import android.app.Activity;
import android.os.Bundle;
import android.view.KeyEvent;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.TextView.OnEditorActionListener;

public class MainActivity extends Activity {

       @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            final EditText text1 = (EditText) findViewById(R.id.text1);
            text1.setOnEditorActionListener(new OnEditorActionListener() {
                public boolean onEditorAction(TextView editText, int actionId, KeyEvent keyEvent) {
                    if (Integer.valueOf(text1.getText().toString())>=50) {
                        text1.setError("Must be less than 50");
                        return true; //keeps this field in focus in case I had >1 entry fields
                    }
                    return false;
                }
            });
       }
}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <EditText
        android:id="@+id/text1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:inputType="number"/>
</LinearLayout>