转换CharSequence []和CharSequence []

Question

I always thought CharSequence[] and String[] were essential the same, however :

I have some code that has the following :

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

When the code runs I get

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

So two questions ?

• why can this cast not happen
• why does the ArrayAdapter not allow for a CharSequence[] in it's constructor.

Answer 1

CharSequence is interface which String , StringBuffer , StringBuilder classes implements. So CharSequence can hold any of this implementation class's Object And CharSequence#toString returns String , try -

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

Answer 2

CharSequence is an Interface. String class implements that interface. So, you cannot cast it to String , the same way you cannot cast List to ArrayList , because it doesn't have to be an instance of that concrete class

Answer 3

You can not cast reference CharSequence[] into String[] .

You can cast it only in that situation:

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

Safe way to solve your problem:

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;
}