java - 启动器上出现空指针异常？安卓

Question

我正在尝试创建一个非常简单的登录页面，但是当我尝试启动该应用程序时出现错误。我根本不明白，因为我基本上已经做到了，就像我拥有另一个发射器一样。我已经将 Intent 过滤器放在这个新 Activity 上。任何帮助将不胜感激，因为我明天必须在考试中展示它。这是代码。关于插入的事情，只是暂时的，我稍后会改变它。

public class LoginActivity extends Activity implements OnClickListener {

EditText loginUsername,loginPassword;
Button loginButton;
SQLiteDatabase db;

public void onCreate(Bundle savedInstanceState)
{ 

    super.onCreate(savedInstanceState);
    setContentView(R.layout.login);
    loginUsername=(EditText)findViewById(R.id.loginUsername);
    loginPassword=(EditText)findViewById(R.id.loginPassword);
    loginButton.setOnClickListener(this);
    db=openOrCreateDatabase("StudentDB", Context.MODE_PRIVATE, null);
    db.execSQL("CREATE TABLE IF NOT EXISTS studentTest5(rollno VARCHAR,name VARCHAR,lastname VARCHAR,password VARCHAR,marks VARCHAR, phone VARCHAR, email VARCHAR);");
    db.execSQL("INSERT INTO studentTest5 VALUES('1','Lars','Hansen','1234','7','60158096','habbahabba@hotmail.com');");
}


@Override
public void onClick(View view) {
    // TODO Auto-generated method stub
    if(view==loginButton)
    {
        if(loginUsername.getText().toString().trim().length()==0||
           loginPassword.getText().toString().trim().length()==0)
        {
            showMessage("Error", "Please enter all values");
            return;
        }
        Cursor c=db.rawQuery("SELECT * FROM studentTest5 WHERE name='"+loginUsername.getText()+"'", null);
        if(c.moveToFirst())
        {
            if(c.getString(3)==loginPassword.getText().toString())
            {
                showMessage("sucess", "user exists");
                //loginPassword.setText(c.getString(3));
                } else {
                    showMessage("Error", "incorrect password");
                }


        }
        else
        {
            showMessage("Error", "Invalid Rollno");
            clearText();
        }

    }

}
 public void showMessage(String title,String message)
    {
        Builder builder=new Builder(this);
        builder.setCancelable(true);
        builder.setTitle(title);
        builder.setMessage(message);
        builder.show();
    }
    public void clearText()
    {
        loginUsername.setText("");
        loginPassword.setText("");

    }

}

这是xml

<TextView
    android:id="@+id/loginWelcome"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="65dp"
    android:text="Welcome!"
    android:textAppearance="?android:attr/textAppearanceLarge" />

<EditText
    android:id="@+id/loginUsername"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/loginWelcome"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="20dp"
    android:ems="10"
    android:hint="Login"
    android:inputType="textPersonName" />

<EditText
    android:id="@+id/loginPassword"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/loginUsername"
    android:layout_alignRight="@+id/loginUsername"
    android:layout_below="@+id/loginUsername"
    android:layout_marginTop="14dp"
    android:ems="10"
    android:hint="Password"
    android:inputType="textPassword" />

<Button
    android:id="@+id/loginButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/loginPassword"
    android:layout_alignRight="@+id/loginPassword"
    android:layout_below="@+id/loginPassword"
    android:layout_marginTop="30dp"
    android:text="Login" />

</RelativeLayout>

这是错误信息

   FATAL EXCEPTION: main
   Process: com.technotalkative.viewstubdemo, PID:     10998
   java.lang.RuntimeException: Unable to start activity      ComponentInfo{com.technotalkative.viewstubdemo/com.technotalkative.viewstubdemo.LoginActivity}:      java.lang.NullPointerException
    at  android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2184)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2233)
    at android.app.ActivityThread.access$800(ActivityThread.java:135)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:136)
    at android.app.ActivityThread.main(ActivityThread.java:5001)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.technotalkative.viewstubdemo.LoginActivity.onCreate(LoginActivity.java:27)
    at android.app.Activity.performCreate(Activity.java:5231)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2148)
    ... 11 more

Answer 1

  loginButton.setOnClickListener(this);

您错过了 loginButton 的 findViewById(对象的初始化)