java - Enum 类中的 Enum 变量

Question

请帮助找到以下情况的最佳方法:

有一个表，大约有 20 列。
每列都有自己的短名称、全名称和类型(数字或字符串)。
每个列类型都可以有自己的运算符 - 例如，字符串 - 包含、等于；数字 - 更多、更少、==、!=
每个操作符都可以有自己的描述。

我必须拥有 Table 类的对象，并且能够查看其所有列，查看每列的短名称和全名，根据列类型使用运算符。

我正在尝试使用枚举，但我不知道如何将特定列连接到特定类型。
例如，如何将“Id”列连接到“StringType”，将“Services”列连接到“NumberType”。请您帮忙。

class Table{

    public enum Column {
        Id("id", "ID number"),
        Services("serv", "Services");

        private final String shortName;
        private final String fullName;

        Column(String shortName, String fullName) {
           this.shortName = shortName;
           this.fullName = fullName;
        }

        public String getShortName() {
            return shortName;
        }

        public String getFullName() {
            return fullName;
        }
    }


    public enum StringType{
        contains("String contain another string"),
        equals("String equal a string");

        private final String placeholder;

        StringType(String fullName) {
            this.placeholder = fullName;
        }

        public String getPlaceholder() {
            return placeholder;
        }
    }

    public enum NumberType{
        more("value that is more than input"),
        less("value that is less than input");

        private final String placeholder;

        NumberType(String fullName) {
            this.placeholder = fullName;
        }

        public String getPlaceholder() {
            return placeholder;
        }
    }

}

Answer 1

与任何其他类一样，枚举类型可以实现接口(interface)。您可以利用这一点来发挥自己的优势:

public interface DataType {
    // Deliberately empty.  This is a marker interface.
}

public enum StringType
implements DataType {
    // ...
}

public enum NumberType
implements DataType {
    // ...
}

public enum Column {
    Id("id", "ID number", StringType.class),
    Services("serv", "Services", NumberType.class);

    private final String shortName;
    private final String fullName;
    private final Class<? extends DataType> type;

    Column(String shortName, String fullName, Class<? extends DataType> type) {
       this.shortName = shortName;
       this.fullName = fullName;
       this.type = type;
    }

    // ...
}

如果您打算实际使用它们来比较数据，您可以向 DataType 接口(interface)添加方法:

public interface DataType<T> {
    Class<T> getDataClass();

    BiPredicate<? super T, ? super T> getTest();

    default boolean test(T value1, T value2) {
        return getTest().test(value1, value2);
    }

    default boolean testObjects(Object value1, Object value2) {
        Class<T> type = getDataClass();
        return test(type.cast(value1), type.cast(value2));
    }
}

public enum StringType
implements DataType<String> {
    contains("String contain another string", String::contains),
    equals("String equal a string", Object::equals);

    private final String placeholder;
    private final BiPredicate<? super String, ? super String> test;

    StringType(String fullName,
               BiPredicate<? super String, ? super String> test) {
        this.placeholder = fullName;
        this.test = test;
    }

    public String getPlaceholder() {
        return placeholder;
    }

    @Override
    public BiPredicate<? super String, ? super String> getTest() {
        return test;
    }

    @Override
    public Class<String> getDataClass() {
        return String.class;
    }
}

public enum NumberType
implements DataType<Number> {
    more("value that is more than input",
        (n1, n2) -> n1.doubleValue() > n2.doubleValue()),
    less("value that is less than input",
        (n1, n2) -> n1.doubleValue() < n2.doubleValue());

    private final String placeholder;
    private final BiPredicate<? super Number, ? super Number> test;

    NumberType(String fullName,
               BiPredicate<? super Number, ? super Number> test) {
        this.placeholder = fullName;
        this.test = test;
    }

    public String getPlaceholder() {
        return placeholder;
    }

    @Override
    public BiPredicate<? super Number, ? super Number> getTest() {
        return test;
    }

    @Override
    public Class<Number> getDataClass() {
        return Number.class;
    }
}

public enum Column {
    Id("id", "ID number", StringType.class),
    Services("serv", "Services", NumberType.class);

    private final String shortName;
    private final String fullName;
    private final Class<? extends DataType<?>> type;

    Column(String shortName, String fullName, Class<? extends DataType<?>> type) {
       this.shortName = shortName;
       this.fullName = fullName;
       this.type = type;
    }

    // ...
}