MainActivity：
package com.example.l3_pre;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private Button btnClear, btnDel, btnDev, btnAdd, btnDot, btnDec, btnMul, btnEqu;//相应的功能键
private TextView txtResult;//显示结果
private StringBuffer digitA = new StringBuffer(), digitB = new StringBuffer();
private boolean isChar = false;//标记是否按下运算符
