpstate_app_error

Answer 1

在您的第二个活动中，在您的 onCreate 方法中添加对 BDone 的调用：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.second_activity);
    BDone();
}

在Main 类中，将public class Done extends MainActivity 替换为public class Done extends Activity。

试试整个代码：

MainActivity 类

package com.rocrider.foodmenue;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        final Done done = new Done();
        Button Bdone = (Button) findViewById(R.id.Bdone);
        Bdone.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                 startActivity(new Intent(MainActivity.this, Done.class));
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }
}

完成课程

package com.rocrider.foodmenue;
import android.os.Bundle;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import java.util.ArrayList;

public class Done extends Activity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second_activity);
        BDone();
    }

    public void BDone(){
        EditText Tfood1amount = (EditText) findViewById(R.id.TFood1Amount);
        EditText Tfood2amount = (EditText) findViewById(R.id.TFood2Amount);
        ListView lvlast = (ListView) findViewById(R.id.LVfinal);
        final ArrayList<String> OrderList = new ArrayList<String>();
        final ArrayAdapter<String> adapter;
        int food1amount = 0;
        int food2amount = 0;
        food1amount = Integer.parseInt(String.valueOf(Tfood1amount.getText()));
        food2amount = Integer.parseInt(String.valueOf(Tfood2amount.getText()));
        adapter = new ArrayAdapter<String>(this,R.id.LVfinal,OrderList);
        lvlast.setAdapter(adapter);
        if(food1amount != 0){
            class alaki{
                public void adder(View v){
                    OrderList.add("Food1: ");
                    adapter.notifyDataSetChanged();
                }
            }
        }
        if(food2amount != 0){
            class alaki{
                public void adder(View v){
                    OrderList.add("Food2: ");
                    adapter.notifyDataSetChanged();
                }
            }
        }
    }
}

Answer 2

只需经历以下场景。您正在调用Done Activity，因此在其onCreate 方法中您正在调用BDone() 函数。在那个函数中，

EditText Tfood1amount = (EditText) findViewById(R.id.TFood1Amount);
EditText Tfood2amount = (EditText) findViewById(R.id.TFood2Amount);
ListView lvlast = (ListView) findViewById(R.id.LVfinal);
final ArrayList<String> OrderList = new ArrayList<String>();
final ArrayAdapter<String> adapter;
int food1amount = 0;
int food2amount = 0;
food1amount = Integer.parseInt(String.valueOf(Tfood1amount.getText()));
food2amount = Integer.parseInt(String.valueOf(Tfood2amount.getText()));

这些行可能会抛出 NullPointerException，因为您正在初始化两个 EditText。问题出在food1amount & food2amount;您正在从 EditText 获取文本并对其进行解析。但是您只是初始化了那些 EditTexts 并从中获取文本。这就是你得到 null 的原因。而您正在尝试解析 null，以便它显示 NullPointerException。抱歉，我在手机上无法发布整洁的代码。我希望你明白。

Answer 3

您的问题在于手动创建第二个活动，因为没有调用 onCreate，并且您收到 NullPoinerException。 BDone() 应该从 Done 活动中的 onCreate 调用。

Answer 4

在 MainActivity 的 OnClick 中，您正在调用导致错误的 done.BDone();。因为那个时候布局还没有在 Done 活动中膨胀，所以 findViewById 是在抛出错误。从 Done 活动中的 onCreate 调用 BDone。

在主类中，这样做：

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button Bdone = (Button) findViewById(R.id.Bdone);
        Bdone.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(MainActivity.this, Done.class));
            }
        });
    }
}

在 Done 类中，执行以下操作：

public class Done extends MainActivity{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.second_activity);
        BDone();
    }

Answer 5

完成活动的超类是MainActivity。应该是Activity。

替换

public class Done extends MainActivity

到

public class Done extends Activity

Answer 6

  Button bdone = (Button)findViewById(id of button);
  bdone.setOnClickListener(new OnlickListener)
  {
      @Override
      public void onClick(View view)
      {
          //Code here
      }
  }