Java多对多关联映射 [英] Java many to many association map

问题描述

我有两个类，ClassA 和 ClassB，以及一个多对多"AssociationClass.我想要一个结构来保存 A 和 B 之间的关联，以便我可以找到 A 或 B 的每个实例的对应物.

I have two classes, ClassA and ClassB, as well as a "many to many" AssociationClass. I want a structure that holds the associations between A and B so that I can find the counterpart for each instance of A or B.

我想过使用带有成对键的 Hashmap:

I thought of using a Hashmap, with pair keys:

Hasmap<Pair<ClassA, ClassB>, AssociationClass> associations;

通过这种方式，我可以添加和删除 ClassA 和 ClassB 的两个实例之间的关联，并且我可以查询两个给定实例的关系.

This way, I can add and remove an association between two instances of ClassA and ClassB, and I can query a relation for two given instances.

然而，我错过了为ClassA 或ClassB 的给定实例定义所有关联的功能.

However, I miss the feature of getting all associations defined for a given instance of ClassA or ClassB.

我可以通过蛮力完成并遍历地图的所有键以搜索给定实例之间的关联，但这效率低下且不优雅.

I could do it by brute force and loop over all keys of the map to search for associations between a given instance, but this is inefficient and not elegant.

你知道有什么数据结构/免费库可以实现这一点吗?我不想重新发明轮子.

Do you know of any data structure / free library that enables this? I don't want to reinvent the wheel.

注意:这不是数据库"问题.这些对象是用于实时计算的纯 POJO，我不需要持久性的东西.

NB: This is not a "database" question. These objects are pure POJO used for live computation, I don't need persistence stuff.

推荐答案

感谢您的建议.

我终于重新发明了轮子......我已经编写了一个用于持有协会的通用类.我用了两张地图，同步的.

I finally reinvented the wheel ... I have written a generic class for holding associations. I use two maps of maps, synchronized.

协会持有者提供以下方法

The associations holder provides the following methods

void setAssociation(LeftClass left, RightClass right, AssociationClass assoc);
AssociationClass getAssociation(LeftClass left, RightClass right);
Map<RightClass, AssociationClass> getAssocationsLeft(LeftClass left);
Map<LeftClass, AssociationClass> getAssocationsRight(RightClass right); 
void removeAssociation(LeftClass left, RightClass right);

代码如下:

import java.util.HashMap;

/** This class holds many to many associations between two classes. */
public class AssociationHolder<LeftClass, RightClass, AssociationClass> {

    // -------------------------------------------------------
    // Attributes
    // -------------------------------------------------------

    private HashMap<LeftClass, HashMap<RightClass, AssociationClass>> associationsLeft = 
        new HashMap<LeftClass, HashMap<RightClass,AssociationClass>>();
    private HashMap<RightClass, HashMap<LeftClass, AssociationClass>> associationsRight = 
        new HashMap<RightClass, HashMap<LeftClass,AssociationClass>>();     

    // -------------------------------------------------------
    // Methods
    // -------------------------------------------------------

    /** 
     *  Set an association between two instance.
     *  Any prior association is overwritten.
     */
    public void setAssociation(LeftClass left, RightClass right, AssociationClass association) {

        // Get the map for the left 
        HashMap<RightClass, AssociationClass> leftMap = this.associationsLeft.get(left);

        // No association defined yet for this left key ? => Create new map
        if (leftMap == null) {
            leftMap = new HashMap<RightClass, AssociationClass>();
            this.associationsLeft.put(left, leftMap);
        }

        // Get the map for the right 
        HashMap<LeftClass, AssociationClass> rightMap = this.associationsRight.get(right);

        // No association defined yet for this right key ? => Create new map
        if (rightMap == null) {
            rightMap = new HashMap<LeftClass, AssociationClass>();
            this.associationsRight.put(right, rightMap);
        }

        // Set the assoication on both maps
        leftMap.put(right, association);
        rightMap.put(left, association);        

    } 

    /** @return null if no association found. */
    public AssociationClass getAssociation(LeftClass left, RightClass right) {

        // Use left maps (could have used the right one as well)
        HashMap<RightClass, AssociationClass> leftMap = this.associationsLeft.get(left);
        if (leftMap == null) return null;
        return leftMap.get(right);
    }

    /** Get all associations defined for a given Left instance.  */
    public HashMap<RightClass, AssociationClass> getAssociationsLeft(LeftClass left) {

        HashMap<RightClass, AssociationClass> leftMap = this.associationsLeft.get(left);

        // No map defined ? return empty one instead of null
        if (leftMap == null) {
            return new HashMap<RightClass, AssociationClass>();
        } else {
            return leftMap;
        }   
    }

    /** Get all associations defined for a given Right instance.  */
    public HashMap<LeftClass, AssociationClass> getAssociationsRight(RightClass right) {

        HashMap<LeftClass, AssociationClass> rightMap = this.associationsRight.get(right);

        // No map defined ? return empty one instead of null
        if (rightMap == null) {
            return new HashMap<LeftClass, AssociationClass>();
        } else {
            return rightMap;
        }   
    }

    /** 
     *  Remove an association between two instances.
     */
    public void removeAssociation(LeftClass left, RightClass right) {
        HashMap<RightClass, AssociationClass> leftMap = this.getAssociationsLeft(left);
        HashMap<LeftClass, AssociationClass> rightMap = this.getAssociationsRight(right);
        leftMap.remove(right);      
        rightMap.remove(left);  
    }
}

我希望这可以在未来帮助某人.

I hope this can help someone in the future.

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