Question

我试图了解ArrayList.indexOf（）为什么返回-1吗？

我创建了一个简单的类，在其中实现了自定义等于，以便可以将类的实例与字符串进行比较。

public class Currency {
    final private String code;
    final private String unicodeHex;
    final private String name;

    public Currency(String code, String unicodeHex, String name) {
        this.code = code;
        this.unicodeHex = unicodeHex;
        this.name = name;
    }

    public String getCode() {
        return code;
    }

    public String getSymbol() {
        return unicodeHex;
    }

    public String getName() {
        return name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null) return false;
        if (o instanceof Currency) {
            Currency currency = (Currency) o;
            return code.equals(currency.code);
        } else if (o instanceof String) {
            String currency = (String) o;
            return code.equals(currency);
        } else return false;
    }

    @Override
    public int hashCode() {
        return Objects.hash(code);
    }

    @Override
    public String toString() {
        return "Currency{" +
                "code='" + code + '\'' +
                ", unicodeHex='" + unicodeHex + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

现在我需要在ArrayList中找到该类的实例。

List<Currency>  currencies = new ArrayList<>(Arrays.asList(
                new Currency("USD","0024","United States Dollar"),
                new Currency("EUR","20ac","Euro Member Countries")));

Log.d(TAG, currencies.toString());
Log.d(TAG,Integer.toString(currencies.indexOf("USD")));

2019-02-13 15:41:50.082 D/CurrencySelector: [Currency{code='USD', unicodeHex='0024', name='United States Dollar'}, Currency{code='EUR', unicodeHex='20ac', name='Euro Member Countries'}]
2019-02-13 15:41:50.082 D/CurrencySelector: -1

我得到： -1

但是：

Currency currency = new Currency("USD","0024","United States Dollar");

Log.d(TAG,Boolean.toString(currency.equals("USD")));

2019-02-13 15:41:50.082 D/CurrencySelector: true

等于应该返回 true 。

ArrayList.indexOf 应使用等于：

public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

Answer 1

尽管currency.equals("USD")可能会返回true（由于您的实现），但是"USD".equals(currency)绝不会返回true，因为它使用了{的equals实现{1}}类，要求所比较的实例的类型为String。

因此，String将返回currencies.indexOf("USD")，因为-1调用o.equals(elementData[i])的{{1}}，而String不是{{1 }}。

具有类的自定义equals（）的List.indexOf（）返回-1

1 个答案: