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;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);Button btnShowToast = findViewById(R.id.btnShowToast);btnShowToast.setOnClickListener(new View.OnClickListener() {@Overridepublic 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() {@Overridepublic void onClick(View view) {tvCount.setText(Integer.toString(++count)) ;}}) ;}
}

activity_main:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayoutxmlns: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"><Buttonandroid: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"/><TextViewandroid: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"/><Buttonandroid: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