但是，将ArrayList转换为HashMap，在ArrayList中选择具有不同变量类的choice对象

Question

我的问题很简单。如何将ArrayList的一部分（特别是“攻击”和“类型”变量）转换为HashMap？

import java.util.*;
import java.io.*;

public class Project6 {

    public static void main(String[] args) throws IOException {
        ArrayList<Pokemon> pokemonList = collectPokemon(args[0]);


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


    }

    // Don't modify this method. If you get errors here, don't forget to add the filename
    // as a command line argument.
    public static ArrayList<Pokemon> collectPokemon(String filename) throws IOException {
        BufferedReader file = new BufferedReader(new FileReader(new File(filename)));
        ArrayList<Pokemon> pokemonList = new ArrayList<Pokemon>();
        file.readLine();
        while(file.ready()) {
            String line = file.readLine();
            String[] split = line.split(",");
            String name = split[0];
            int attack = Integer.parseInt(split[1]);
            int defense = Integer.parseInt(split[2]);
            double height = Double.parseDouble(split[3]);
            double weight = Double.parseDouble(split[6]);
            String type = split[5];
            Pokemon current = new Pokemon(name, attack, defense, height, weight, type);
            pokemonList.add(current);
        }
        return pokemonList;
    }
}

口袋妖怪类

import java.util.*;

public class Pokemon {

    private String name;
    private int attack;
    private int defense;
    private double height;
    private double weight;
    private String type;

    public Pokemon(String inName, int inAttack, int inDefense, double inHeight, double inWeight, String inType) {
        name = inName;
        attack = inAttack;
        defense = inDefense;
        height = inHeight;
        weight = inWeight;
        type = inType;
    }

    public String getName() {
        return name;
    }

    public int getAttack() {
        return attack;
    }

    public int getDefense() {
        return defense;
    }

    public double getHeight() {
        return height;
    }

    public double getWeight() {
        return weight;
    }

    public String getType() {
        return type;
    }

    public String toString() {
        return "Pokemon: '" + name + "' Atk: " + attack + " Def: " + defense + " Ht: " + height + "m Wt: " + weight + "Kg Type: " + type;
    }
}

Answer 1

您可以尝试使用一个简单的for each循环：

// Note the Map type change to Double!
HashMap<String, Double> typeToAvgAttack = new HashMap<String, Double>();

// Intermediate map to store list of all attack values per type
HashMap<String, List<Integer>> typeToAttack = new HashMap<String, List<Integer>>();

for (Pokemon pokemon: pokemonList) {
    String type = pokemon.getType();
    int attack = pokemon.getAttack();

    // the map is empty at start, we need to check for the keys (pokemon type) existance
    List<Integer> attackValues = typeToAttack.get(type); 
    if (attackValues == null) {
        typeToAttack.put(type, attackValues = new ArrayList());
    }
    attackValues.add(attack);
}

// Iterate over map keys to calculate the average
for (String type : typeToAttack.keySet()) {
    List<Integer> attackValues = typeToAttack.get(type);
    double average = calculateAverage(attackValues);
    typeToAvgAttack.put(type, average);
}

helper函数，从这里复制：

public static double calculateAverage(List <Integer> values) {
    double sum = 0d;
    if(!values.isEmpty()) {
        for (Integer value: values) {
            sum += value;
        }
        return sum / values.size();
    }
    return sum;
}

但要注意，这种方法既不是最佳的，也不是优雅的。我更喜欢使用streamAPI，但这可能并不适合您当前的熟练程度。