android - 无法启动 Activity ComponentInfo : NullPointerException

Question

我不是 android 开发的新手，但我无法弄清楚为什么会出现这个愚蠢的问题。我所做的只是在应用程序启动时启动一个 Activity ，它崩溃并给出 nullpointerException。请在下面检查我的代码。

可能是最后办公时间让我失明了。

日志:

08-23 18:19:19.359: E/AndroidRuntime(798): FATAL EXCEPTION: main
08-23 18:19:19.359: E/AndroidRuntime(798): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.click4tab.orderformnew/com.click4tab.orderformnew.Login}: java.lang.NullPointerException

登录.java :

public class Login extends Activity {

    Button loginButton;
    EditText id, password;
    String sID, sPassword;
    TestAdapter obj;
    Context loginContext;
    public static int loginSuccessful;

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

        //Initialising variables 

        loginButton = (Button)findViewById(R.id.button1);
        id = (EditText)findViewById(R.id.editText1);
        password = (EditText)findViewById(R.id.editText2);

        obj = new TestAdapter(loginContext);


        sID = id.getText().toString();
        sPassword = password.getText().toString();

        StringBuffer buf = new StringBuffer();
        buf.append(sID + ":::" + sPassword);


        TestAdapter.params = new ArrayList<NameValuePair>();
        TestAdapter.params.add(new BasicNameValuePair("tag", "login"));
        TestAdapter.params.add( new BasicNameValuePair("query", buf.toString()));

        loginButton.setOnClickListener( new View.OnClickListener() {

            @Override
            public void onClick(View v) {

                //send login information to authenticate user

                obj.new Read().execute("");

//              Intent i = new Intent("mainActivity");
//              startActivity(i);

            }
        });

    }



}

list :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.click4tab.orderformnew"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="15" />
    <uses-permission android:name="android.permission.INTERNET"/>

    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="mainActivity" />

                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>

        <activity
            android:name=".Login"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>

登录.xml:

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

        <TextView
            android:id="@+id/textView1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_alignParentTop="true"
            android:layout_marginTop="194dp"
            android:text="ID"
            android:textAppearance="?android:attr/textAppearanceMedium" />


        <EditText
            android:id="@+id/editText1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBaseline="@+id/textView1"
            android:layout_alignBottom="@+id/textView1"
            android:layout_alignLeft="@+id/button1"
            android:ems="10" >

            <requestFocus />
        </EditText>

        <TextView
            android:id="@+id/textView2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_below="@+id/editText1"
            android:layout_marginTop="29dp"
            android:text="Password"
            android:textAppearance="?android:attr/textAppearanceMedium" />




        <EditText
            android:id="@+id/editText2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBaseline="@+id/textView2"
            android:layout_alignBottom="@+id/textView2"
            android:layout_alignLeft="@+id/button1"
            android:layout_alignParentRight="true"
            android:ems="10"
            android:inputType="textPassword" />


        <Button
            android:id="@+id/button1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/editText2"
            android:layout_centerHorizontal="true"
            android:layout_marginLeft="19dp"
            android:layout_marginTop="16dp"
            android:text="Login" />

    </RelativeLayout>

Answer 1

你还没有实例化loginContext

做

loginContext = this;

之前

obj = new TestAdapter(loginContext);