How to convert ArrayList to String[] in java, Arraylist contains VO objects

问题

Please help me to convert ArrayList to String[]. The ArrayList contains values of type Object(VO).

For example,

The problem is that I need to convert a country List to String Array, sort it and then put it in a list. However I am getting a ClassCastException.

回答1:

String [] countriesArray = countryList.toArray(new String[countryList.size()]);

I have assumed that your country List name is countryList.

So to convert ArrayList of any class into array use following code. Convert T into the class whose arrays you want to create.

List<T> list = new ArrayList<T>();    
T [] countries = list.toArray(new T[list.size()]);

回答2:

Please help me to convert ArrayList to String[], ArrayList Contains Values Object(VO) as Values.

As you mentioned that list contains Values Object i.e. your own class you need toString() overridden to make this work correctly.

This code works. Assuming VO is your Value Object class.

    List<VO> listOfValueObject = new ArrayList<VO>();
    listOfValueObject.add(new VO());
    String[] result = new String[listOfValueObject.size()];
    for (int i = 0; i < listOfValueObject.size(); i++) {
        result[i] = listOfValueObject.get(i).toString();
    }
    Arrays.sort(result);
    List<String> sortedList = Arrays.asList(result);

The snippet of

    List<VO> listOfValueObject = new ArrayList<VO>();
    listOfValueObject.add(new VO());
    String[] countriesArray = listOfValueObject.toArray(new String[listOfValueObject.size()]);

will give you ArrayStoreException due VO is not the String type as required by native method arraycopy subsequently called from toArray one.

回答3:

In case your ArrayList contains Strings, you can simply use the toArray method:

String[] array = list.toArray( new String[list.size()] );

If that is not the case (as your question is not completely clear on this), you will have to manually loop over all elements

List<MyRandomObject> list;
String[] array = new String[list.size() ];
for( int i = 0; i < list.size(); i++ ){
  MyRandomObject listElement = list.get(i);
  array[i] = convertObjectToString( listElement );
}

回答4:

String[] array = list.toArray(new String[list.size()]);

What are we doing here:

String[] array is the String array you need to convert your ArrayList to
list is your ArrayList of VO objects that you have in hand
List#toArray(String[] object) is the method to convert List objects to Array objects

回答5:

As correctly suggested by Viktor, I have edited my snippet.

The is a method in ArrayList(toArray) like:

List<VO> listOfValueObject // is your value object
String[] countries  = new String[listOfValueObject.size()];
for (int i = 0; i < listOfValueObject.size(); i++) {
    countries[i] = listOfValueObject.get(i).toString();
}

Then to sort you have::

Arrays.sort(countries);

Then re-converting to List like ::

List<String> countryList = Arrays.asList(countries);

回答6:

Prior to Java 8 we have the option of iterating the list and populating the array, but with Java 8 we have the option of using stream as well. Check the following code:

   //Populate few country objects where Country class stores name of country in field name.
    List<Country> countries  = new ArrayList<>();
    countries.add(new Country("India"));
    countries.add(new Country("USA"));
    countries.add(new Country("Japan"));

    // Iterate over list
    String[] countryArray = new String[countries.size()];
    int index = 0;
    for (Country country : countries) {
        countryArray[index] = country.getName();
        index++;
    }

    // Java 8 has option of streams to get same size array
    String[] stringArrayUsingStream = countries.stream().map(c->c.getName()).toArray(String[]::new);

来源：https://stackoverflow.com/questions/11464706/how-to-convert-arraylist-to-string-in-java-arraylist-contains-vo-objects

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java

arrays

arraylist

classcastexception