所有要改的代码如下： 

MainActivity代码：

package com.example.code02;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private int count=0;
    private TextView tvCount;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button btnShowToast = findViewById(R.id.btnShowToast);
        btnShowToast.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this, "Hello World!",Toast.LENGTH_SHORT).show();
            }
        });

        tvCount=findViewById(R.id.tvCount);
        Button btnCount = findViewById(R.id.btnCount);
        btnCount . setOnClickListener (new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                tvCount.setText(Integer.toString(++count)) ;
            }
        }) ;

    }
}

activity_main:

<?xml version="1.0" encoding="utf-8"?>
<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:layout_margin="16dp"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/btnShowToast"
        android:text="Show Toast"
        android:textColor="@android:color/white"
        android:background="@color/colorPrimary"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

    <TextView
        android:id="@+id/tvCount"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:text="0"
        android:layout_weight="1"
        android:gravity="center"
        android:textSize="160sp"
        android:textColor="@color/colorAccent"

       />

    <Button
        android:id="@+id/btnCount"
        android:text="Count"
        android:textColor="@android:color/white"
        android:background="@color/colorPrimary"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />



</LinearLayout>

colors:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="black">#FF000000</color>
    <color name="white">#FFFFFFFF</color>
    <color name="background_color">#009688</color>
    <color name="white_grey">#EEEEEE</color>
    <color name="button_selector_color">#9E9E9E</color>
    <color name="colorPrimary">#008577</color>
    <color name="colorPrimaryDark">#004A57</color>
    <color name="colorAccent">#C550DA</color>
</resources>

strings部分我没动，但是代码可以在这里再改进，不过不影响功能