转换CharSequence[]和CharSequence[]

Question

然而，我一直认为CharSequence[]和String[]同样重要：

我有一些代码，它有以下内容：

CharSequence[] mEntries;
...
String[] mEntriesString = (String[]) mEntries;
ListAdapter adapter = new ArrayAdapter<String>(getContext(), R.layout.two_lines_list_preference_row, mEntriesString)

当代码运行时，我得到

java.lang.ClassCastException: java.lang.CharSequence[] cannot be cast to java.lang.String[]

那么两个问题？

• 为什么这样的演员不会发生
• 为什么ArrayAdapter不允许在其构造函数中使用CharSequence[]。

Answer 1

CharSequence是String、StringBuffer、StringBuilder类实现的接口。所以CharSequence可以保存这个实现类的任何一个对象，CharSequence#toString返回String，尝试-

String[] mEntriesString = new String[mEntries.length];
int i=0;
for(CharSequence ch: mEntries){
    mEntriesString[i++] = ch.toString(); 
}

Answer 2

CharSequence是一个接口。String类实现了该接口。因此，不能将其转换为String，就像不能将List转换为ArrayList一样，因为它不必是该具体类的实例

Answer 3

不能将引用CharSequence[]转换为String[]。

只有在这种情况下你才能投出它：

        CharSequence[] charSequencesAsString = new String[] { "test" };
        String[] result = (String[]) charSequencesAsString;
        System.out.println(Arrays.toString(result));

解决问题的安全方法：

public static void main(String[] args) {

    CharSequence[] charSequencesAsString = new String[] { "test" };
    CharSequence[] charSequencesAsCharSequence = new CharSequence[] { "test" };
    CharSequence[] charSequencesAsStringBuilder = new StringBuilder[] { new StringBuilder("Test") };

    String[] stringsFromStrings = convertToStringArray(charSequencesAsString);
    String[] stringsFromCharSequence = convertToStringArray(charSequencesAsCharSequence);
    String[] stringsFromStringBuilder = convertToStringArray(charSequencesAsStringBuilder);

    System.out.println("Same array after conversion: " + (stringsFromStrings == charSequencesAsString));
    System.out.println("Same array after conversion: " + (stringsFromCharSequence == charSequencesAsCharSequence));
    System.out.println("Same array after conversion: " + (stringsFromStringBuilder == charSequencesAsStringBuilder));
}

public static String[] convertToStringArray(CharSequence[] charSequences) {
    if (charSequences instanceof String[]) {
        return (String[]) charSequences;
    }

    String[] strings = new String[charSequences.length];
    for (int index = 0; index < charSequences.length; index++) {
        strings[index] = charSequences[index].toString();
    }

    return strings;
}