最近在学安卓的相关知识，第一个demo做了一个简易计算器，功能仿手机上自带的计算器，没有加括号，简单的四则运算，支持长表达式运算。此篇贴出本次简易计算器完整逻辑代码。
代码如下：
package com.example.zwkkkk1.caculator1;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.view.View;import android.widget.TextView;import java.math.BigDecimal;import java.util.Stack;import java.util.regex.Pattern;public class MainActivity extends AppCompatActivity implements View.OnClickListener {private static String TAG = "CACULATOR";TextView txt_result, txt_edit;boolean isOperateDown = false;//运算符是否已经按过一次，默认没有按过 falseboolean isDotDown = false;//. 是否已经按过一次，默认没有按过 false@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);txt_edit = (TextView)findViewById(R.id.txt_edit);txt_result = (TextView)findViewById(R.id.txt_result);findViewById(R.id.btn_0).setOnClickListener(this);findViewById(R.id.btn_1).setOnClickListener(this);findViewById(R.id.btn_2).setOnClickListener(this);findViewById(R.id.btn_3).setOnClickListener(this);findViewById(R.id.btn_4).setOnClickListener(this);findViewById(R.id.btn_5).setOnClickListener(this);findViewById(R.id.btn_6).setOnClickListener(this);findViewById(R.id.btn_7).setOnClickListener(this);findViewById(R.id.btn_8).setOnClickListener(this);findViewById(R.id.btn_9).setOnClickListener(this);findViewById(R.id.btn_divide).setOnClickListener(this);findViewById(R.id.btn_multi).setOnClickListener(this);findViewById(R.id.btn_plus).setOnClickListener(this);findViewById(R.id.btn_sub).setOnClickListener(this);findViewById(R.id.btn_equal).setOnClickListener(this);findViewById(R.id.btn_clear).setOnClickListener(this);findViewById(R.id.btn_back).setOnClickListener(this);findViewById(R.id.btn_equal).setOnClickListener(this);findViewById(R.id.btn_dot).setOnClickListener(this);}public void onClick(View v) {switch (v.getId()) {case R.id.btn_0:num_down("0");break;case R.id.btn_1:num_down("1");break;case R.id.btn_2:num_down("2");break;case R.id.btn_3:num_down("3");break;case R.id.btn_4:num_down("4");break;case R.id.btn_5:num_down("5");break;case R.id.btn_6:num_down("6");break;case R.id.btn_7:num_down("7");break;case R.id.btn_8:num_down("8");break;case R.id.btn_9:num_down("9");break;case R.id.btn_plus:operator_down("+");break;case R.id.btn_sub:operator_down("-");break;case R.id.btn_divide:operator_down("÷");break;case R.id.btn_multi:operator_down("×");break;case R.id.btn_clear:isDotDown = false;isOperateDown = false;txt_edit.setText("0");txt_result.setText("");break;case R.id.btn_back: {String strEdit = txt_edit.getText().toString();int length = strEdit.length();if (Pattern.matches("^=[0-9].*", strEdit)) {txt_edit.setText("0");txt_result.setText("");} else {if (length > 0) {String word = strEdit.substring(length - 1, length);if(word.equals("."))isDotDown = false;if(word.equals("+") || word.equals("-") || word.equals("×") || word.equals("÷"))isOperateDown = false;txt_edit.setText(strEdit.substring(0, length - 1));}}break;}case R.id.btn_dot: {String strEdit = txt_edit.getText().toString();if (!isDotDown) {isDotDown = true;if(Pattern.matches("^=[0-9].*", strEdit))strEdit = "0";txt_edit.setText(strEdit + ".");}break;}case R.id.btn_equal:equal();break;}}//按下数字函数private void num_down(String num) {String strEdit = txt_edit.getText().toString();isOperateDown = false;if (strEdit.equals("0") || Pattern.matches("^=[0-9].*", strEdit)) {txt_edit.setText(num);txt_result.setText("");} else {txt_edit.setText(strEdit + num);}}// 按下运算符函数private void operator_down(String operator) {if(!isOperateDown) {String strEdit = txt_edit.getText().toString();isOperateDown = true;isDotDown = false;if(Pattern.matches("^=[0-9].*", strEdit))strEdit = strEdit.substring(1, strEdit.length());txt_edit.setText(strEdit + operator);}}private void equal() {String strEdit = txt_edit.getText().toString();int length = strEdit.length();if(!Pattern.matches("^=[0-9].*", strEdit)){txt_result.setText(strEdit);if(Pattern.matches(".*[\\+\\-\\×\\÷\\.]$", strEdit)) {strEdit = strEdit.substring(0, length - 1);}String postfixExp = getPostfixExp(strEdit);txt_edit.setText("=" + calPostfix(postfixExp));}}//将中缀表达式转换为后缀表达式private String getPostfixExp(String str) {String postfix = "";String numString = ""; //因数字 不止一位需要String存储Stack numStack = new Stack();Stack opStack = new Stack();for(int i = 0; i < str.length(); i++) {char ch = str.charAt(i);if(Character.isDigit(ch) || ch == '.') { //判定ch 是否是数字 或者是 .numString += String.valueOf(ch); //将数字和 .放入numString，等待下一个运算符} else { //ch为运算符时if(numString.length() > 0) {numStack.push(numString);//将此运算符前数字压入数字栈numString = ""; //压入栈后，初始化 numString}opPush(opStack, numStack, ch);}}//最后判定numString是否为空，因为最后一个可能是数字，没有运算符进行判定if(numString.length() > 0)numStack.push(numString);//检测完后，将运算符栈中转入到数字栈中while(!opStack.empty()) {numStack.push(opStack.pop());}//将数字栈打印出来得到后缀表达式//此处需要将字符串逆序，才得到后缀表达式，但是有小数点的存在，不能直接用 reverse 的逆序函数//通过两个栈的先进后出特点，得到栈的逆序while(!numStack.empty()) {opStack.push(numStack.pop());}while(!opStack.empty()) {postfix = postfix + String.valueOf(opStack.pop()) + " ";}return postfix;}//计算后缀表达式private String calPostfix(String str) {String result = "";Stack numStack = new Stack();for(int i = 0; i < str.length(); i++) {char ch = str.charAt(i);if(ch == ' ') {//运算符时if(result.length() > 0 && (result.equals("+") || result.equals("-") || result.equals("×") || result.equals("÷"))){double num = 0;double secondNum = Double.parseDouble(String.valueOf(numStack.pop()));double firstNum = Double.parseDouble(String.valueOf(numStack.pop()));switch (result) {case "+":num = firstNum + secondNum;break;case "-":num = firstNum - secondNum;break;case "×":num = firstNum * secondNum;break;case "÷":num = firstNum / secondNum;break;}numStack.push(num);}else if(result.length() > 0) {numStack.push(result);}result = "";} else {result += String.valueOf(ch);}}return BigDecimal.valueOf(Double.valueOf(String.valueOf(numStack.pop()))).stripTrailingZeros().toPlainString();}//获取运算符权重private int getOpWeight(char ch) {// + - 权重为1if(ch == '+' || ch == '-') return 1;//× ÷ 权重为2if(ch == '×' || ch == '÷') return 4;return -1;}//将运算符压入栈private void opPush(Stack opStack, Stack numStack, char ch) {if(canOpPush(opStack, ch)) { //判定能否将运算符压入栈内opStack.push(ch); //true则压入栈内} else { //false（即 待压入运算符优先级 <= 栈顶运算符优先级）//将栈顶运算符取出压入数字栈numStack.push(String.valueOf(opStack.pop()));//此处需要递归判定，弹出所有优先级 >= 该运算符的栈顶元素opPush(opStack, numStack, ch);}}//判定运算符能否压入运算符栈private Boolean canOpPush(Stack opStack, char ch) {//当运算符栈为空时，返回true；或当待压入运算符权重大于栈顶权重，返回trueif(opStack.empty() || (getOpWeight(ch) > getOpWeight(String.valueOf(opStack.peek()).charAt(0))))return true;return false; //其他情况返回false}}
