<?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"
    android:orientation="vertical"
    tools:context=".HomeActivity">
    <TextView
        android:textSize="50dp"
        android:id="@+id/tvshow"
        android:layout_width="match_parent"
        android:layout_height="100dp"/>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <Button
            android:text="1"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="2"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="3"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="4"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <Button
            android:text="5"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="6"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="7"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="8"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <Button
            android:text="9"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="0"
            android:onClick="saveNum"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:onClick="saveSymbol"
            android:text="+"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:text="-"
            android:onClick="saveSymbol"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
    </LinearLayout>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <Button
            android:onClick="saveSymbol"
            android:text="*"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:onClick="saveSymbol"
            android:text="/"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>
        <Button
            android:onClick="clear"
            android:text="清空"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>

        <Button
            android:text="="
            android:onClick="calc"
            android:layout_width="0dp"
            android:layout_height="80dp"
            android:layout_weight="1"/>


    </LinearLayout>

</LinearLayout>

public class HomeActivity extends AppCompatActivity {
    public TextView textView;
    private double num1 = 0;
    private double num2 = 0;
    private String symbol = "";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        textView =  findViewById(R.id.tvshow);
    }
    public void saveNum(View v){
        Button btnNum = (Button)v;
        if(symbol.equals("")){
            num1 = num1 * 10 + Integer.valueOf((String) btnNum.getText());
            textView.setText(String.valueOf(num1));
        }else{
            num2 = num2 * 10 + Integer.valueOf((String) btnNum.getText());
            textView.setText(String.valueOf(num2));
        }
    }

    public void saveSymbol(View v){
        Button v1 = (Button)v;
        symbol = (String) v1.getText();
        textView.setText(symbol);
    }
    public void clear(View v){
        num1 = 0;
        num2 = 0;
        symbol ="";
        textView.setText("");
    }

    public void calc(View v){
        if (symbol.equals("")){
            return;
        }
        double result = 0;
        switch (symbol){
            case "*":
                result = num1 * num2;
                break;
            case "+":
                result = num1 + num2;
                break;
            case "-":
                result = num1 - num2;
                break;
            case "/":
                if(num2 == 0) return;
                result = num1 / num2;
                break;
        }
        clear(v);
        textView.setText(String.valueOf(result));
    }
}