Android应用程序直接意外崩溃

Question

我正在研究android应用程序的登录和注册功能。 用户名和密码保存在ms sql数据库中。 最初，建立了连接，应用程序运行正常。 但是，尽管我没有进行任何更改，但现在它直接在运行时崩溃 。.这是我的活动和我收到的Logcat的代码。 有人可以帮忙吗？

活动：

public class Login extends Activity implements OnClickListener{
    private static final String TAG = Login.class.getSimpleName();
    EditText user;
    EditText pass;
    Button btnlog;
    Button btncan;
    String query;
    Connection connection;
    ArrayList<String> userlist = new ArrayList<String>();
    ArrayList<String> passlist = new ArrayList<String>();

    private void declarer(){
        user = (EditText)findViewById(R.id.editText_name);
        pass = (EditText)findViewById(R.id.editText_pass);
        btnlog = (Button)findViewById(R.id.login);
        //btncan = (Button)findViewById(R.id.cancel);
        btnlog.setOnClickListener(this);
        btncan.setOnClickListener(this);

        query = "SELECT * FROM table ";
        connection = CONN("databse","password","db","server");
    }
    //method to establish a connection with the database
    private Connection CONN(String _user, String _pass, String _DB, String _server){

        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
        StrictMode.setThreadPolicy(policy);
        Connection conn = null;
        String ConnURL = null;
        try{
            Class.forName("net.sourceforge.jtds.jdbc.Driver");
            ConnURL = "jdbc:jtds:sqlserver://" + _server + ";" + "databaseName=" + _DB + ";user=" + _user + ";password=" + _pass + ";";
            //ConnURL ="jdbc:sqlserver://"+_server+":1433/"+_DB+";";
            conn = DriverManager.getConnection(ConnURL);
        }
        catch (SQLException se){
            Log.e("Error", se.getMessage());
        }
        catch(ClassNotFoundException e){
            Log.e("Error",e.getMessage());
        }
        catch (Exception e){
            Log.e("Error",e.getMessage());
        }
        return conn;
    }

    //Method to execute sql queries on the database
    public void QuerySQL(String CommandDoSQL){
        ResultSet rs;
        try{
            Statement statement = connection.createStatement();
            rs = statement.executeQuery(CommandDoSQL);

            int i =0;
            while(rs.next()){
                userlist.add(rs.getString("username"));
                passlist.add(rs.getString("password"));
                i++;
            }
        }
        catch (Exception e){
            Log.e("Error",e.getMessage());
        }
    }

    // Have to implement with the OnClickListner
    // onClick is called when a view has been clicked.
    @Override
    public void onClick(View v){
       String un = user.getText().toString();
       String pas = pass.getText().toString();

        switch (v.getId()){

            case R.id.login:

                if(isValidUser(un)!= -1 && isValidPassword(pas)!= -1 && isValidUser(un)==isValidPassword(pas)){
                    Intent i = new Intent(this,Activity2.class);
                    startActivity(i);
                }
                else if (isValidPassword(pas) == -1 && isValidUser(un)!= -1) {
                    pass.setError("Invalid Password");
                }
                else if (isValidUser(un) == -1 && isValidPassword(pas)!= -1) {
                    pass.setError("Invalid Username");
                }
                else if (isValidUser(un)== -1 && isValidPassword(pas)== -1) {
                    user.setError("Invalid Username");
                    pass.setError("Invalid Password");
                }
                break;

            case R.id.cancel:
                user.setText("");
                pass.setText("");
                break;

            default:
                break;
        }
    }

    private int isValidUser(String un) {

        for(int i =0;i<userlist.size();i++){
            if (un.equals(userlist.get(i))) {
                return i;
            }
        }
        return -1;
    }
    private int isValidPassword(String pass) {

        for(int i =0;i<passlist.size();i++){
            if (pass.equals(passlist.get(i))) {
                return i;
            }
        }
        return -1;
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);

        declarer();
        QuerySQL(query);
    }
}

XML：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".Login"
    android:id="@+id/Login">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Username"
        android:id="@+id/textView4"
        android:layout_marginTop="31dp"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Password"
        android:id="@+id/textView5"
        android:layout_marginTop="53dp"
        android:layout_below="@+id/textView4"
        android:layout_toLeftOf="@+id/editText_name"
        android:layout_toStartOf="@+id/editText_name" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:inputType="textPersonName"
        android:hint="Username"
        android:ems="10"
        android:id="@+id/editText_name"
        android:layout_above="@+id/textView5"
        android:layout_toRightOf="@+id/textView4"
        android:layout_toEndOf="@+id/textView4"
        android:layout_marginLeft="27dp"
        android:layout_marginStart="27dp" />

    <EditText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:hint="Password"
        android:inputType="textPassword"
        android:ems="10"
        android:id="@+id/editText_pass"
        android:layout_alignBottom="@+id/textView5"
        android:layout_alignLeft="@+id/editText_name"
        android:layout_alignStart="@+id/editText_name" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Login"
        android:id="@+id/login"
        android:layout_marginTop="43dp"
        android:layout_below="@+id/editText_pass"
        android:layout_alignRight="@+id/editText_pass"
        android:layout_alignEnd="@+id/editText_pass" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Cancel"
        android:id="@+id/cancel"
        android:layout_alignTop="@+id/login"
        android:layout_centerHorizontal="true" />

</RelativeLayout>

logcat的：

3156-3156/com.developer.user.application D/AndroidRuntime﹕ Shutting down VM
3156-3156/com.developer.user.application E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.developer.user.application, PID: 3156
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.developer.user.application/com.developer.user.application.Login}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)                 at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)                 at android.app.ActivityThread.access$800(ActivityThread.java:144)                 at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.developer.user.application.Login.declarer(Login.java:45)
at com.developer.user.application.Login.onCreate(Login.java:154)
at android.app.Activity.performCreate(Activity.java:5937)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer 1

您正在尝试设置侦听器btncan.setOnClickListener(this); 上没有参考的对象，因为您用设置参考注释了行。 尝试取消注释此行：

    //btncan = (Button)findViewById(R.id.cancel);

在

private void declarer(){
    user = (EditText)findViewById(R.id.editText_name);
    pass = (EditText)findViewById(R.id.editText_pass);
    btnlog = (Button)findViewById(R.id.login);
    //btncan = (Button)findViewById(R.id.cancel);
    btnlog.setOnClickListener(this);
    btncan.setOnClickListener(this);

    query = "SELECT * FROM table ";
    connection = CONN("databse","password","db","server");
}

Answer 2

// btncan =（Button）findViewById（R.id.cancel）; 您怎么看，btncan的价值是什么？ 似乎btcan保持为空，并且您尝试按异常说明在空对象上调用setOnClickListener（）。

Answer 3

取消注释：

//btncan = (Button)findViewById(R.id.cancel);