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