http://stackoverflow.com/questions/5690351/java-stringlist-toarray-gives-classcastexception

編譯并不會報錯，但是運行時會檢查類型，這個時候在數(shù)組做強制轉(zhuǎn)換的時候出現(xiàn)問題

The class cast exception happened just because toArray() returns Object[]. Surely Object[] cannot cast to String[].
JDK8 API: List toArray

import java.util.HashSet;
import java.util.Set;

public class ToArrayGivesClassCastException {
    public static void main(String[] args) {
        // java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
        Set<String> recipientSet1 = new HashSet<>();
        recipientSet1.add("wdxxlanswer@gmail.com");
        String[] recipients1 = (String[]) recipientSet1.toArray(); // 問題出在這里
        System.out.println(recipients1.length);

        // Solution 解決方案
        Set<String> recipientSet2 = new HashSet<>();
        recipientSet2.add("wdxxlanswer@gmail.com");
        String[] recipients2 = recipientSet2.toArray(new String[recipientSet2.size()]);
        System.out.println(recipients2.length);
    }
}