如何处理嵌套结构？

Question

我的问题是将问题中的嵌套答案映射到我的Question.class。我尝试在我的Question类中使用Map，以便DataSnapshot.getValue将使用所要求的参数填充该类。我在嵌套结构之上得到了一切。但没有答案，例如A，B，C和D。

这是我的Firebase结构的示例，

我创建的用于映射参数的类。

public  class Question {

    private String ID;
    private String description;
    private String resource;
    private String source;


    private Map<Answer, String> A;
    private Map<Answer, String> B;
    private Map<Answer, String> C;
    private Map<Answer, String> D;

    public Map<Answer, String> getA() {
        return A;
    }
    ...

    public Question(){}

    public Question(String ID, String description, String resource, String source) {
        this.ID = ID;
        this.description = description;
        this.resource = resource;
        this.source = source;

    }

    public String getID() {
        return ID;
    }

    public void setID(String ID) {
        this.ID = ID;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getResource() {
        return resource;
    }

    public void setResource(String resource) {
        this.resource = resource;
    }

    public String getSource() {
        return source;
    }

    public void setSource(String source) {
        this.source = source;
    }
}