无法解决Android Studio 2.1上的运行时错误

Question

I'm an Android beginner and I'building a test app to know what I can do and what I can't, and I'm trying to solve this Runtime error and don't know what to do. Can anyone help me please? I post my code and the error

MainActvity.java:

package com.example.android.myapplication;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

    EditText uname;

    {
        uname = (EditText) findViewById(R.id.username);
    }

    EditText pword;

    {
        pword = (EditText) findViewById(R.id.password);
    }

    Button btn;

    {
        btn = (Button) findViewById(R.id.button);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        OnClickButtonListener();
    }
    public void OnClickButtonListener() {
        btn.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        Intent intent = new Intent("com.example.android.myapplication.Activity2");
                        startActivity(intent);
                    }
                }
        );
    }
}

Log:

05-01 14:32:21.725 32378-32378/com.example.android.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.android.myapplication, PID: 32378
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.myapplication/com.example.android.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2555)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2773)
    at android.app.ActivityThread.access$900(ActivityThread.java:177)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1434)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5930)
    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:1405)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
    at android.app.Activity.findViewById(Activity.java:2172)
    at com.example.android.myapplication.MainActivity.<init>(MainActivity.java:12)
    at java.lang.reflect.Constructor.newInstance(Native Method)
    at java.lang.Class.newInstance(Class.java:1690)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2545)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2773) 
    at android.app.ActivityThread.access$900(ActivityThread.java:177) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1434) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:135) 
    at android.app.ActivityThread.main(ActivityThread.java:5930) 
    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:1405) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1200) 

Answer 1

Move all three of your statements like this one:

uname = (EditText) findViewById(R.id.username);

into onCreate() , after your setContentView() call, for two reasons:

1. In general, you cannot safely call methods that you inherit from Activity until after super.onCreate() has been called

2. You specifically cannot call findViewById() until you actually have the views, such as by calling setContentView()

Answer 2

Take these three line of code to oncreate method after the setcontentview method recall. That initializes your view and your NULLPOINTEREXCEPTION will not happen.

uname = (EditText) findViewById(R.id.username);
pword = (EditText) findViewById(R.id.password);
btn = (Button) findViewById(R.id.button);