NullPointerException轉到另一個活動，Android Studio

Question

更新后，我發布了新的LogCat報告，並且我知道第二個類“ Done”返回null導致此錯誤：

致命異常：main java.lang.RuntimeException：無法啟動活動ComponentInfo {com.rocrider.foodmenue / com.rocrider.foodmenue.Done}：java.lang.NullPointerException

我認為問題出在類Done與按鈕BDone和second_activity XML文件的關聯中。

僅供參考，在我的應用啟動時，我確實給food1amount和food2amount賦予了多個值。 我按下按鈕“ BDone”后，它崩潰了。

這個程序應該是供侍應生從平板電腦上的顧客那里接單的，然后該程序應該列出一份訂購量大於0的食品清單，“后來，不在此代碼中”可以將其發送到餐廳廚房中的打印機。

我的主類名為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;
    }
}

我的第二個類名為Done，應該與second_activity.xml相關聯：

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報告：

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

在第二個活動中，在onCreate方法中添加對BDone的調用：

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

在Main類中，將public class Done extends MainActivity替換為public class Done extends Activity 。

試試下面的整個代碼：

MainActivity類

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;
    }
}

完成課程

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

只需經歷以下情形。 您正在調用Done活動”，因此在其onCreate方法中正在調用BDone()函數。 在該功能中

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()));

這些行可能會拋出NullPointerException ，因為您正在初始化兩個EditTexts。 問題出在food1amount和food2amount ； 您從EditText獲取文本並進行解析。 但是您只是初始化了那些EditText並從中獲取文本。 這就是為什么你得到null的原因 。 而且您正在嘗試解析null，以便顯示NullPointerException 。 抱歉，我無法在手機上發布整潔的代碼。 希望你能理解。

Answer 3

您的問題在於手動創建第二個活動，因為沒有調用onCreate，並且您收到了NullPoinerException。 BDone()應該在Done活動中從onCreate調用。

Answer 4

在MainActivity的OnClick中，您正在調用done.BDone(); 這是導致錯誤的原因。 由於當時Done活動中的布局尚未膨脹，因此findViewById引發錯誤。 在“完成”活動中，從onCreate調用BDone 。

在主類中，執行以下操作：

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“完成”類中，執行以下操作：

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

Answer 5

您的Done活動的超類是MainActivity 。 應該是Activity 。

更換

public class Done extends MainActivity

至

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
      }
  }