获取Map&＃39; s键的数组 [英] Get array of Map's keys

问题描述

我正在尝试使用Python学习Java，所以请多多包涵。

I am trying to learn Java with a Python basis, so please bear with me.

我正在实现一种Eratosthenes筛方法（我在Python中有一个；尝试将其转换为Java）：

I am implementing a Sieve of Eratosthenes method (I have one in Python; trying to convert it to Java):

def prevPrimes(n):
    """Generates a list of primes up to 'n'"""
    primes_dict = {i : True for i in range(3, n + 1, 2)}
    for i in primes_dict:
        if primes_dict[i]:
            num = i
        while (num * i <= n):
            primes_dict[num*i] = False
            num += 2
    primes_dict[2] = True
    return [num for num in primes_dict if primes_dict[num]]

此我是试图将其转换为Java：

This is my attempt to convert it to Java:

import java.util.*;
public class Sieve {
    public static void sieve(int n){
        System.out.println(n);
        Map primes = new HashMap();
        for(int x = 0; x < n+1; x++){
            primes.put(x, true);
        }
        Set primeKeys = primes.keySet();
        int[] keys = toArray(primeKeys);  // attempt to convert the set to an array
        System.out.println(primesKeys); // the conversion does not work
        for(int x: keys){
            System.out.println(x);
        }
        // still have more to add
        System.out.println(primes);
    }
}

我得到的错误是它找不到方法 toArray（java.util.Set）。我该如何解决？

The error I get is that it cannot find the method toArray(java.util.Set). How can I fix this?

推荐答案

首先，使用泛型：

Map<Integer, Boolean> map = new HashMap<Integer, Boolean>();
Set<Integer> keys = map.keySet();

第二，要将集合转换为数组，可以使用 toArray（ T [] a）：

Second, to convert the set to an array, you can use toArray(T[] a):

Integer[] array = keys.toArray(new Integer[keys.size()]);

，如果要 int 而不是 Integer ，然后遍历每个元素：

and if you want int instead of Integer, then iterate over each element:

int[] array = new int[keys.size()];
int index = 0;
for(Integer element : keys) array[index++] = element.intValue();

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