import android.app.Activity;
    import android.app.AlertDialog.Builder;
    import android.content.Context;
    import android.content.DialogInterface;
    import android.content.Intent;
    import android.content.SharedPreferences;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;
    import android.app.AlertDialog;
    import android.widget.EditText;

    public class userPreferences extends Activity {
    EditText editText;
    Button save;
    public static final String NAME="UserName";
    SharedPreferences sharedPreferences;
    SharedPreferences.Editor editor;
    String n;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.userpreferences);
        editText=(EditText)findViewById(R.id.editText);
        save=(Button)findViewById(R.id.button);
        sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
      save.setOnClickListener(new View.OnClickListener() {
          final AlertDialog.Builder alertDialog = new AlertDialog.Builder(userPreferences.this);

          @Override
          public void onClick(View arg0) {

                      alertDialog.setTitle("Alert Dialog");
              alertDialog.setMessage("Are you sure you want to save?");
              alertDialog.setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                          @Override
                          public void onClick(DialogInterface dialog, int which) {
                              n=editText.getText().toString();
                              sharedPreferences=getPreferences(MODE_PRIVATE);
                              editor=sharedPreferences.edit();
                              editor.putString(NAME,n);
                              editor.apply();
                              startActivity(new Intent(userPreferences.this,Welcome.class));

                          }
                      });
                      alertDialog.setNegativeButton("No", null);
                      alertDialog.show();


          }
      });

    }


}

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class Welcome extends Activity {

TextView textView;
SharedPreferences sharedPreferences;
public static final String NAME="UserName";
SharedPreferences.Editor editor;
String n;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.welcome);
    textView=(TextView)findViewById(R.id.textView);
    textView.setText("Welcome," + editor.putString(NAME, n));
    sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
    editor.apply();

}

@Override
public void onBackPressed() {
    super.onBackPressed();
    startActivity(new Intent(Welcome.this, MainActivity.class));
    return;


}

Attempt to invoke interface method 'android.content.SharedPreferences$Editor android.content.SharedPreferences$Editor.putString(java.lang.String, java.lang.String)' on a null object reference

sharedPreferences=getPreferences(MODE_PRIVATE);

sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
if(!sharedPreferences.getString(NAME,"Default value").equals("Default value")){
  startActivity(new Intent(userPreferences.this, Welcome.class));
  finish();
}

sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
textView=(TextView)findViewById(R.id.textView);
textView.setText("Welcome," + sharedPreferences.getString(NAME,"Default value"));
editor = = sharedPreferences.edit();
editor.putString(NAME, sharedPreferences.getString(NAME,"Default value"));
editor.apply();

// open your preferences using the name you originally used to create it
sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);

// open for editing
editor = sharedPreferences.edit();

// you probably don't want to use the same name here as preferences name
editor.putString(NAME,n);
editor.apply();

sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);
String name = sharedPreferences.getString(NAME);

textView.setText("Welcome," + name);

@Override
protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.welcome);
     textView=(TextView)findViewById(R.id.textView);

     sharedPreferences = getSharedPreferences(NAME, MODE_PRIVATE);
     String name = sharedPreferences.getString(NAME);

     textView.setText("Welcome," + name);
     editor = sharedPreferences.edit();
     editor.putString(NAME, name);
     editor.apply();

}

TextView textView;
SharedPreferences sharedPreferences;
public static final String NAME = "UserName";
public static final String defaultName = "";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.welcome);
    textView = (TextView) findViewById(R.id.textView);

    sharedPreferences = getSharedPreferences(NAME, Context.MODE_PRIVATE);
    textView.setText("Welcome," + sharedPreferences.getString(NAME, defaultName));
}

EditText editText;
Button save;
public static final String NAME="UserName";
public static final String default = "";
SharedPreferences sharedPreferences;
SharedPreferences.Editor editor;
String n;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.userpreferences);
    editText = (EditText) findViewById(R.id.editText);
    save = (Button) findViewById(R.id.button);
    sharedPreferences=getSharedPreferences(NAME, Context.MODE_PRIVATE);
    if (sharedPreferences.getString(NAME, default).length() > 0) {
        // The user has already entered a username, so start the Welcome activity
        startActivity(new Intent(userPreferences.this, Welcome.class));
        finish();
    }
    // put the rest of your onCreate code here
}