activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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="com.codinginflow.radiobuttonexample.MainActivity">

    <RadioGroup
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:id="@+id/radioGroup">

        <RadioButton
            android:id="@+id/radio_one"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="One"
            android:onClick="checkButton"
            android:checked="true"/>

        <RadioButton
            android:id="@+id/radio_two"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Two"
            android:onClick="checkButton"/>

        <RadioButton
            android:id="@+id/radio_three"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Three"
            android:onClick="checkButton"/>

    </RadioGroup>

    <TextView
        android:id="@+id/text_view_selected"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/radioGroup"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="27dp"
        android:text="Your selection: "
        android:textSize="20sp" />

    <Button
        android:id="@+id/button_apply"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/text_view_selected"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="37dp"
        android:text="apply" />

</RelativeLayout>
package com.codinginflow.radiobuttonexample;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RadioButton;
import android.widget.RadioGroup;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    RadioGroup radioGroup;
    RadioButton radioButton;
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        radioGroup = findViewById(R.id.radioGroup);
        textView = findViewById(R.id.text_view_selected);

        Button buttonApply = findViewById(R.id.button_apply);
        buttonApply.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int radioId = radioGroup.getCheckedRadioButtonId();

                radioButton = findViewById(radioId);

                textView.setText("Your choice: " + radioButton.getText());
            }
        });
    }

    public void checkButton(View v) {
        int radioId = radioGroup.getCheckedRadioButtonId();

        radioButton = findViewById(radioId);

        Toast.makeText(this, "Selected Radio Button: " + radioButton.getText(),
                Toast.LENGTH_SHORT).show();
    }
}