[英]Generifying with static factory methods and constants in Java
In my Android app, I have some very similar classes, let's call them FooA
and FooB
.在我的 Android 应用程序中,我有一些非常相似的类,我们称它们为FooA
和FooB
。
For each of these classes, I have a schema class that contains constants for the columns of the table - FooASchema
and FooBSchema
:对于这些类中的每一个,我都有一个架构类,其中包含表列的常量 - FooASchema
和FooBSchema
:
public final class FooASchema {
public static final String TABLE_NAME = "foo_a_table";
public static final String COL_CATEGORY_ID = "category_id";
public static final String COL_PROPERTY_A = "property_a";
public static final String COL_PROPERTY_B = "property_b";
// COL_PROPERTY_C = ...
}
public final class FooBSchema {
public static final String TABLE_NAME = "foo_b_table";
public static final String COL_CATEGORY_ID = "category_id";
public static final String COL_OTHER_PROPERTY_A = "other_property_a";
// COL_OTHER_PROPERTY_B = ...
}
Both FooA
and FooB
have a static factory method that enables me to create them using a Cursor
: FooA
和FooB
都有一个静态工厂方法,使我能够使用Cursor
创建它们:
public static FooA from(Cursor cursor) {
int categoryId = cursor.getInt(cursor.getColumnIndex(FooASchema.COL_CATEGORY_ID));
String propertyA = cursor.getString(cursor.getColumnIndex(FooASchema.COL_PROPERTY_A));
String propertyB = cursor.getString(cursor.getColumnIndex(FooASchema.COL_PROPERTY_B));
// int propertyC = ...
return FooA(id, propertyA, propertyB, ...);
}
public static FooB from(Cursor cursor) {
int categoryId = cursor.getInt(cursor.getColumnIndex(FooBSchema.COL_CATEGORY_ID));
int otherA = cursor.getInt(cursor.getColumnIndex(FooASchema.COL_OTHER_PROPERTY_A));
// String otherB = ...
return FooB(id, otherA, otherB, ...);
}
Finally, I have two util classes that I use to retrieve data from the tables:最后,我有两个 util 类可用于从表中检索数据:
public final class FooAUtils {
public static ArrayList<FooA> getFooAs(Context context, int categoryId) {
ArrayList<FooA> fooAs = new ArrayList<>();
Cursor cursor = MyDbHelper.getInstance(context).getReadableDatabase.query(
FooASchema.TABLE_NAME,
null,
FooASchema.COL_CATEGORY_ID + "=?",
new String[] {String.valueOf(categoryId)},
null,
null,
null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
fooAs.add(FooA.from(cursor));
cursor.moveToNext();
}
cursor.close();
return fooAs;
}
// ...
}
public final class FooBUtils {
public static ArrayList<FooA> getFooBs(Context context, int categoryId) {
ArrayList<FooB> fooBs = new ArrayList<>();
Cursor cursor = MyDbHelper.getInstance(context).getReadableDatabase.query(
FooBSchema.TABLE_NAME,
null,
FooBSchema.COL_CATEGORY_ID + "=?",
new String[] {String.valueOf(categoryId)},
null,
null,
null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
fooBs.add(FooB.from(cursor));
cursor.moveToNext();
}
cursor.close();
return fooBs;
}
// ...
}
You can see that most of the code between FooA
-related classes and FooB
-related classes are very similar, and especially in the util classes - where the code is almost identical.您可以看到FooA
相关类和FooB
相关类之间的大多数代码非常相似,尤其是在 util 类中 - 代码几乎相同。
I want to try to reduce this duplication, and I have been trying to do so using generics (I've read about them, but I haven't yet used them in a project).我想尝试减少这种重复,并且我一直在尝试使用泛型(我已经阅读过它们,但我还没有在项目中使用它们)。
For example, I want to be able to have a generic util class.例如,我希望能够拥有一个通用的 util 类。 Here's how I thought I could implement it:这是我认为我可以实现它的方式:
public final class FooUtils {
public static <T> get(Context context, int categoryId) {
ArrayList<T> items = new ArrayList<>();
Cursor cursor = MyDbHelper.getInstance(context).getReadableDatabase.query(
BaseSchema.TABLE_NAME,
null,
BaseSchema.COL_CATEGORY_ID + "=?",
new String[] {String.valueOf(categoryId)},
null,
null,
null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
items.add(T.from(cursor)); // ??
cursor.moveToNext();
}
cursor.close();
}
// ...
}
Where:在哪里:
public interface BaseSchema {
public static final String TABLE_NAME; // can't make this abstract?
public static final String COL_CATEGORY_ID = "category_id";
}
public final class FooASchema implements BaseSchema { ... }
public final class FooBSchema implements BaseSchema { ... }
But as you can see, I can't do T.from(cursor)
, and I can't have an abstract constant TABLE_NAME
that the subclasses can implement.但是正如你所看到的,我不能做T.from(cursor)
,我不能有一个子类可以实现的抽象常量TABLE_NAME
。
How can I call my static factory method in this way?如何以这种方式调用我的静态工厂方法?
Is there a better way of approaching this and reducing code duplication?有没有更好的方法来解决这个问题并减少代码重复?
In your actual code you don't use an instance of the class to invoke the form()
factory, you use a static method of the class :在您的实际代码中,您不使用类的实例来调用form()
工厂,而是使用类的静态方法:
fooAs.add(FooA.from(cursor));
With generics, you cannot use the parameterized type to invoke a method on it like that items.add(T.from(cursor));
使用泛型,你不能使用参数化类型来调用它的方法,比如items.add(T.from(cursor));
since the generic was erased after the compilation.因为泛型在编译后被删除了。
In your case, I see two ways of handling the problem :在您的情况下,我看到两种处理问题的方法:
introducing a abstract base class with the common method and an abstract method that subclasses have to implement to create a Foo
instance ( FooA
, FooB
).引入具有公共方法的抽象基类和子类必须实现以创建Foo
实例( FooA
, FooB
)的抽象方法。
keeping your way of doing and introducing an interface to create a Foo
instance.保持你的方式并引入一个接口来创建一个Foo
实例。 You would have two implementation of it.你会有两个实现它。 One for FooA
and another one for FooB
and you could provide a instance of it in the FooUtils.get()
method.一个用于FooA
,另一个用于FooB
,您可以在FooUtils.get()
方法中提供它的一个实例。
With the first option you could do the following.使用第一个选项,您可以执行以下操作。
Base class基类
public abstract class AbstractFooProcessing<T extends Foo> {
public abstract T createFooInstance(Cursor cursor);
public ArrayList<T> get(Context context, int categoryId) {
ArrayList<T> items = new ArrayList<>();
Cursor cursor = MyDbHelper.getInstance(context).getReadableDatabase.query(
BaseSchema.TABLE_NAME,
null,
BaseSchema.COL_CATEGORY_ID + "=?",
new String[] {String.valueOf(categoryId)},
null,
null,
null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
items.add(createFooInstance(cursor));
cursor.moveToNext();
}
cursor.close();
}
// ...
}
FooAProcessing FooA处理
public class FooAProcessing extends AbstractFooProcessing<FooA>{
@Override
public FooA createFooInstance(Cursor cursor) {
return FooA.from(cursor);
}
}
FooBProcessing FooB处理
public class FooBProcessing extends AbstractFooProcessing<FooB>{
@Override
public FooB createFooInstance(Cursor cursor) {
return FooB.from(cursor);
}
}
With the second option you could do the following.使用第二个选项,您可以执行以下操作。
FooProcessing interface FooProcessing 接口
public interface FooProcessing<T extends Foo> {
T createFooInstance(Cursor cursor);
}
FooProcessingA FooProcessingA
public class FooAProcessing implements FooProcessing<FooA>{
@Override
public FooA createFooInstance(Cursor cursor) {
return FooA.from(cursor);
}
}
FooProcessingB FooProcessingB
public class FooBProcessing implements FooProcessing<FooB>{
@Override
public FooB createFooInstance(Cursor cursor) {
return FooB.from(cursor);
}
}
FooUtils updated so that get()
takes as argument a FooProcessing
factory instance. FooUtils已更新,以便get()
将FooProcessing
工厂实例作为参数。
public final class FooUtils {
public static <T extends Foo> ArrayList<T> get(Context context, int categoryId, FooProcessing<T> fooProcessing) {
ArrayList<T> items = new ArrayList<>();
Cursor cursor = MyDbHelper.getInstance(context).getReadableDatabase.query(
BaseSchema.TABLE_NAME,
null,
BaseSchema.COL_CATEGORY_ID + "=?",
new String[] {String.valueOf(categoryId)},
null,
null,
null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
items.add(fooProcessing.createFooInstance(cursor)); // ??
cursor.moveToNext();
}
cursor.close();
}
// ...
return items;
}
You can now call the FooUtils.get() method in this way :您现在可以通过以下方式调用 FooUtils.get() 方法:
...
FooProcessing fooAProcessing = new FooAProcessing();
...
ArrayList<FooA> fooAs = FooAUtils.getFoo(context, category, fooAProcessing);
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