从 ArrayList 的字符串中获取变量名

Question

I was wondering if it was possible to retrieve a variable name from a string in Java, but that can be used in an ArrayList. I have read the countless posts on StackOverflow about doing this, using maps, for example, but I keep getting "The constructor ArrayList(String) is undefined." All I want to do is this:

 int first = 1; //These can change between each other, depending on the circumstances, 
                  like second could equal 3.
 int second = 2;
 int third = 3;
 int fourth = 4;

            List<String> player1List = new LinkedList<String>(Arrays.asList(player1));
            List<String> player2List = new LinkedList<String>(Arrays.asList(player2));
            List<String> player3List = new LinkedList<String>(Arrays.asList(player3));
            List<String> player4List = new LinkedList<String>(Arrays.asList(player4));

                String FirstString = "player" + first + "List";
                String SecondString = "player" + second + "List";
                String ThirdString = "player" + third + "List";
                String FourthString = "player" + fourth + "List";

                List<String> fusedPlayer1 = new ArrayList<String>(FirstString);
                fusedPlayer1.addAll(FourthString);

                List<String> fusedPlayer1 = new ArrayList<String>(SecondString);
                fusedPlayer1.addAll(ThirdString);

player1 , player2 , player3 and player4 are all strings. Now, you may be wondering why I want to do this, but this is just an example, and in my actual program, there is a better reason for using this method. I'm a very beginner at Java, so please forgive my lack of knowledge... Thanks!

UPDATE

New code :

int first = 1; //These can change between each other, depending on the circumstances, 
                      like second could equal 3.
     int second = 2;
     int third = 3;
     int fourth = 4;

                List<String> player1List = new LinkedList<String>(Arrays.asList(player1));
                List<String> player2List = new LinkedList<String>(Arrays.asList(player2));
                List<String> player3List = new LinkedList<String>(Arrays.asList(player3));
                List<String> player4List = new LinkedList<String>(Arrays.asList(player4));

                    String FirstString = "player" + first + "List";
                    String SecondString = "player" + second + "List";
                    String ThirdString = "player" + third + "List";
                    String FourthString = "player" + fourth + "List";

             Map<String, String> map = new HashMap<String, String>();
             map.put(FirstString,  "player" + first   + "List");
             map.put(SecondString, "player" + second  + "List");
             map.put(ThirdString,  "player" + third   + "List");
             map.put(FourthString, "player" + fourth  + "List");

        List<String> fusedPlayer1 = new ArrayList<String>();
        fusedPlayer1.add(map.get(FirstString));
        fusedPlayer1.add(map.get(FourthString));

        List<String> fusedPlayer2 = new ArrayList<String>();
        fusedPlayer2.add(map.get(SecondString));
        fusedPlayer2.add(map.get(ThirdString));

Answer 1

This line:

List<String> fusedPlayer1 = new ArrayList<String>(FirstString);

Is telling Java that you want to invoke the ArrayList constructor that takes a String as an argument. The problem is that ArrayList has no such constructor, hence "The constructor ArrayList(String) is undefined.".

The ArrayList class provides three constructors, and they have the following signatures:

public ArrayList(int initialCapacity) { ... }

public ArrayList() { ... }

public ArrayList(Collection<? extends E> c) { ... }

What I suspect you're trying to do is to create the ArrayList with the argument String as an initial element. As it's a single String rather than a group of String s, your best bet is to do the following:

List<String> fusedPlayer1 = new ArrayList<String>();
fusedPlayer1.add(FirstString);

It should also be noted that as of Java 7, you no longer need to specify the type twice, so you can write the following:

List<String> fusedPlayer1 = new ArrayList<>();

The compiler then uses type inference to work out what the generic type of the ArrayList should be.

Based on some of your comments, a Map construct seems more suited to your needs:

Map<Integer, LinkedList<String>> players = new HashMap<>();

players.put(1, player1List);
players.put(2, player2List);
players.put(3, player3List);
players.put(4, player4List);

Then when you need to access, for example, player3List , you simply do:

players.get(3); // This will return the LinkedList associated with the Integer 3.

Lastly, as you can see from the syntax colouring above, the conventions for Java are that your variable names should either be:

• namedInCamelCase , that is, the first word is all lower case and the first letter of subsequent words is upper case, or
• NAMED_IN_CAPS_WITH_UNDERSCORES if the variable is both final and static .

Class names should be in CamelCaseWithAnInitialCapital .

Answer 2

You either need to store your lists in a map or you use a two dimmensional array where the first dimension is the index. Using variable names would also be possible with reflection (at least if your variables are fields), but this is extremly bad practize.

This is how you would generate 4 lists and address them with an integer variable:

int first = 0;
List<String>[] lists = new List<String>[4];
lists[0] = new ArrayList<String>();
lists[1] = new ArrayList<String>(); ...
List<String> firstList = lists[first];