Eclipse Android SDK中的Android崩溃

Question

I'm beginner in android developement. And i have a problem with my code. The application is totally crashed. It is Java error in main.

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>

Catlog:

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

Answer 1

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);

Put this in the oncreate method after setContentView(R.layout.activity_main); and you should be good. You cant do it from that location :)

Answer 2

You should initialize your Views after setting the layout in your onCreate() method. You are getting the error because you are trying to access the views before setting the layout that is why the application is not able to find the view's id.

Change your code as below:

  @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); } 

Answer 3

You're trying to find views before your layout is setup. Move

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);

to your onCreate method. Just after your setContentView call.

Answer 4

First of all you should learn Android life cycle you should find your all button and edit text in the oncreate method

@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);
}   

Answer 5

Initialize you Button and EditText outside the onCreate method like this

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

Then insert this

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);

inside your onCreate method after the setContentView(R.layout.activity_main);

Hope it helps.