java - 工厂方法重载

Question

我有一个子类列表，它们都是父类(super class)的实例。我的目标是创建一个工厂，它将根据子类返回不同的结果。

public class SuperClass {
    ...
}

public class SubClass1 extends SuperClass {
    ...
}

public class SubClass2 extends SuperClass {
    ...
}

public class Factory {
    public static getInstance(SubClass1 item) {
        return new EditText();
    }

    public static getInstance(SubClass2 item) {
        return new CheckBox();
    }
}

public class Generator {
    public Generator() {
        List<SuperClass> list = getList();

        for (SuperClass item : list) {
            Factory.getInstance(item);
        }
    }

    List<SuperClass> getList() {
        ...
    }
}

new Generator();

此代码在编译过程中会失败，因为它需要 getInstance(SuperClass item) 重载，但如果我添加它，它将始终被调用。

有没有办法在不触及 SuperClass、SubClass1、SubClass2 的情况下做到这一点？

更新。这里为了说明我要存档的是原代码:

import android.content.Context;
import android.view.View;
import android.widget.CheckBox;
import android.widget.EditText;
import android.widget.RadioButton;

import com.tom_roush.pdfbox.cos.COSArray;
import com.tom_roush.pdfbox.cos.COSDictionary;
import com.tom_roush.pdfbox.cos.COSName;
import com.tom_roush.pdfbox.pdmodel.PDDocument;
import com.tom_roush.pdfbox.pdmodel.PDDocumentCatalog;
import com.tom_roush.pdfbox.pdmodel.PDPage;
import com.tom_roush.pdfbox.pdmodel.PDPageTree;
import com.tom_roush.pdfbox.pdmodel.common.PDRectangle;
import com.tom_roush.pdfbox.pdmodel.interactive.annotation.PDAnnotation;
import com.tom_roush.pdfbox.pdmodel.interactive.form.PDAcroForm;
import com.tom_roush.pdfbox.pdmodel.interactive.form.PDCheckbox;
import com.tom_roush.pdfbox.pdmodel.interactive.form.PDField;
import com.tom_roush.pdfbox.pdmodel.interactive.form.PDRadioButton;
import com.tom_roush.pdfbox.pdmodel.interactive.form.PDTextField;


public class Page {
    private View view;
    private Context context;
    private PDDocument file;

    public Page(Context _context, View _view, PDDocument _document) {
        view = _view;
        context = _context;
        document = _document;

        renderFields();
    }

    private void renderFields() {
        PDDocumentCatalog docCatalog = document.getDocumentCatalog();
        RelativeLayout ll = view.findViewById(R.id.pageFields);
        ll.removeAllViews();

        PDPageTree pageTree = docCatalog.getPages();
        PDPage page = pageTree.get(pageIndex);

        PDAcroForm acroForm = docCatalog.getAcroForm();
        List<PDField> fields = acroForm.getFields();

        for (PDField field : fields) {
            String fieldName = field.getFullyQualifiedName();
            COSDictionary fieldDict = field.getCOSObject();

            COSArray fieldAreaArray = (COSArray) fieldDict.getDictionaryObject(COSName.RECT);
            PDRectangle mediaBox = new PDRectangle(fieldAreaArray);

            int fieldColor = Color.argb(180, 220, 228, 254);
            // Factory
            View fieldView = FieldFactory.getViewFromPDField(context, field);
            RelativeLayout.LayoutParams fieldLayoutParams = new RelativeLayout.LayoutParams(
                (int) (mediaBox.getWidth() * posRatio),
                (int) (mediaBox.getHeight() * posRatio)
            );
            fieldLayoutParams.addRule(RelativeLayout.ALIGN_PARENT_LEFT);
            fieldLayoutParams.leftMargin = (int) (left * posRatio);
            fieldLayoutParams.topMargin = (int) (top * posRatio);
            fieldView.setBackgroundColor(fieldColor);
            fieldView.setLayoutParams(fieldLayoutParams);

            ll.addView(fieldView, fieldLayoutParams);
        }
    }
}

...

public class FieldFactory {
    public static View getViewFromPDField(Context context, PDTextField field) {
        return new EditText(context);
    }

    public static View getViewFromPDField(Context context, PDCheckbox field) {
        return new CheckBox(context);
    }

    public static View getViewFromPDField(Context context, PDRadioButton field) {
        return new RadioButton(context);
    }
}

Answer 1

不幸的是，没有干净的方法来做到这一点，因为在编译期间我们不知道要调用哪个方法。为此，您可以使用 instanceof 关键字或使用反射。

假设您的模型大致如下所示:

class SuperClass {
    //...
}

class SubClass1 extends SuperClass {
    //...
}

class SubClass2 extends SuperClass {
    //...
}

class Context {
    //...
}

abstract class View {
    private final Context context;

    public View(Context context) {
        this.context = context;
    }

    @Override
    public String toString() {
        return getClass().getSimpleName();
    }
}

class TextBox extends View {

    public TextBox(Context context) {
        super(context);
    }
}

class CheckBox extends View {

    public CheckBox(Context context) {
        super(context);
    }
}

你可以像下面这样实现工厂:

class FieldFactory {
    static Map<Class<? extends SuperClass>, Class<? extends View>> fieldEditorMap = new HashMap<>();

    static {
        fieldEditorMap.put(SubClass1.class, TextBox.class);
        fieldEditorMap.put(SubClass2.class, CheckBox.class);
    }

    public static View getViewFromPDField(Context context, SuperClass field) {
        Class<? extends View> editorClass = fieldEditorMap.get(field.getClass());
        try {
            return editorClass.getConstructor(Context.class).newInstance(context);
        } catch (Exception e) {
            throw new IllegalArgumentException("Can not create view for " + field.getClass().getSimpleName(), e);
        }
    }
}

示例用法:

public class Main {

    public static void main(String[] args) throws Exception {
        List<SuperClass> fields = Arrays.asList(new SubClass1(), new SubClass2(), new SubClass1());
        for (SuperClass field : fields) {
            System.out.println(FieldFactory.getViewFromPDField(new Context(), field));
        }
    }
}

以上打印:

TextBox
CheckBox
TextBox

当然这只是一个示例，您应该使用一些反射库或实用程序更好地实现它。我们假设每个 View 实现都有给定的构造函数等。