[英]How to genericize a Java enum with static members?
I am refactoring a part of our legacy app which handles exporting and importing of DB tables from/to Excel sheets. 我正在重构遗留应用程序的一部分,该应用程序处理从/到Excel工作表的数据库表的导出和导入。 We have a
Formatter
subclass for each table, to provide the definition of that table: how many columns it has, and what is the name, format and validator of each column. 我们为每个表都有一个
Formatter
子类,以提供该表的定义:它具有多少列,以及每列的名称,格式和验证器是什么。 The getters which supply this data are then called by a Template Method which exports/imports the table. 然后,通过导出/导入表的模板方法调用提供此数据的吸气剂。 I have extracted the column data into an enum, which greatly simplified the code.
我已经将列数据提取到一个枚举中,这大大简化了代码。 A formatter now looks like this (some details omitted for brevity):
格式化程序现在看起来像这样(为简洁起见,省略了一些详细信息):
public class DamageChargeFormatter extends BaseFormatter {
public static final int NUM_COLUMNS = 7;
public enum Column {
VEHICLE_GROUP(0, "Vehicle Group", /* more params */),
NAME_OF_PART(1, "Name of Part", /* more params */),
//...
LOSS_OF_USE(6, "Loss of Use", /* more params */);
private static final Map<Integer, Column> intToColumn = new HashMap<Integer, Column>();
static {
for (Column type : values()) {
intToColumn.put(type.getIndex(), type);
}
}
public static TableColumn valueOf(int index) {
return intToColumn.get(index);
}
private int index;
private String name;
Column(int index, String name, /* more params */) {
this.index = index;
this.name = name;
//...
}
public int getIndex() { return index; }
public String getName() { return name; }
// more members and getters...
}
protected String getSheetName() {
return "Damage Charges";
}
public String getColumnName(int columnNumber) {
TableColumn column = Column.valueOf(columnNumber);
if (column != null) {
return column.getName();
}
return null;
}
// more getters...
protected int getNumColumns() {
return NUM_COLUMNS;
}
protected boolean isVariableColumnCount() {
return false;
}
}
Now, I have about a dozen such classes, each of which containing exactly the same code except that NUM_COLUMNS
and the enum values of Column
are different. 现在,我大约有十二个这样的类,除了
NUM_COLUMNS
和Column
的枚举值不同之外,每个类都包含完全相同的代码。 Is there any way to genericize this somehow? 有什么办法可以某种方式将其泛化吗? The main obstacle to this is the static
Column.valueOf()
method and the static constant NUM_COLUMNS
. 这样做的主要障碍是静态
Column.valueOf()
方法和静态常量NUM_COLUMNS
。 Another concern with latter is that it really belongs to an abstraction one level higher, ie to the table, not to an individual column - it would be nice to somehow incorporate this into the generic solution. 后者的另一个担心是,它确实属于更高一级的抽象,即属于表,而不是单个列-最好以某种方式将其合并到通用解决方案中。
Technically I could solve this with a base interface ( TableColumn
below) and reflection, but I don't really like that, as apart from trading compile time errors to runtime errors, it makes the code ugly (to me): 从技术上讲,我可以使用基本接口(下面的
TableColumn
)和反射来解决此问题,但是我不是很喜欢,除了将编译时错误换成运行时错误外,这对我来说代码也很丑陋:
public class GenericFormatter<E extends TableColumn> extends BaseFormatter {
private Method valueOfMethod;
public GenericFormatter(Class<E> columnClass) {
try {
valueOfMethod = columnClass.getDeclaredMethod("valueOf", Integer.class);
} catch (NoSuchMethodException e) {
throw new RuntimeException(e);
}
}
public String getColumnName(int columnNumber) {
try {
@SuppressWarnings("unchecked")
E elem = (E) valueOfMethod.invoke(columnNumber);
if (elem != null) {
return elem.getName();
}
} catch (Exception e) {
throw new RuntimeException(e);
}
return null;
}
//...
}
Note that this code is purely experimental, as yet untested... 请注意,此代码纯粹是实验性的,尚未经过测试...
Is there a nicer, cleaner, safer way? 有没有更好,更清洁,更安全的方法?
May be, something like this: 可能是这样的:
public class TableMetadata<E extends Enum & TableColumn> {
private Map<Integer, TableColumn> columns = new HashMap<Integer, TableColumn>();
public TableMetadata(Class<E> c) {
for (E e: c.getEnumConstants()) {
columns.put(e.getIndex(), e);
}
}
public String getColumnName(int index) {
return columns.get(index).getName();
}
}
public class GenericFormatter<E extends TableColumn> extends BaseFormatter {
private TableMetadata<E> m;
public GenericFormatter(TableMetadata<E> m) {
this.m = m;
}
public String getColumnName(int columnNumber) {
return m.getColumnName(index);
}
//...
}
EDIT: Enum
added to the type parameter for more compile-time safety 编辑:将
Enum
添加到type参数以获得更多的编译时安全性
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