在進行一個表達式的計算時，先將表達式分割成數字和字符串然後利用出入棧將分割後的表達式進行中綴轉後綴，再將後綴表達式進行計算得到結果(思想在上一篇寫過)現在貼下Java語言的代碼實現。(學習Java時間不長所以可能會有很多不足的地方，我會改進也歡迎大神可以給我一些意見和建議~謝謝啦
)
我將這部分分成三個方法完成功能，並在getResult方法調用(getResult方法被主方法調用)
private String getResult(String str) {
//分割
String[] Str = segment(str);
//中綴轉後綴
String newStr = infToSuf(Str);
//後綴計算
String result = sufToRes(newStr);
return sufToRes(result);
}
1.字符串分割，爲避免在TextView上顯示帶空格刪除時不方便而且顯示屏就那麼大佔地方，錄入時的字符串中沒有空格然後就手動分割了
private static String[] segment(String str) {
String[] exp = new String[str.length()+1];
//找最近的索引並截取字符串
int l = str.length();
for(int i = 0;i < l+1;i++) {
int index;
int[] ind = new int[6];
ind[0] = str.indexOf('+');
ind[1] = str.indexOf('-');
ind[2] = str.indexOf('*');
ind[3] = str.indexOf('/');
ind[4] = str.indexOf('(');
ind[5] = str.indexOf(')');
if(ind[1] == 0) {
Arrays.sort(ind);
int t;
for(t = 0;t <6;t++) {
if(ind[t] >= 0)
break;
}
int r = ind[t+1];
exp[i] = str.substring(0,r);
i++;
exp[i] = str.substring(r,r+1);
str = str.substring(r+1);
}else if(((ind[1]-ind[4]) == 1) && (ind[4]==0)) {
Arrays.sort(ind);
int t ;
for(t = 0;t <6;t++) {
if(ind[t] >= 0)
break;
}
int r = ind[t+1];
exp[i] = str.substring(0,1);
i++;
exp[i] = str.substring(1,r+2);
i++;
exp[i] = str.substring(r+2,r+3);
str = str.substring(r+3);
}else {
Arrays.sort(ind);
int t;
for(t = 0;t <6;t++) {
if(ind[t] >= 0)
break;
}
if(t==6)
break;
index = ind[t];
if(index!=0) {
exp[i] = str.substring(0,index);
i++;
}
exp[i] = str.substring(index,index+1);
str = str.substring(index+1);
}
}
int j = 0;
int k = 0;
for(; exp[j]!=null ;j++){}
if(!exp[j-1].equals(")")) {
exp[j]=str;
str = "";
k = j;
}else {
k = j-1;
}
String[] expp = new String[k+1];
for(int t = 0; t < k+1;t++) {
expp[t] = exp[t];
}
return expp;
//System.out.println("分割的字符串:");
}
2.中綴轉後綴
private static String infToSuf(String[] exp) {
String newStrs = "";
//初始化棧
Stack<String> stack = new Stack<>();
/*
判斷並放入後綴表達式中：
for循環遍歷整個str進行判斷
循環結束若棧不爲空全部出棧
*/
int l = exp.length;
for(int i = 0; i < l; i++) {
if ((stack.empty()) && (exp[i].equals("+") || exp[i].equals("-") || exp[i].equals("*") || exp[i].equals("/"))) {
stack.push(exp[i]);
} else if (exp[i].equals("(")) {
stack.push(exp[i]);
} else if (exp[i].equals("*") || exp[i].equals("/")) {
while (stack.peek().equals("*") || stack.peek().equals("/")) {
newStrs = newStrs.concat(stack.pop()+" ");
if(stack.isEmpty()) {
break;
}
}
stack.push(exp[i]);
} else if (exp[i].equals("+") || exp[i].equals("-")) {
while (!(stack.isEmpty())&&((stack.peek()).equals("*") || (stack.peek()).equals("/") || (stack.peek()).equals("+") || (stack.peek()).equals("-"))) {
newStrs = newStrs.concat(stack.pop()+" ");
if(stack.isEmpty()) {
break;
}
}
stack.push(exp[i]);
} else if (exp[i].equals(")")) {
int t = stack.search("(");
for (int k = 1; k < t; k++) {
newStrs = newStrs.concat(stack.pop()+" ");
}
String tstr = stack.pop();
} else {
newStrs = newStrs.concat(exp[i]+ " ");
}
}
while (!stack.empty()) {
if (!stack.peek().equals("(") || !stack.peek().equals(")")) {
newStrs = newStrs.concat(stack.pop()+" ");
} else if (stack.peek().equals("(") || stack.peek().equals(")")) {
String tstr = stack.pop();
}
}
// System.out.println("後綴:"+newStrs);
return newStrs;
}
3.後綴的計算
private static String sufToRes(String sufStr) {
String[] exp = sufStr.split(" ");
Stack<String> stack = new Stack<>();
String Res = "";
for(int i = 0;i < exp.length; i++) {
if(!exp[i].equals("+") && !exp[i].equals("-") && !exp[i].equals("*") && !exp[i].equals("/")){
stack.push(exp[i]);
}else if(exp[i].equals("+")) {
BigDecimal b2 = new BigDecimal(stack.pop());
BigDecimal b1 = new BigDecimal(stack.pop());
BigDecimal b3 = b1.add(b2);
stack.push(b3.toString());
}else if(exp[i].equals("-")) {
BigDecimal b2 = new BigDecimal(stack.pop());
BigDecimal b1 = new BigDecimal(stack.pop());
BigDecimal b3 = b1.subtract(b2);
stack.push(b3.toString());
}else if(exp[i].equals("*")) {
BigDecimal b2 = new BigDecimal(stack.pop());
BigDecimal b1 = new BigDecimal(stack.pop());
BigDecimal b3 = new BigDecimal(0);
if(b1.compareTo(BigDecimal.ZERO)== 0|| b2.compareTo(BigDecimal.ZERO) == 0) {
b3 = BigDecimal.ZERO;
}else {
b3 = b1.multiply(b2);
}
stack.push(b3.toString());
}else if(exp[i].equals("/")){
BigDecimal b2 = new BigDecimal(stack.pop());
BigDecimal b1 = new BigDecimal(stack.pop());
BigDecimal b3 = new BigDecimal(0);
double d1 = b1.doubleValue();
double d2 = b2.doubleValue();
if(d1%d2 == 0){
b3 = (b1.divide(b2));
stack.push(b3.toString());
}else {
b3 = b1.divide(b2,10, RoundingMode.HALF_UP);
stack.push(b3.toString());
}
}
}
Res = stack.pop();
boolean flag = false;
for (int m = 0; m < Res.length() - 1;m++) {
if(Res.charAt(m) == '.'){
flag = true;
}
}
if(flag) {
for(int m = Res.length()-1;m >= 0;m--) {
if(Res.charAt(m) == '0'){
}else {
Res = Res.substring(0,m+1);
break;
}
}
if(Res.charAt(Res.length()-1) == '.') {
Res = Res.substring(0,Res.length()-1);
}
}
return Res;
}