<span >SharedPreferences sharedPreferences=getSharedPreferences("YUNCHAT", MODE_PRIVATE);
    if(!sharedPreferences.contains("userName")){
      new Thread(){
        public void run(){
          Intent intent=new Intent(MainActivity.this,LoginActivity.class);
          startActivity(intent);
        }
      }.start();
    }</span>

<span >btn_login.setOnClickListener(new OnClickListener() {
      
      @Override
      public void onClick(View arg0) {
        // TODO Auto-generated method stub
        userName=username.getText().toString();
        userPwd=password.getText().toString();
        SharedPreferences sp=getSharedPreferences("YUNCHAT", MODE_PRIVATE);
        Editor editor=sp.edit();
        editor.putString("userName", userName);
        editor.commit();
        Intent intent=new Intent(LoginActivity.this,MainActivity.class);
        startActivity(intent);
      }
    });</span>