Meine Android-Login-App leider gestoppt

Question

Meine einfache App bekommen leider gestoppt Fehler.
Bitte lösen, bitte.

Ich brauche keine Berechtigungen

Hier ist meine Android-Hauptaktivität:

package vk.com.login_simple; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.widget.Button; 
import android.widget.*; 


public class MainActivity extends AppCompatActivity { 
private EditText username=(EditText) findViewById(R.id.username); 
private EditText password=(EditText) findViewById(R.id.password); 
private Button login=(Button) findViewById(R.id.login); 
private Button reset=(Button) findViewById(R.id.reset); 
private Button exit=(Button) findViewById(R.id.exit); 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setTitle("Login App"); 
     setContentView(R.layout.activity_main); 

    } 
    public void login(View view){ 
if(username.getText().toString().equals("admin")&&password.getText().toString().equals("password")){ 
    Toast.makeText(getApplicationContext(),"Login Success",Toast.LENGTH_LONG).show(); 
//your code here 
}else{ 
    Toast.makeText(getApplicationContext(),"Wrong Username/Password",Toast.LENGTH_LONG).show(); 
} 
    } 

    public void reset(View view) { 
     username.setText(""); 
     password.setText(""); 
     Toast.makeText(this,"Reset success",Toast.LENGTH_LONG).show(); 
    } 
    public void exit(){ 
     //empty 
    } 
} 

Answer 1

findViewById nur funktionieren, die die Stammansicht des Layouts definieren, zu zeigen, wo die Ansichten gefunden werden können. Wenn du es außerhalb von Android anrufst, weißt du nicht, wie und wo du Views für dich findest.

Also, was Sie tun müssen, ist die gesamte findViewById() Code innerhalb onCreate() und nach setContentView().

private EditText username; 
private EditText password; 
private Button login; 
private Button reset; 
private Button exit; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     setTitle("Login App"); 

     username=(EditText) findViewById(R.id.username); 
     password=(EditText) findViewById(R.id.password); 
     login=(Button) findViewById(R.id.login); 
     reset=(Button) findViewById(R.id.reset); 
     exit=(Button) findViewById(R.id.exit); 

    } 

Answer 2

package vk.com.login_simple; 

import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.View; 
import android.widget.Button; 
import android.widget.*; 


public class MainActivity extends AppCompatActivity { 
private EditText username; 
private EditText password; 
private Button login; 
private Button reset; 
private Button exit; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     username=(EditText) findViewById(R.id.username); 
     password=(EditText) findViewById(R.id.password); 
     login=(Button) findViewById(R.id.login); 
     reset=(Button) findViewById(R.id.reset); 
     exit=(Button) findViewById(R.id.exit); 
     setTitle("Login App"); 


    } 
    public void login(View view){ 
if(username.getText().toString().equals("admin")&&password.getText().toString().equals("password")){ 
    Toast.makeText(getApplicationContext(),"Login Success",Toast.LENGTH_LONG).show(); 
//your code here 
}else{ 
    Toast.makeText(getApplicationContext(),"Wrong Username/Password",Toast.LENGTH_LONG).show(); 
} 
    } 

    public void reset(View view) { 
     username.setText(""); 
     password.setText(""); 
     Toast.makeText(this,"Reset success",Toast.LENGTH_LONG).show(); 
    } 
    public void exit(){ 
     //empty 
    } 
} 

Sie können findViewById verwenden, nachdem sie in Ihrem onCreate() -Methode erstellt.

Answer 3

Setzen Sie Ihr Parsing innerhalb onCreate (Methode), nachdem Sie setContentView()

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

    username=(EditText) findViewById(R.id.username); 
    password=(EditText) findViewById(R.id.password); 
    login=(Button) findViewById(R.id.login); 
    reset=(Button) findViewById(R.id.reset); 
    exit=(Button) findViewById(R.id.exit); 
} 

Answer 4

Parse innen onCreate Methode

public class MainActivity extends AppCompatActivity { 
private EditText username; 
private EditText password; 
private Button login; 
private Button reset; 
private Button exit; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setTitle("Login App"); 
     setContentView(R.layout.activity_main); 
     username=(EditText) findViewById(R.id.username); 
     password=(EditText) findViewById(R.id.password); 
     login=(Button) findViewById(R.id.login); 
     reset=(Button) findViewById(R.id.reset); 
     exit=(Button) findViewById(R.id.exit); 
    } 
} 

statt Parsing Verwendung butterknife

public class MainActivity extends AppCompatActivity { 
    @BindView(R.id.username) 
    private EditText username; 
    @BindView(R.id.password) 
    private EditText password; 
    @BindView(R.id.login) 
    private Button login; 
    @BindView(R.id.reset) 
    private Button reset; 
    @BindView(R.id.exit) 
    private Button exit; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setTitle("Login App"); 
     setContentView(R.layout.activity_main); 
    } 
} 

Answer 5

Sie haben wie bitte belllow

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

    EditText username=(EditText) findViewById(R.id.username); 
    EditText password=(EditText) findViewById(R.id.password); 
    Button login=(Button) findViewById(R.id.login); 
    Button reset=(Button) findViewById(R.id.reset); 
    Button exit=(Button) findViewById(R.id.exit); 


}