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When I run my code i get a error when the SendCredentials void is executed.

The error is:-

 E/AndroidRuntime: FATAL EXCEPTION: main
 Process: com.example.messager, PID: 16131
 java.lang.IllegalStateException: Could not execute method for android:onClick
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:390)
 at android.view.View.performClick(View.java:6614)
 at android.view.View.performClickInternal(View.java:6591)
 at android.view.View.access$3100(View.java:786)
 at android.view.View$PerformClick.run(View.java:25948)
 at android.os.Handler.handleCallback(Handler.java:873)
 at android.os.Handler.dispatchMessage(Handler.java:99)
 at android.os.Looper.loop(Looper.java:201)
 at android.app.ActivityThread.main(ActivityThread.java:6806)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)
 Caused by: java.lang.reflect.InvocationTargetException
 at java.lang.reflect.Method.invoke(Native Method)
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385)
 at android.view.View.performClick(View.java:6614) 
 at android.view.View.performClickInternal(View.java:6591) 
 at android.view.View.access$3100(View.java:786) 
 at android.view.View$PerformClick.run(View.java:25948) 
 at android.os.Handler.handleCallback(Handler.java:873) 
 at android.os.Handler.dispatchMessage(Handler.java:99) 
 at android.os.Looper.loop(Looper.java:201) 
 at android.app.ActivityThread.main(ActivityThread.java:6806) 
 at java.lang.reflect.Method.invoke(Native Method) 
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873) 
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.io.DataInputStream.readLine()'
 on a null object reference
 at com.example.messager.MainActivity.SendCredentials(MainActivity.java:69)
 at com.example.messager.MainActivity.SendMessage(MainActivity.java:26)
 at java.lang.reflect.Method.invoke(Native Method) 
 at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385) 
 at android.view.View.performClick(View.java:6614) 
 at android.view.View.performClickInternal(View.java:6591) 
 at android.view.View.access$3100(View.java:786) 
 at android.view.View$PerformClick.run(View.java:25948) 
 at android.os.Handler.handleCallback(Handler.java:873) 
 at android.os.Handler.dispatchMessage(Handler.java:99) 
 at android.os.Looper.loop(Looper.java:201) 
 at android.app.ActivityThread.main(ActivityThread.java:6806) 
 at java.lang.reflect.Method.invoke(Native Method) 
 at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547) 
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

My Code is

package com.example.messager;

import android.support.design.widget.TextInputLayout;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import java.net.*;
import java.io.*;

public class MainActivity extends AppCompatActivity 

 EditText stopPort;

 @Override
 protected void onCreate(Bundle savedInstanceState) 
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 
 public void SendMessage(View view)throws IOException
 EditText emailText = (EditText) findViewById(R.id.Email);
 String Username = emailText.getText().toString();
 EditText passwordText = (EditText) findViewById(R.id.Password);
 String Password = passwordText.getText().toString();

 System.out.println("Your username is " + Username + "Your Password is " + Password);
 SendCredentials();
 
 public void SendCredentials()throws IOException 
 System.out.println("Work");
 Socket socket = new Socket("192.168.0.114", 4321);
 System.out.println("Work");
 DataOutputStream dOut = new DataOutputStream(socket.getOutputStream());

 dOut.writeByte(1);
 dOut.writeUTF("This is a test");
 dOut.flush();
 System.out.println("Work");
 
 public void Check()



 

`

Looking to above code and exception trace 2 things can be wrong.

1. check your code for
   java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.io.DataInputStream.readLine()' on a null object reference at




2. In xml layout file, have you declared "check" method on android:onClick
   i.e. android:onClick="check", if yes then your check method should look like this

public void Check(View view)

Can you share you xml layout file as well.

yea my xml code is '

xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 tools:context=".MainActivity">

 <TextView
 android:id="@+id/Welcome"
 android:layout_width="198dp"
 android:layout_height="52dp"
 android:layout_marginStart="104dp"
 android:layout_marginTop="68dp"
 android:text="Welcome"
 android:textAlignment="center"
 android:textAllCaps="false"
 android:textColor="@color/colorPrimaryDark"
 android:textSize="36sp"
 android:textStyle="bold"
 app:layout_constraintStart_toStartOf="parent"
 app:layout_constraintTop_toTopOf="parent" />

 <android.support.design.widget.TextInputLayout
 android:id="@+id/passwordWrapper"
 android:layout_width="387dp"
 android:layout_height="56dp"
 android:layout_marginStart="8dp"
 android:layout_marginTop="444dp"
 android:hint="Password"
 app:hintEnabled="true"
 app:layout_constraintStart_toStartOf="parent"
 app:layout_constraintTop_toTopOf="parent">

 <EditText
 android:id="@+id/Password"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:hint="Password"
 android:inputType="textPassword"
 android:textSize="18sp"
 tools:text="" />
 </android.support.design.widget.TextInputLayout>

 <android.support.design.widget.TextInputLayout
 android:id="@+id/emailWrapper"
 android:layout_width="397dp"
 android:layout_height="56dp"
 android:layout_marginStart="8dp"
 android:layout_marginTop="368dp"
 android:hint="Email"
 app:hintEnabled="true"
 app:layout_constraintStart_toStartOf="parent"
 app:layout_constraintTop_toTopOf="parent">

 <EditText
 android:id="@+id/Email"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:hint="Email"
 android:inputType="textWebEmailAddress"
 android:textAlignment="textStart"
 android:textSize="18sp"
 tools:text=""/>

 </android.support.design.widget.TextInputLayout>

 <Button
 android:id="@+id/LoginButton"
 android:layout_width="348dp"
 android:layout_height="45dp"
 android:layout_marginStart="28dp"
 android:layout_marginTop="520dp"
 android:text="LOGIN"
 android:onClick="SendMessage"
 app:layout_constraintStart_toStartOf="parent"
 app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

'

public void SendMessage(View view) throws IOException 
 EditText emailText = (EditText) findViewById(R.id.Email);
 String Username = emailText.getText().toString();
 EditText passwordText = (EditText) findViewById(R.id.Password);
 String Password = passwordText.getText().toString();

 System.out.println("Your username is " + Username + "Your Password is " + Password);
 SendCredentials();


public void SendCredentials() throws IOException 
 new MyAsyncTask().execute();


public void Check() 



private class MyAsyncTask extends AsyncTask<Void, Void, Void> 

 Activity activity;
 IOException ioException;

 MyAsyncTask() 
 super();
 

 @Override
 protected Void doInBackground(Void... voids) 
 StringBuilder sb = new StringBuilder();
 try 
 System.out.println("Work");
 Socket socket = new Socket("192.168.0.114", 4321);
 System.out.println("Work");
 DataOutputStream dOut = new DataOutputStream(socket.getOutputStream());

 dOut.writeByte(1);
 dOut.writeUTF("This is a test");
 dOut.flush();
 System.out.println("Work");

 socket.close();

 catch (IOException e) 
 this.ioException = e;