关于迭代器

以下代码模拟了迭代器的创建和使用。

先创建一个类，并定义迭代器常有的hasNext（）和next（）方法。

package caculator;

import java.util.LinkedList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Formula {
private String str;

public Formula(String str) {
super();
this.str = str.trim();
}

public class Iterator{

int from;

String regex = "\\d+(\\.\\d+)?|[+/\\-*]";
//Matcher m = Pattern.compile(regex).matcher(str);

public String next(){
Matcher m = Pattern.compile(regex).matcher(str);
m.find(from);
String s = m.group();
from = m.end();
return s;
}

public Boolean hasNext(){
return from < str.length();
}
}

public double caculator(){
LinkedList<Double> list1 = new LinkedList<>();
LinkedList<String> list2 = new LinkedList<>();
Iterator it = new Iterator();
while(it.hasNext()){
String s = it.next();
if(s.matches("\\d+(\\.\\d+)?")){
list1.add(Double.parseDouble(s));
}else if(s.matches("[+\\-]")){
list2.add(s);
}else{
double a = list1.removeLast();
double b = Double.parseDouble(it.next());
double c = caculate(a,s,b);

list1.add(c);
}
}

while(list2.size()!=0){
double a = list1.removeFirst();
double b = list1.removeFirst();
String s = list2.removeFirst();

double c = caculate(a,s,b);

list1.addFirst(c);
}
return list1.get(0);
}

private double caculate(double a, String s, double b) {
double r = 0;
switch (s.charAt(0)) {
case '+':
r = a+b;
break;
case '-':
r=a-b;
break;
case '*':
r=a*b;
break;
case '/':
r=a/b;
break;
default:
break;
}
return r;
}
}

再创建一个测试类，并引上述类的方法计算用户输入的公式。

package caculator;

import java.util.Scanner;
import caculator.Formula.Iterator;

public class Test1 {
public static void main(String[] args) {
System.out.println("请输入计算公式：");
String s = new Scanner(System.in).nextLine();
Formula f = new Formula(s);

Double d = f.caculator();
System.out.println(s+"="+d);
}
}

以上程序是实现的是根据用户输入的算式输出准确的计算结果。如输入：6+8/2+3*2,输入结果为：6+8/2+3*2=16.0