Java 获取 Map 键的数组
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Get array of Map's keys
提问by Rushy Panchal
I am trying to learn Java with a Python basis, so please bear with me.
我正在尝试以 Python 为基础学习 Java，所以请耐心等待。
I am implementing a Sieve of Eratosthenes method (I have one in Python; trying to convert it to Java):
我正在实施一种 Eratosthenes 方法（我在 Python 中有一个方法；试图将其转换为 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]]
This is my attempt to convert it to Java:
这是我将其转换为 Java 的尝试：
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);
}
}
The error I get is that it cannot find the method toArray(java.util.Set). How can I fix this?
我得到的错误是它找不到方法toArray(java.util.Set)。我怎样才能解决这个问题？
采纳答案by Eng.Fouad
First of all, use generics:
首先，使用泛型：
Map<Integer, Boolean> map = new HashMap<Integer, Boolean>();
Set<Integer> keys = map.keySet();
Second, to convert the set to an array, you can use toArray(T[] a):
其次，要将集合转换为数组，您可以使用toArray(T[] a)：
Integer[] array = keys.toArray(new Integer[keys.size()]);
and if you want intinstead of Integer, then iterate over each element:
如果你想要int而不是Integer，那么迭代每个元素：
int[] array = new int[keys.size()];
int index = 0;
for(Integer element : keys) array[index++] = element.intValue();
回答by dan04
Use primeKeys.toArray()instead of toArray(primeKeys).
使用primeKeys.toArray()代替toArray(primeKeys)。
回答by Ray Stojonic
toArray()is a member of the Collectionclass, so just put Collection.toArray(...)and import java.util.Collection;
toArray()是Collection类的成员，所以只需放入Collection.toArray(...)并导入 java.util.Collection；
Note: toArray()returns an Object[], so you'll have to cast it to Integer[] and assign it to an Integer[] reference:
注意：toArray()返回一个Object[]，因此您必须将其强制转换为 Integer[] 并将其分配给 Integer[] 引用：
Integer[] array = (Integer[])Collection.toArray( someCollection );
Thanks to autoboxing, Integers now work like ints, most of the time.
多亏了自动装箱，整数现在大部分时间都像整数一样工作。
edit: dan04's solution is pretty cool, wish I'd thought of that...anyway, you'll still have to cast and assign to a Object[] type.
编辑：dan04 的解决方案非常酷，希望我能想到……无论如何，您仍然必须强制转换并分配给 Object[] 类型。
