他のアクティビティの起動時にアプリケーションがクラッシュする

Question

食費を計算するアプリを作成しましたが、ボタンを押してアプリケーションを計算するアクティビティに私を連れて行くとクラッシュします。私は実際の計算を行うメソッドを追加するまで、Appはうまく変わるだろう。私はこれを初めて知ったので、コードがちょっと混乱しているかもしれません。

ランチャー活動：

package com.example.foodcostcalculator; 

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

public class ActivityHome extends Activity { 

private TextView mMainHeader; 
private Button mStartCalc; 

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


    mStartCalc = (Button)findViewById(R.id.calc_button); 
    mStartCalc.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      final Intent intent1 = new Intent(ActivityHome.this, StartCalc.class); 
      if (intent1 != null) { 
       startActivity(intent1); 
      } 
     } 
    }); 

} 

@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_home, 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); 
} 
} 

StartCalc活動：

package com.example.foodcostcalculator; 

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

public class StartCalc extends Activity { 

private Button mBackButton; 
private EditText mItemName; 
private EditText mItemCost; 
private EditText mSellPercent; 
private EditText mSellAmount; 
private Button mCalculate; 

String item = mItemName.getText().toString();          
String itcost = mItemCost.getText().toString();          
int cost = Integer.parseInt(itcost); 
String sellperc = mSellPercent.getText().toString();          
int no2 = Integer.parseInt(sellperc); 
String sellamou = mSellAmount.getText().toString();        
int no3 = Integer.parseInt(sellamou); 

private String name1 = "Menu Item: $" + item; 
private String cost1 = "Item Cost: $" + cost; 
private String perc1 = "Sell Percent: %" + no2; 
private String amount1 = "Sell Price: $" + no3; 



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

    final AlertDialog.Builder builder2 = new AlertDialog.Builder(this); 
    builder2.setMessage("Required Fields aren't filled!"); 
    builder2.setCancelable(true); 




    mBackButton = (Button)findViewById(R.id.back_button1); 
    mBackButton.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      final Intent intent1 = new Intent(StartCalc.this, ActivityHome.class); 
      if (intent1 != null) { 
       startActivity(intent1); 
      } 
     } 
    }); 

    mItemName = (EditText)findViewById(R.id.edit_item_name); 
    mItemCost = (EditText)findViewById(R.id.item_cost); 
    mSellPercent = (EditText)findViewById(R.id.profit_percent); 
    mSellAmount = (EditText)findViewById(R.id.profit_amount); 

    mCalculate = (Button)findViewById(R.id.calculate_button); 
    mCalculate.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 

      if (mSellPercent.getText().toString().trim().length() == 0) { 
       int no5 = Calculator.getPerc(cost, no3); 
       String percstring = "Item Percent: %" + no5; 
       final AlertDialog.Builder builder1 = new AlertDialog.Builder(StartCalc.this); 
       builder1.setMessage(name1 + "\n" + cost + "\n" + percstring + "\n" + amount1); 
       builder1.setCancelable(true); 
       AlertDialog alert11 = builder1.create(); 
       alert11.show(); 
      } 
      else if (mSellAmount.getText().toString().trim().length() == 0) { 
       int no6 = Calculator.getPerc(cost, no3); 
       String amountstring = "Item Amount: $" + no6; 
       final AlertDialog.Builder builder1 = new AlertDialog.Builder(StartCalc.this); 
       builder1.setMessage(name1 + "\n" + cost1 + "\n" + perc1 + "\n" + amountstring); 
       builder1.setCancelable(true); 
       AlertDialog alert11 = builder1.create(); 
       alert11.show(); 
      } 

      else { 
       AlertDialog alert2 = builder2.create(); 
       alert2.show(); 
      } 
     } 
    }); 


} 

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

電卓Javaクラス：

package com.example.foodcostcalculator; 

public class Calculator { 
    private static int cost3; 
    private static int percent; 
    private static int amountsell; 

    public static void setCost(int cd) { 
     cost3 = cd; 
    } 

    public static int getCost() { 
     return cost3; 
    } 

    public static void setPerc(int p) { 
     percent = p; 
    } 

    public static void setAmount(int am) { 
     amountsell = am; 
    } 

    public static int getPerc(int a, int b) { 

     percent = (b/a) * 100; 
     return percent; 
    } 

    public static int getAmount(int c, int e) { 
     amountsell = c * e; 
     return amountsell; 
    } 
} 

ログイン：

04-09 01:34:37.812: E/AndroidRuntime(14908): FATAL EXCEPTION: main 
04-09 01:34:37.812: E/AndroidRuntime(14908): Process: com.example.foodcostcalculator, PID: 14908 
04-09 01:34:37.812: E/AndroidRuntime(14908): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.foodcostcalculator/com.example.foodcostcalculator.StartCalc}: java.lang.NullPointerException 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2124) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2257) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread.access$800(ActivityThread.java:139) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1210) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.os.Handler.dispatchMessage(Handler.java:102) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.os.Looper.loop(Looper.java:136) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread.main(ActivityThread.java:5097) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at java.lang.reflect.Method.invokeNative(Native Method) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at java.lang.reflect.Method.invoke(Method.java:515) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at dalvik.system.NativeStart.main(Native Method) 
04-09 01:34:37.812: E/AndroidRuntime(14908): Caused by: java.lang.NullPointerException 
04-09 01:34:37.812: E/AndroidRuntime(14908): at com.example.foodcostcalculator.StartCalc.<init>(StartCalc.java:22) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at java.lang.Class.newInstanceImpl(Native Method) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at java.lang.Class.newInstance(Class.java:1208) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.Instrumentation.newActivity(Instrumentation.java:1084) 
04-09 01:34:37.812: E/AndroidRuntime(14908): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2115) 
04-09 01:34:37.812: E/AndroidRuntime(14908): ... 11 more 

Answer 1

StartCalcアクティビティでは、以下の宣言がデフォルトでnullに初期化されます。

private Button mBackButton; 
private EditText mItemName; 
private EditText mItemCost; 
private EditText mSellPercent; 
private EditText mSellAmount; 
private Button mCalculate; 

だから、あなたがしようとすると：

String item = mItemName.getText().toString();          
String itcost = mItemCost.getText().toString();          
int cost = Integer.parseInt(itcost); 
String sellperc = mSellPercent.getText().toString();          
int no2 = Integer.parseInt(sellperc); 
String sellamou = mSellAmount.getText().toString();        
int no3 = Integer.parseInt(sellamou); 

アンドロイドはjava.lang.NullPointerExceptionをスローします。アクティビティコードとレイアウトでViewコンポーネント間の接続は、あなたがfindViewById()メソッドを介してそれらをリンクのみ後を確立していることに注意してください。

findviewById()の呼び出しの後、上記のコードを次のように記述します。

... 
mItemName = (EditText)findViewById(R.id.edit_item_name); 
mItemCost = (EditText)findViewById(R.id.item_cost); 
mSellPercent = (EditText)findViewById(R.id.profit_percent); 
mSellAmount = (EditText)findViewById(R.id.profit_amount); 

mCalculate = (Button)findViewById(R.id.calculate_button); 
item = mItemName.getText().toString();          
itcost = mItemCost.getText().toString();          
cost = Integer.parseInt(itcost); 
sellperc = mSellPercent.getText().toString();          
no2 = Integer.parseInt(sellperc); 
sellamou = mSellAmount.getText().toString();        
no3 = Integer.parseInt(sellamou); 

Answer 2

String item = mItemName.getText().toString();          
String itcost = mItemCost.getText().toString(); 
String sellperc = mSellPercent.getText().toString(); 
String sellamou = mSellAmount.getText().toString(); 

mItemName、mItemCost、mSellPercent、およびmSellAmountはnullあり、あなたはそれらのメソッドにアクセスすることはできません。 findViewByIdメソッドを使用して初期化した後にアクセスしてください。 mItemNameは、mItemCost、mSellPercent、およびmSellAmountがitializedさ

String item = mItemName.getText().toString();          
String itcost = mItemCost.getText().toString();          
int cost = Integer.parseInt(itcost); 
String sellperc = mSellPercent.getText().toString();          
int no2 = Integer.parseInt(sellperc); 
String sellamou = mSellAmount.getText().toString();        
int no3 = Integer.parseInt(sellamou); 

private String name1 = "Menu Item: $" + item; 
private String cost1 = "Item Cost: $" + cost; 
private String perc1 = "Sell Percent: %" + no2; 
private String amount1 = "Sell Price: $" + no3; 

後：このような

mItemName = (EditText)findViewById(R.id.edit_item_name); 
mItemCost = (EditText)findViewById(R.id.item_cost); 
mSellPercent = (EditText)findViewById(R.id.profit_percent); 
mSellAmount = (EditText)findViewById(R.id.profit_amount); 

：

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

    final AlertDialog.Builder builder2 = new AlertDialog.Builder(this); 
    builder2.setMessage("Required Fields aren't filled!"); 
    builder2.setCancelable(true); 

    mBackButton = (Button)findViewById(R.id.back_button1); 
    mBackButton.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      final Intent intent1 = new Intent(StartCalc.this, ActivityHome.class); 
      if (intent1 != null) { 
       startActivity(intent1); 
      } 
     } 
    }); 

    mItemName = (EditText)findViewById(R.id.edit_item_name); 
    mItemCost = (EditText)findViewById(R.id.item_cost); 
    mSellPercent = (EditText)findViewById(R.id.profit_percent); 
    mSellAmount = (EditText)findViewById(R.id.profit_amount); 

    String item = mItemName.getText().toString();          
    String itcost = mItemCost.getText().toString();          
    int cost = Integer.parseInt(itcost); 
    String sellperc = mSellPercent.getText().toString();          
    int no2 = Integer.parseInt(sellperc); 
    String sellamou = mSellAmount.getText().toString();        
    int no3 = Integer.parseInt(sellamou); 

    String name1 = "Menu Item: $" + item; 
    String cost1 = "Item Cost: $" + cost; 
    String perc1 = "Sell Percent: %" + no2; 
    String amount1 = "Sell Price: $" + no3; 
    mCalculate = (Button)findViewById(R.id.calculate_button); 

Answer 3

問題はであるあなたのStartCalc活動で は、このコードを配置しますStartCalcアクティビティは、最初にインスタンス化されると、可変item、itcost、cost、この時点でヌルであるmItemName、mItemCost、mSellPercent、mSellAmountに依存する値にsellperc（onCreate()が呼び出されるまで）。

あなたは（あなたがやろうとしているかに応じて）例外を乗り越えるためにあなたがmItemNameに値を代入した後、onCreate()にこれらの割り当ての操作を移動してみmItemCost、mSellPercent、mSellAmountことができます。しかし、item（およびその他の変数）の値は、mItemName EditTextが変更されたときに変更されないことに注意してください。だから、私はあなたがそれらを使用する直前にこれらの値を割り当てる方が良いと思います。