レイアウトさえもJavaで記述する方法

MainActivity.java

package com.example.todoappli;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    TextView textName;
    EditText editName;
    TextView textAge;
    EditText editAge;

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

        LinearLayout linearLayout = new LinearLayout(MainActivity.this);
        linearLayout.setLayoutParams(new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT));
        linearLayout.setOrientation(LinearLayout.VERTICAL);
        textName = new TextView(MainActivity.this);
        textName.setText("名前：");
        editName = new EditText(MainActivity.this);
        textAge = new TextView(MainActivity.this);
        textAge.setText("年齢（数字のみ)");
        editAge = new EditText(MainActivity.this);
        linearLayout.addView(textName);
        linearLayout.addView(editName);
        linearLayout.addView(textAge);
        linearLayout.addView(editAge);
        setContentView(linearLayout);
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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=".MainActivity">

</LinearLayout>