轉換CharSequence []和CharSequence []

Question

我一直認為CharSequence []和String []是必不可少的，但是：

我有一些具有以下內容的代碼：

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

當代碼運行時，我得到

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

有兩個問題？

• 為什么這種轉換不會發生
• 為什么ArrayAdapter不允許在其構造函數中使用CharSequence []。

Answer 1

CharSequence是String ， StringBuffer ， StringBuilder類實現的接口。 因此， CharSequence可以容納此實現類的任何Object，並且CharSequence#toString返回String ，嘗試-

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

Answer 2

CharSequence是一個接口。 String類實現該接口。 因此，您不能將其String為String ，也無法將List ArrayList為ArrayList ，因為它不必是該具體類的實例

Answer 3

您不能將引用CharSequence[]轉換為String[] 。

您只能在這種情況下進行投射：

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

解決問題的安全方法：

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;
}