Java - 初始化 HashMap 的 HashMap [英] Java - Initialize a HashMap of HashMaps

问题描述

我是 Java 新手，我通过创建一个简单的 NaiveBayes 分类器来练习.我还是对象实例化的新手，想知道如何初始化 HashMap 的 HashMap.在向分类器中插入新的观察值时，我可以为给定类中未见过的特征名称创建一个新的 HashMap，但我需要初始化吗?

I am new to java and practicing by creating a simplistic NaiveBayes classifier. I am still new to object instantiation, and wonder what to do to initialize a HashMap of HashMaps. When inserting new observations into the classifier, I can create a new HashMap for an unseen feature name in a given class, but do I need to initialize?

import java.util.HashMap;

public class NaiveBayes {

    private HashMap<String, Integer> class_counts;
    private HashMap<String, HashMap<String, Integer>> class_feature_counts;

    public NaiveBayes() {
        class_counts = new HashMap<String, Integer>();
        // do I need to initialize class_feature_counts?
    }

    public void insert() {
        // todo
        // I think I can create new hashmaps on the fly here for class_feature_counts
    }

    public String classify() {
        // stub 
        return "";
    }

    // Naive Scoring:
    // p( c | f_1, ... f_n) =~ p(c) * p(f_1|c) ... * p(f_n|c)
    private double get_score(String category, HashMap features) {
       // stub
       return 0.0;
    }

    public static void main(String[] args) {
        NaiveBayes bayes = new NaiveBayes();
       // todo
     }
}

请注意，此问题并非特定于朴素贝叶斯分类器，只是想我会提供一些上下文.

Note this question is not specific to Naive Bayes classifiers, just thought I would provide some context.

推荐答案

是的，你需要初始化它.

Yes, you need to initialize it.

class_feature_counts = new HashMap<String, HashMap<String, Integer>>();

当你想给 class_feature_counts 添加一个值时，你也需要实例化它:

When you want to add a value to class_feature_counts, you need to instantiate it too:

HashMap<String, Integer> val = new HashMap<String, Integer>();
// Do what you want to do with val
class_feature_counts.put("myKey", val);

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