Java 中带有 Document 和 BasicDBObject 的通用方法

Question

I'm learning generics and trying to write generic method in a project that I'm working on. I have a use case where I have a method which is recursive, which is exactly same but the parameters type are either Document or BasicDBObject. These parameters have nested list of objects so each of them again have objects of type Document or BasicDBObject inside them. So I have overloaded the methods with both the types. Can I use generics to change this method to accept any type?

public <T> String convertToRules(T rulesObj) {
    final StringBuilder expressionBuilder = new StringBuilder("${");
    if (rulesObj.getString("condition") != null) {
        String condition = rulesObj.getString("condition");
        if (rulesObj.get("rules") != null) {
           ArrayList<BasicDBObject> rules = (ArrayList<BasicDBObject>) rulesObj.get("rules");
           // rules can be ArrayList<BasicDBObject> or ArrayList<Document> so just mentioning them 
           // both here in question to avoid confusion of what type they are.
           ArrayList<Document> rules = (ArrayList<Document>) rulesObj.get("rules");
           rules.forEach(rule -> {
            // Some code
            expressionBuilder.append(convertToRules(rule));
       }
    }
 }

There are compiler errors in the above code as I was trying to convert the existing method to generic method. Below are the actual methods.

public String convertToRules(BasicDBObject rulesObj) {
    final StringBuilder expressionBuilder = new StringBuilder("${");
    if (rulesObj.getString("condition") != null) {
        String condition = rulesObj.getString("condition");
        if (rulesObj.get("rules") != null) {
           ArrayList<BasicDBObject> rules = (ArrayList<BasicDBObject>) rulesObj.get("rules");
           rules.forEach(rule -> {
            // Some code
            expressionBuilder.append(convertToRules(rule));
       }
    }
 }


public String convertToRules(Document rulesObj) {
    final StringBuilder expressionBuilder = new StringBuilder("${");
    if (rulesObj.getString("condition") != null) {
        String condition = rulesObj.getString("condition");
        if (rulesObj.get("rules") != null) {
           ArrayList<Document> rules = (ArrayList<Document>) rulesObj.get("rules");
           rules.forEach(rule -> {
            // Some code
            expressionBuilder.append(convertToRules(rule));
       }
    }
 }

I'm pretty sure this is not the right way to do it, need some guidance on how this can be achieved. I want to know if I'm thinking the right way about generics or this implementation is wrong.

Answer 1

You can make an abstract superclass or interface for both of the class. Let's say SuperClass with the common methods like getString() of the BasicDBObject and Document. Then, override the methods in the two class. Then for the generic method, you can make it like <? extends SuperClass. Then you can read the property of SuperClass easily. I think it should work fine.

Answer 2

RuleType - Contract of Two objects.

import java.util.List;

public interface RuleType<T> {
    String getString(String parameter);
    List<T> get(String param);
}

BasicDBObject

import java.util.ArrayList;
import java.util.List;

public class BasicDBObject implements RuleType<BasicDBObject> {

    @Override
    public String getString(String parameter) {
        return "Something";
    }

    @Override
    public List<BasicDBObject> get(String param) {
        return new ArrayList<>();
    }
}

Doument

import java.util.ArrayList;
import java.util.List;

public class Document implements RuleType<Document> {

    @Override
    public String getString(String parameter) {
        return "Something";
    }

    @Override
    public List<Document> get(String param) {
        return new ArrayList<>();
    }
}

Generic Method (Logic implementation)

public <T extends RuleType> String convertToRules(T rulesObj) {
        final StringBuilder expressionBuilder = new StringBuilder("${");
        if (rulesObj.getString("condition") != null) {
            String condition = rulesObj.getString("condition");
            if (rulesObj.get("rules") != null) {
                List<T> rules = rulesObj.get("rules");
                rules.forEach(rule ->
                    // Some code
                    expressionBuilder.append(convertToRules(rule))
                );
            }
        }
        return expressionBuilder.toString();
    }