java - java.lang.NullPointerException 制作我的应用程序

Question

应用程序崩溃，甚至不打开并给出错误 java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on an null object reference

package com.example.andriod.hostelapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

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


    EditText userId = (EditText) findViewById(R.id.user_name);
    EditText userPassword = (EditText) findViewById(R.id.password);

    public void login()
    {
        if (userId.getText().toString().equals("ritwik7101") && 
            userPassword.getText().toString().equals("ritwik123"))

        {
        }
        else{
            Toast.makeText(this, "INCORRECT USERNAME OR PASSWORD!",
                    Toast.LENGTH_LONG).show();
       }
    }
}

标签： javaandroid

Answer 1

您必须将findViewById调用放在onCreate方法内部和setContent调用之后。