editText field is required before moving on to another Activity

Try this:

bt.setOnClickListener(new OnClickListener() {
    public void onClick(View arg0)
    {
        String  str=et.getText().toString();

        if(str.equalsIgnoreCase(""))
        {
            et.setHint("please enter username");//it gives user to hint
            et.setError("please enter username");//it gives user to info message //use any one //
        }
        else
        {
            Intent in=new Intent(getApplicationContext(),second.class);
            startActivity(in);
        }
    }
});

You can also try this:

 if ( ( userName.getText().toString().trim().equals("")) ) 
 {
      Toast.makeText(getApplicationContext(), "User name is empty", Toast.LENGTH_SHORT).show();
 }
 else
 {
      Intent i = new Intent(getApplicationContext(), Login.class);
      startActivity(i);
 }

I faced a similiar problem. In my case, I had a lot of Edittexts to verify, some them was child of another layouts. Tip: all views must be in same root view. So, to be simple:

...
LinearLayout viewGroup = (LinearLayout) findViewById(R.id.viewGroup);
checkFieldsRequired(viewGroup);
...

public boolean checkFieldsRequired(ViewGroup viewGroup){

    int count = viewGroup.getChildCount();
    for (int i = 0; i < count; i++) {
        View view = viewGroup.getChildAt(i);
        if (view instanceof ViewGroup)
            checkFieldsRequired((ViewGroup) view);
        else if (view instanceof EditText) {
            EditText edittext = (EditText) view;
            if (edittext.getText().toString().trim().equals("")) {                  
                edittext.setError("Required!");                 
            }
        }
    }

    return false;
}

It's easy...check if your EditText is empty as in below example below.

if( TextUtils.isEmpty(userName.getText())){
   /**
    *   You can Toast a message here that the Username is Empty    
    **/
    
   userName.setError( "First name is required!" );

}else{
   Intent i = new Intent(getApplicationContext(), Login.class);
   startActivity(i);
}

Here is a Kotlin version:

userName.takeIf { it.isEmpty() }?.let {
  userName.error = "First name is required!"
} ?: run {
   startActivity(Intent(applicationContext, this))
}