Android Java错误：空指针异常无法查看原因？

嗨，我有一个应用程序，它从一个登录页面转到另一个带有进度条的活动，然后转到另一个带有选项卡主机的活动。如果我摆脱了加载页面，从登录转到选项卡主机，效果很好，但是如果我尝试从登录转到加载，应用程序会停止并说“不幸的是，应用程序已停止”。我检查了logcat，发现加载页面存在空指针错误，但我不明白原因。为了从登录转到加载，我正确地调用了加载，并且加载中的set content视图与其xml加载匹配。下面是我的代码，Thanks：

主要登录活动： 包com.example.loginscreen

import android.app.Activity;
import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {

   private EditText  username=null;
   private EditText  password=null;
   private TextView attempts;
   private Button login;
   int counter = 3;


   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      username = (EditText)findViewById(R.id.editText1);
      password = (EditText)findViewById(R.id.editText2);
      attempts = (TextView)findViewById(R.id.textView5);
      attempts.setText(Integer.toString(counter));
      login = (Button)findViewById(R.id.button1);
   }

   public void login(View view){
      if(username.getText().toString().equals("mara") && 
      password.getText().toString().equals("mara")){
      Toast.makeText(getApplicationContext(), "Login Successful!", 
      Toast.LENGTH_LONG).show();
      startActivity(new Intent(MainActivity.this,Loading.class)); 
       }    
   else{
      Toast.makeText(getApplicationContext(), "Wrong Credentials",
      Toast.LENGTH_SHORT).show();
      attempts.setBackgroundColor(Color.RED);   
      counter--;
      attempts.setText(Integer.toString(counter));
      if(counter==0){
         login.setEnabled(false);
      }

   }

}
   @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;
   }

}

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.widget.Button;
import android.view.View;
import android.view.View.OnClickListener;

public class Loading extends Activity {

    Button btnStartProgress;
    ProgressDialog progressBar;
    private int progressBarStatus = 0;
    private Handler progressBarHandler = new Handler();

    private long fileSize = 0;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.loading_main);

        addListenerOnButton();

    }

    public void addListenerOnButton() {

        btnStartProgress = (Button) findViewById(R.id.button1);
        btnStartProgress.setOnClickListener( // <== This is line 33
                new OnClickListener() {

           @Override
           public void onClick(View v) {

            // prepare for a progress bar dialog
            progressBar = new ProgressDialog(v.getContext());
            progressBar.setCancelable(true);
            progressBar.setMessage("Searching for Driver Card...");
            progressBar.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            progressBar.setProgress(0);
            progressBar.setMax(100);
            progressBar.show();

            //reset progress bar status
            progressBarStatus = 0;

            //reset filesize
            fileSize = 0;

            new Thread(new Runnable() {
              public void run() {
                while (progressBarStatus < 100) {

                  // process some tasks
                  progressBarStatus = doSomeTasks();

                  // your computer is too fast, sleep 1 second
                  try {
                    Thread.sleep(1000);
                  } catch (InterruptedException e) {
                    e.printStackTrace();
                  }

                  // Update the progress bar
                  progressBarHandler.post(new Runnable() {
                    public void run() {
                      progressBar.setProgress(progressBarStatus);
                    }
                   });
                }

                // driver card is found
                if (progressBarStatus >= 100) {

                   // sleep 2 seconds, so that you can see the 100%
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    // close the progress bar dialog
                    progressBar.dismiss();
                 }
              }
               });
            startActivity(new Intent(Loading.this,LinkTabs.class));

               }

                });

        }
    // file download simulator
        public int doSomeTasks() {

            while (fileSize <= 1000000) {

                fileSize++;

                if (fileSize == 100000) {
                return 10;
                } else if (fileSize == 200000) {
                return 20;
                } else if (fileSize == 300000) {
                    return 30;
                }

            }

            return 100;

        }

     }

import android.app.TabActivity;
import android.content.Intent;
import android.content.res.Resources;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.TabHost.TabSpec;

@SuppressWarnings("deprecation")
public class LinkTabs extends TabActivity {

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.link_main);

        Resources ressources = getResources(); 
        TabHost tabHost = getTabHost(); 

        // First tab
        Intent intentAndroid = new Intent().setClass(this, HomePage.class);
        TabSpec tabSpecHomePage = tabHost
          .newTabSpec("HomePage")
          .setIndicator("", ressources.getDrawable(R.drawable.overview))
          .setContent(intentAndroid);

        // Second tab
        Intent intentApple = new Intent().setClass(this, HomePage2.class);
        TabSpec tabSpecHomePage2 = tabHost
          .newTabSpec("HomePage2")
          .setIndicator("", ressources.getDrawable(R.drawable.card_summary))
          .setContent(intentApple);

        // Third tab
        Intent intentWindows = new Intent().setClass(this, HomePage3.class);
        TabSpec tabSpecHomePage3 = tabHost
          .newTabSpec("HomePage3")
          .setIndicator("", ressources.getDrawable(R.drawable.details))
          .setContent(intentWindows);


        // add all tabs 
        tabHost.addTab(tabSpecHomePage);
        tabHost.addTab(tabSpecHomePage2);
        tabHost.addTab(tabSpecHomePage3);


        //set Windows tab as default (zero based)
        tabHost.setCurrentTab(2);
    }

}

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Overview");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage2 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Card Summary");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage3 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Details");
        setContentView(textview);
    }
}

加载活动： 包com.example.loginscreen

import android.app.Activity;
import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {

   private EditText  username=null;
   private EditText  password=null;
   private TextView attempts;
   private Button login;
   int counter = 3;


   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      username = (EditText)findViewById(R.id.editText1);
      password = (EditText)findViewById(R.id.editText2);
      attempts = (TextView)findViewById(R.id.textView5);
      attempts.setText(Integer.toString(counter));
      login = (Button)findViewById(R.id.button1);
   }

   public void login(View view){
      if(username.getText().toString().equals("mara") && 
      password.getText().toString().equals("mara")){
      Toast.makeText(getApplicationContext(), "Login Successful!", 
      Toast.LENGTH_LONG).show();
      startActivity(new Intent(MainActivity.this,Loading.class)); 
       }    
   else{
      Toast.makeText(getApplicationContext(), "Wrong Credentials",
      Toast.LENGTH_SHORT).show();
      attempts.setBackgroundColor(Color.RED);   
      counter--;
      attempts.setText(Integer.toString(counter));
      if(counter==0){
         login.setEnabled(false);
      }

   }

}
   @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;
   }

}

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.widget.Button;
import android.view.View;
import android.view.View.OnClickListener;

public class Loading extends Activity {

    Button btnStartProgress;
    ProgressDialog progressBar;
    private int progressBarStatus = 0;
    private Handler progressBarHandler = new Handler();

    private long fileSize = 0;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.loading_main);

        addListenerOnButton();

    }

    public void addListenerOnButton() {

        btnStartProgress = (Button) findViewById(R.id.button1);
        btnStartProgress.setOnClickListener( // <== This is line 33
                new OnClickListener() {

           @Override
           public void onClick(View v) {

            // prepare for a progress bar dialog
            progressBar = new ProgressDialog(v.getContext());
            progressBar.setCancelable(true);
            progressBar.setMessage("Searching for Driver Card...");
            progressBar.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            progressBar.setProgress(0);
            progressBar.setMax(100);
            progressBar.show();

            //reset progress bar status
            progressBarStatus = 0;

            //reset filesize
            fileSize = 0;

            new Thread(new Runnable() {
              public void run() {
                while (progressBarStatus < 100) {

                  // process some tasks
                  progressBarStatus = doSomeTasks();

                  // your computer is too fast, sleep 1 second
                  try {
                    Thread.sleep(1000);
                  } catch (InterruptedException e) {
                    e.printStackTrace();
                  }

                  // Update the progress bar
                  progressBarHandler.post(new Runnable() {
                    public void run() {
                      progressBar.setProgress(progressBarStatus);
                    }
                   });
                }

                // driver card is found
                if (progressBarStatus >= 100) {

                   // sleep 2 seconds, so that you can see the 100%
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    // close the progress bar dialog
                    progressBar.dismiss();
                 }
              }
               });
            startActivity(new Intent(Loading.this,LinkTabs.class));

               }

                });

        }
    // file download simulator
        public int doSomeTasks() {

            while (fileSize <= 1000000) {

                fileSize++;

                if (fileSize == 100000) {
                return 10;
                } else if (fileSize == 200000) {
                return 20;
                } else if (fileSize == 300000) {
                    return 30;
                }

            }

            return 100;

        }

     }

import android.app.TabActivity;
import android.content.Intent;
import android.content.res.Resources;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.TabHost.TabSpec;

@SuppressWarnings("deprecation")
public class LinkTabs extends TabActivity {

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.link_main);

        Resources ressources = getResources(); 
        TabHost tabHost = getTabHost(); 

        // First tab
        Intent intentAndroid = new Intent().setClass(this, HomePage.class);
        TabSpec tabSpecHomePage = tabHost
          .newTabSpec("HomePage")
          .setIndicator("", ressources.getDrawable(R.drawable.overview))
          .setContent(intentAndroid);

        // Second tab
        Intent intentApple = new Intent().setClass(this, HomePage2.class);
        TabSpec tabSpecHomePage2 = tabHost
          .newTabSpec("HomePage2")
          .setIndicator("", ressources.getDrawable(R.drawable.card_summary))
          .setContent(intentApple);

        // Third tab
        Intent intentWindows = new Intent().setClass(this, HomePage3.class);
        TabSpec tabSpecHomePage3 = tabHost
          .newTabSpec("HomePage3")
          .setIndicator("", ressources.getDrawable(R.drawable.details))
          .setContent(intentWindows);


        // add all tabs 
        tabHost.addTab(tabSpecHomePage);
        tabHost.addTab(tabSpecHomePage2);
        tabHost.addTab(tabSpecHomePage3);


        //set Windows tab as default (zero based)
        tabHost.setCurrentTab(2);
    }

}

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Overview");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage2 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Card Summary");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage3 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Details");
        setContentView(textview);
    }
}

选项卡活动： 包com.example.loginscreen

import android.app.Activity;
import android.content.Intent;
import android.graphics.Color;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {

   private EditText  username=null;
   private EditText  password=null;
   private TextView attempts;
   private Button login;
   int counter = 3;


   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      username = (EditText)findViewById(R.id.editText1);
      password = (EditText)findViewById(R.id.editText2);
      attempts = (TextView)findViewById(R.id.textView5);
      attempts.setText(Integer.toString(counter));
      login = (Button)findViewById(R.id.button1);
   }

   public void login(View view){
      if(username.getText().toString().equals("mara") && 
      password.getText().toString().equals("mara")){
      Toast.makeText(getApplicationContext(), "Login Successful!", 
      Toast.LENGTH_LONG).show();
      startActivity(new Intent(MainActivity.this,Loading.class)); 
       }    
   else{
      Toast.makeText(getApplicationContext(), "Wrong Credentials",
      Toast.LENGTH_SHORT).show();
      attempts.setBackgroundColor(Color.RED);   
      counter--;
      attempts.setText(Integer.toString(counter));
      if(counter==0){
         login.setEnabled(false);
      }

   }

}
   @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;
   }

}

import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.widget.Button;
import android.view.View;
import android.view.View.OnClickListener;

public class Loading extends Activity {

    Button btnStartProgress;
    ProgressDialog progressBar;
    private int progressBarStatus = 0;
    private Handler progressBarHandler = new Handler();

    private long fileSize = 0;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.loading_main);

        addListenerOnButton();

    }

    public void addListenerOnButton() {

        btnStartProgress = (Button) findViewById(R.id.button1);
        btnStartProgress.setOnClickListener( // <== This is line 33
                new OnClickListener() {

           @Override
           public void onClick(View v) {

            // prepare for a progress bar dialog
            progressBar = new ProgressDialog(v.getContext());
            progressBar.setCancelable(true);
            progressBar.setMessage("Searching for Driver Card...");
            progressBar.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            progressBar.setProgress(0);
            progressBar.setMax(100);
            progressBar.show();

            //reset progress bar status
            progressBarStatus = 0;

            //reset filesize
            fileSize = 0;

            new Thread(new Runnable() {
              public void run() {
                while (progressBarStatus < 100) {

                  // process some tasks
                  progressBarStatus = doSomeTasks();

                  // your computer is too fast, sleep 1 second
                  try {
                    Thread.sleep(1000);
                  } catch (InterruptedException e) {
                    e.printStackTrace();
                  }

                  // Update the progress bar
                  progressBarHandler.post(new Runnable() {
                    public void run() {
                      progressBar.setProgress(progressBarStatus);
                    }
                   });
                }

                // driver card is found
                if (progressBarStatus >= 100) {

                   // sleep 2 seconds, so that you can see the 100%
                    try {
                        Thread.sleep(2000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    // close the progress bar dialog
                    progressBar.dismiss();
                 }
              }
               });
            startActivity(new Intent(Loading.this,LinkTabs.class));

               }

                });

        }
    // file download simulator
        public int doSomeTasks() {

            while (fileSize <= 1000000) {

                fileSize++;

                if (fileSize == 100000) {
                return 10;
                } else if (fileSize == 200000) {
                return 20;
                } else if (fileSize == 300000) {
                    return 30;
                }

            }

            return 100;

        }

     }

import android.app.TabActivity;
import android.content.Intent;
import android.content.res.Resources;
import android.os.Bundle;
import android.widget.TabHost;
import android.widget.TabHost.TabSpec;

@SuppressWarnings("deprecation")
public class LinkTabs extends TabActivity {

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.link_main);

        Resources ressources = getResources(); 
        TabHost tabHost = getTabHost(); 

        // First tab
        Intent intentAndroid = new Intent().setClass(this, HomePage.class);
        TabSpec tabSpecHomePage = tabHost
          .newTabSpec("HomePage")
          .setIndicator("", ressources.getDrawable(R.drawable.overview))
          .setContent(intentAndroid);

        // Second tab
        Intent intentApple = new Intent().setClass(this, HomePage2.class);
        TabSpec tabSpecHomePage2 = tabHost
          .newTabSpec("HomePage2")
          .setIndicator("", ressources.getDrawable(R.drawable.card_summary))
          .setContent(intentApple);

        // Third tab
        Intent intentWindows = new Intent().setClass(this, HomePage3.class);
        TabSpec tabSpecHomePage3 = tabHost
          .newTabSpec("HomePage3")
          .setIndicator("", ressources.getDrawable(R.drawable.details))
          .setContent(intentWindows);


        // add all tabs 
        tabHost.addTab(tabSpecHomePage);
        tabHost.addTab(tabSpecHomePage2);
        tabHost.addTab(tabSpecHomePage3);


        //set Windows tab as default (zero based)
        tabHost.setCurrentTab(2);
    }

}

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Overview");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage2 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Card Summary");
        setContentView(textview);
    }
}



package com.example.loginscreen;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HomePage3 extends Activity {
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView textview = new TextView(this);
        textview.setText("Details");
        setContentView(textview);
    }
}

以下是xml文件： //activity_main.xml

---

很明显，错误在xml中，您没有在此处添加按钮：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content" >
    </LinearLayout>

    <ProgressBar
        android:id="@+id/progressBar1"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="413dp"
        android:layout_height="382dp" />

</LinearLayout>

尝试在加载的\u main.xml中添加以下内容

            <Button
                android:id="@+id/button1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"/>

完整的内容应该如下所示：

NullPointerException

这是因为使用空对象或未定义对象的值

在Loading.java中，您使用了

setContentView(R.layout.loading_main);

那没有钮扣1

---

在loading_main.xml中添加按钮1或在setContentViewR.layout.other_布局中更改布局

您在LoadingMain活动中使用setContentView引用了以下布局：

   setContentView(R.layout.loading_main);

但是这个布局没有任何按钮1。相反，您试图引用另一个布局中的按钮，即activity_main.xml中的按钮

   btnStartProgress = (Button) findViewById(R.id.button1);

---

此按钮1位于activity_main.xml内，您不能在布局中引用未按setContentView附加的按钮。因此，您需要做的是，在加载_main.xml中添加一个按钮，但是，不要给它与活动主按钮1中相同的id。当然，只要引用了正确的布局，就可以提供相同的id，但id应该是唯一的。如果activity_布局中的按钮应该保留，请在loading_main.xml中为按钮的id指定另一个名称，可能android:id=@+id/loading_main_button_1。

post loading_main.xmlde R.layout.loading_main包含button1？您应该只发布相关的代码片段。loading main中没有button1，这是您的空指针的原因！^^我也数了数：谢谢DHi，我想知道其他版面/活动中的按钮是如何引用的。我不希望任何按钮在带有进度条的加载活动中可见，而是希望登录活动中的按钮移动到加载活动激活进度条并移动到选项卡主机活动。你的建议能做到这一点吗？然后你只需从活动中删除你不想要的按钮，并将其放在登录屏幕中即可。另外，删除活动中的所有引用，只引用登录屏幕中的按钮。谢谢，我想知道其他版面/活动中的引用按钮。我不希望任何按钮在带有进度条的加载活动中可见，而是希望登录活动中的按钮移动到加载活动激活进度条并移动到选项卡主机活动。你的建议能做到这一点吗？我不知道你的问题的复杂性，但是你的空指针异常的解决方案就在上面的答案中。