Question

我用一个静态arraylist制作一个Coin类，它存储了所创建的类的每个实例，但是我需要用初始实例启动该列表，而且我还没想出如何在不添加它的情况下执行它（因为冗余代码），有什么建议吗？

public class Coin {
    private static ArrayList<String> coinNames = new ArrayList<>();
    private static ArrayList<String> coinAbbreviations = new ArrayList<>(Arrays.asList("CLP"));
    private static ArrayList<Coin> coins =
            new ArrayList<>(Arrays.asList(new Coin("Pesos chilenos", "CLP", 1f, "CLP")));
    private static HashMap<String,Float> exchangeRates;
    private String coinName;
    private String coinAbbreviation;
    private Float coinValue;
    private String unit;


    public Coin(String coinName, String coinAbbreviation, Float coinValue, String unit) {
        assert !coinAbbreviations.contains(coinAbbreviation) : "Coin abbreviation already used";
        assert coinAbbreviations.contains(unit) : "Coin unit non existent.";
        assert !coinNames.contains(coinName) : "Coin name already used.";
        this.coinName = coinName;
        this.coinAbbreviation = coinAbbreviation;
        this.coinValue = coinValue;
        this.unit = unit;

        coins.add(this);
    }
}

Answer 1

如果你坚持要有可变的静态变量 - 那么做这样的事情通常不是一个好主意 - 你可以做到

private static ArrayList<Coin> coins =
        new ArrayList<>();

static {
  new Coin("Pesos chilenos", "CLP", 1f, "CLP");
}

...将元素立即添加到列表中。

Answer 2

什么阻止你在声明中初始化列表然后只是将每个实例添加到构造函数中的列表？

Answer 3

您也可以使用一些最佳实践模式设计您的应用程序。您想要保留所有创建的硬币的注册表。这最好保留在Coin类本身之外。您可以拥有一个管理硬币创建的类，并保留其创建的列表。如果您愿意，Coin类本身可以是一个接口，因为您可以确保除了CoinFactory之外不能创建它。

public interface Coin {
    String name();
    String abbreviation();
    BigDecimal value();
    String unit();
}

和硬币工厂类：

public class CoinFactory {

    // Concrete coin is an internal implementation class whose details don't
    // need to be known outside of the CoinFactory class.
    // Users just see it as interface Coin.
    private static class ConcreteCoin implements Coin {
        private final String name;
        private final String abbreviation;
        private final BigDecimal value;
        private final String unit;

        ConcreteCoin(String name, String abbreviation, BigDecimal value, String unit) {
            this.abbreviation = abbreviation;
            this.name = name;
            this.value = value;
            this.unit = unit;
        }

        public String name() { return name; }
        public String abbreviation() { return abbreviation; }
        public BigDecimal value() { return value; }
        public String unit() { return unit; }
    }

    // Sets for enforcing uniqueness of names and abbreviations
    private Set<String> names = new HashSet<>();
    private Set<String> abbreviations = new HashSet<>();

    // All coins must have one of the following ISO currency codes as the 'unit' field.
    private final Set<String> allIsoCurrencyCodes =
            Set.of("CLP", "GBP", "EUR", "CAD", "USD", "XXX" /* , ... */);

    private List<Coin> allCoins = new ArrayList<>(
            List.of(createCoin("Pesos chilenos", "CLP", BigDecimal.ONE, "CLP")));

    private List<Coin> unmodifiableListOfAllCoins =
            Collections.unmodifiableList(allCoins);


    public Coin createCoin(String name, String abbreviation, BigDecimal value, String unit) {
        if (!names.add(name))
            throw new IllegalArgumentException("Name already exists: " + name);
        if (!abbreviations.add(abbreviation))
            throw new IllegalArgumentException("Abbreviation already exists: " + abbreviation);
        if (!allIsoCurrencyCodes.contains(unit))
            throw new IllegalArgumentException("Coin unit is not a recognised ISO currency code: " + unit);

        Coin coin = new ConcreteCoin(name, abbreviation, value, unit);
        allCoins.add(coin);
        return coin;
    }

    public Collection<Coin> allCoins() {
        return unmodifiableListOfAllCoins;
    }
}

带有初始值的Class实例的静态arraylist

3 个答案: