Android实现加法计算器

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package cn.itcast.totalizator;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import android.content.DialogInterface;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
 //输入框input1 input2
 // 结果框input3
 private EditText input1 = null;
 private EditText input2 = null;
 private EditText input3 = null;
 //等于按钮btn1
 //归零按钮btn2
 private Button btn1 = null;
 private Button btn2 = null;


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

 input1 = (EditText) findViewById(R.id.input1);
 input2 = (EditText) findViewById(R.id.input2);
 input3 = (EditText) findViewById(R.id.input3);

 btn1 = (Button) findViewById(R.id.btn1);
 btn2 = (Button) findViewById(R.id.btn2);

 //点击等于按钮触发监听事件
 btn1.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 String input1Str = input1.getText().toString();
 String input2Str = input2.getText().toString();

 if (! input1Str.equals("") && ! input2Str.equals("")) {
 double number1 = Double.parseDouble(input1Str);
 double number2 = Double.parseDouble(input2Str);
 double result = number1 + number2;

 input3.setText(result + "");
 }
 else{
 Toast.makeText(getApplicationContext(),"请输入合法的数字",Toast.LENGTH_SHORT).show();
 }
 }
 });
 //点击归零按钮触发监听事件
 btn2.setOnClickListener(new View.OnClickListener() {
 @Override
 public void onClick(View v) {
 //把三个输入框都设置为空
 input1.setText("");
 input2.setText("");
 input3.setText("");
 }
 });


 }

 //点击退出按钮，弹出是否确认退出对话框
 @Override
 public void onBackPressed() {
 //声明对象
 AlertDialog dialog;
 //绑定当前界面窗口，设置标题
 dialog = new AlertDialog.Builder(this)
 .setTitle("Dialog对话框")
 .setMessage("是否确定退出")
 .setIcon(R.mipmap.ic_launcher)
 .setPositiveButton("确定",listener)
 .setNegativeButton("取消", listener)
 .create();
 dialog.show();

 }

 //监听对话框里面的button点击事件
 DialogInterface.OnClickListener listener = new DialogInterface.OnClickListener()
 {
 public void onClick(DialogInterface dialog, int which)
 {
 switch (which)
 {
 case AlertDialog.BUTTON_POSITIVE:// "确认"按钮退出程序
 finish();
 break;
 case AlertDialog.BUTTON_NEGATIVE:// "取消"第二个按钮取消对话框
 break;
 default:
 break;
 }
 }
 };
}