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我是android开发的初学者。我的代码有问题。应用程序完全崩溃了。这是主要的Java错误。

MainActivity.java：

package hu.cgkni6.uniobuda.testapplication;

import hu.cgkni6.uniobuda.testapplication.R;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;


public class MainActivity extends Activity {

    final String username = "diak";
    final String pass = "diak";
    Button okbutton = (Button) findViewById(R.id.btn_OK);
    Button cancelbutton = (Button) findViewById(R.id.btn_Cancel);
    final EditText usrname = (EditText) findViewById(R.id.editText1);
    final EditText passwd = (EditText) findViewById(R.id.editText2);





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

        okbutton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if (usrname.getText().toString().equals(username))
                {
                    if (passwd.getText().toString().equals(pass))
                    {
                        String text2 = "Success!";
                        Toast.makeText(MainActivity.this, text2, Toast.LENGTH_LONG).show();

                    }
                    else {
                        Toast.makeText(MainActivity.this, "Invalid Password", Toast.LENGTH_LONG).show();
                    }
                }
                else {
                    Toast.makeText(MainActivity.this, "Invalid username", Toast.LENGTH_LONG).show();
                }
            }
        });

        cancelbutton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                finish();
            }
        });

    }


    @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;
    }

}

Activity_main.xml：

<LinearLayout android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" xmlns:android="http://schemas.android.com/apk/res/android"
    android:background="#45B7FA" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="15dp"
        android:layout_marginTop="15dp"
        android:gravity="center_horizontal"
        android:textColor="#FFFFFF"
        android:textSize="30sp"
        android:text="@string/login_interface" />


    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:gravity="right"
        android:orientation="horizontal" >


            <TextView
                android:id="@+id/textView1"
                android:layout_width="100dp"
                android:layout_height="wrap_content"
                android:text="@string/username"
                android:textAppearance="?android:attr/textAppearanceMedium" />

            <EditText
                android:id="@+id/editText1"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:inputType="textEmailAddress"
                android:hint="@string/username" >

                <requestFocus />
            </EditText>       
         </LinearLayout>


    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_marginBottom="5dp"
        android:gravity="right"
        android:orientation="horizontal" >


            <TextView
                android:id="@+id/textView2"
                android:layout_width="100dp"
                android:layout_height="wrap_content"
                android:text="@string/password"
                android:textAppearance="?android:attr/textAppearanceMedium" />

            <EditText
                android:id="@+id/editText2"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:inputType="textPassword"
                android:hint="@string/password"  >

            </EditText>       
         </LinearLayout>



    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:gravity="right"
        android:orientation="horizontal" >

        <Button
            android:id="@+id/btn_OK"
            android:layout_marginLeft="10dp"
            android:layout_marginRight="10dp"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="45dp"
            android:background="#FFFFFF"
            android:textColor="#000000"
            android:textSize="20sp"
            android:text="@string/ok" />
        <Button
            android:id="@+id/btn_Cancel"
            android:layout_marginLeft="10dp"
            android:layout_marginRight="10dp"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="45dp"
            android:background="#FFFFFF"
            android:textColor="#000000"
            android:textSize="20sp"
            android:text="@string/cancel" />
    </LinearLayout>

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent" >

       <TableRow
           android:id="@+id/tableRow1"
           android:layout_width="fill_parent"
           android:layout_height="wrap_content"

           android:layout_alignParentBottom="true"
           android:layout_marginBottom="3dp" >

           <Button
               android:id="@+id/button1"
               android:layout_width="0dp"
               android:layout_height="35dp"
               android:layout_weight="1"
               android:layout_marginLeft="5dp"
               android:background="#FFFFFF"
               android:textColor="#000000"
               android:text="@string/about_us"

 />

           <Button
               android:id="@+id/button2"
               android:layout_width="0dp"
               android:layout_height="35dp"
               android:layout_weight="1"
               android:layout_marginLeft="5dp"
               android:layout_marginRight="5dp"
               android:background="#FFFFFF"
               android:textColor="#000000"
               android:text="@string/contact_us" />

           <Button
               android:id="@+id/button3"
               android:layout_width="0dp"
               android:layout_height="35dp"
               android:layout_weight="1"
               android:layout_marginRight="5dp"
               android:background="#FFFFFF"
               android:textColor="#000000"
               android:text="@string/help" />
       </TableRow>

    </RelativeLayout>

</LinearLayout>

目录：

10-09 09:18:26.244: E/AndroidRuntime(302): FATAL EXCEPTION: main
10-09 09:18:26.244: E/AndroidRuntime(302): java.lang.RuntimeException: Unable to instantiate activity     ComponentInfo{hu.cgkni6.uniobuda.testapplication/hu.cgkni6.uniobuda.testapplication.MainActivity}: java.lang.NullPointerException
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.os.Looper.loop(Looper.java:123)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread.main(ActivityThread.java:4627)
10-09 09:18:26.244: E/AndroidRuntime(302):  at java.lang.reflect.Method.invokeNative(Native Method)
10-09 09:18:26.244: E/AndroidRuntime(302):  at java.lang.reflect.Method.invoke(Method.java:521)
10-09 09:18:26.244: E/AndroidRuntime(302):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
10-09 09:18:26.244: E/AndroidRuntime(302):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
10-09 09:18:26.244: E/AndroidRuntime(302):  at dalvik.system.NativeStart.main(Native Method)
10-09 09:18:26.244: E/AndroidRuntime(302): Caused by: java.lang.NullPointerException
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.Activity.findViewById(Activity.java:1637)
10-09 09:18:26.244: E/AndroidRuntime(302):  at hu.cgkni6.uniobuda.testapplication.MainActivity.<init>(MainActivity.java:17)
10-09 09:18:26.244: E/AndroidRuntime(302):  at java.lang.Class.newInstanceImpl(Native Method)
10-09 09:18:26.244: E/AndroidRuntime(302):  at java.lang.Class.newInstance(Class.java:1429)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
10-09 09:18:26.244: E/AndroidRuntime(302):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
10-09 09:18:26.244: E/AndroidRuntime(302):  ... 11 more
4

5 回答 5

0 
Button okbutton = (Button) findViewById(R.id.btn_OK);
Button cancelbutton = (Button) findViewById(R.id.btn_Cancel);
final EditText usrname = (EditText) findViewById(R.id.editText1);
final EditText passwd = (EditText) findViewById(R.id.editText2);

把它放在 setContentView(R.layout.activity_main); 之后的 oncreate 方法中 你应该很好。你不能从那个位置做到这一点:)

于 2013-10-09T09:37:08.413 回答
0

在您的onCreate()方法中设置布局后，您应该初始化您的视图。您收到错误是因为您在设置布局之前尝试访问视图，这就是应用程序无法找到视图 ID 的原因。

更改您的代码如下：

  @Override
   protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
     Button okbutton = (Button) findViewById(R.id.btn_OK);
    Button cancelbutton = (Button) findViewById(R.id.btn_Cancel);
     final EditText usrname = (EditText) findViewById(R.id.editText1);
    final EditText passwd = (EditText) findViewById(R.id.editText2);
   }
于 2013-10-09T09:39:14.130 回答
0

您正在尝试在设置布局之前查找视图。移动

Button okbutton = (Button) findViewById(R.id.btn_OK);
Button cancelbutton = (Button) findViewById(R.id.btn_Cancel);
final EditText usrname = (EditText) findViewById(R.id.editText1);
final EditText passwd = (EditText) findViewById(R.id.editText2);

到你的onCreate方法。就在你的setContentView电话之后。

于 2013-10-09T09:39:54.670 回答
0

首先你应该学习Android生命周期你应该找到你的所有按钮并在oncreate方法中编辑文本

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Button okbutton = (Button) findViewById(R.id.btn_OK);
           Button cancelbutton = (Button) findViewById(R.id.btn_Cancel);
          final EditText usrname = (EditText) findViewById(R.id.editText1);
          final EditText passwd = (EditText) findViewById(R.id.editText2);
}
于 2013-10-09T09:53:21.240 回答
0

像这样初始化你Button和EditText外部onCreate method

Button okbutton;
Button cancelbutton;
EditText usrname;
EditText passwd;

然后插入这个

okbutton = (Button) findViewById(R.id.btn_OK);
cancelbutton = (Button) findViewById(R.id.btn_Cancel);
EditText usrname = (EditText) findViewById(R.id.editText1);
EditText passwd = (EditText) findViewById(R.id.editText2);

在你的onCreate方法之后setContentView(R.layout.activity_main);

希望能帮助到你。

于 2013-10-09T09:58:20.427 回答