如何從通用ArrayList查找最小值，最大值和平均值

Question

我有3類寵物，貓和狗課程。 Cat和Dog類是Pet類的子類，我已經重寫了toString方法。 我在Dog類中重寫的toString方法將打印Dog類的名稱字段和權重字段。 我已經創建了一個名為mainList的ArrayList，並且需要一個方法來計算ArrayList中權重的最小，最大和平均值。 您能告訴我如何創建一種方法來計算ArrayList中Dog對象權重的最小，最大和平均值嗎？

我的課程低於...

寵物課：

public class Pet
{
    private String name;



    public String getName()
    {
        return name;
    }

    protected void setName(String newName)
    {
        name = newName;
    }

    public Pet(String petName) {
        name = petName;
    }
} 

狗類：

public class Dog extends Pet
{

    private Double weight;

    public double getWeight()
    {
        return weight;
    }

    protected void setWeight(double newWeight)
    {
        weight = newWeight;
    }

    public String toString() {
        return String.format("%s , %f", getName(), getWeight());
    }

    public Dog(String petName, double dogWeight)
    {
        super(petName);
        weight = dogWeight;

    }
}

PetTest類別：

import java.util.*;

public class PetTest
{
    static ArrayList<Pet> mainList = new ArrayList<Pet>();
    static Iterator<Pet> mainIter = mainList.iterator();
    static Scanner keyboard = new Scanner(System.in);

    public static void eighthCase()
    {
      // I need a method here
    }

    public static void listThemAll()
    {
        for(Pet pet: mainList)
        {
            System.out.println(pet.toString());
        }
    }

    public static void listDogs()
    {
            for(Pet pet: mainList)
            {
                if (pet instanceof Dog)
                {
                    System.out.println(pet.toString());
                }
            }//end of for loop
    }//end of listDogs method

    public static void listCats()
    {
            for(Pet pet: mainList)
            {
                if(pet instanceof Cat)
                {
                    System.out.println(pet.toString());
                }
            }//end of for loop

    }//end of listCats method

    public static void addDog(String dogName,Double dogWeight)
    {
        Pet dog = new Dog(dogName, dogWeight);
        mainList.add(dog);
    }//end of addDog method

    public static void addCat(String catName, String furColor)
    {
        Pet cat = new Cat(catName, furColor);
        mainList.add(cat);
    }//end of addCat method

    public static void removeDog(String dogName)
    {
        for(Pet pet : mainList)
            {
                if((pet instanceof Dog) && (pet.getName().equals(dogName)))
                {
                    mainList.remove(pet);
                }//end of if statement
            }//end of for loop

    }//end of removeDog method


    public static void removeCat(String catName)
    {
            for(Pet pet : mainList)
            {
                if((pet instanceof Cat) && (pet.getName().equals(catName)))
                {
                    mainList.remove(pet);

                }//end of if statement

            }//end of for loop

    }//end of removeCat method

    public static void showMenu()
    {
        System.out.println("1. Add dog ");
        System.out.println("2. Add cat");
        System.out.println("3. Remove dog");
        System.out.println("4. Remove cat");
        System.out.println("5. List dogs");
        System.out.println("6. List cats");
        System.out.println("7. List all pets");
        System.out.println("8. Show min, max and average weight of dogs");
        System.out.println("0. Quit");
        int action = keyboard.nextInt();

        Scanner parameter1 = new Scanner(System.in);
        Scanner parameter2 = new Scanner(System.in);

        while(action > 0 && action <= 8)
        {
        switch(action)
        {
        case 1: 
            System.out.println("Type in the name of the dog that you want to add.");
            String dogName = parameter1.next();
            System.out.println("Type in the weight of the dog that you want to add.");
            Double dogWeight = parameter2.nextDouble();
            addDog(dogName,dogWeight);
            showMenu();

            break;

        case 2:
            System.out.println("Type in the name of the cat that you want to add.");
            String catName = parameter1.next();
            System.out.println("Type in the color of the cat that you want to add.");
            String furColor = parameter2.next();
            addCat(catName,furColor);
            showMenu();

            break;

        case 3:
            System.out.println("Type in the name of dog that you want to remove.");
            String dogToRemove = parameter1.next();
            removeDog(dogToRemove);
            showMenu();

            break;

        case 4:
            System.out.println("Type in the name of dog that you want to remove.");
            String catToRemove = parameter1.next();
            removeDog(catToRemove);
            showMenu();

            break;

        case 5:
            listDogs();
            showMenu();

            break;

        case 6:
            listCats();
            showMenu();
            break;

        case 7:
            listThemAll();
            showMenu();
            break;

        case 8:
            eighthCase();
            break;


        }//end of switch statement

        }//end of while loop
    }//end of showMenu method

    public static void main(String[] args)
    {

        showMenu();



        }//end of main method



}//end of the class

Answer 1

在Java 8中，這可以通過Stream來完成：

mainList.stream()
    .filter (animal -> animal instanceof Dog)
    .mapToDouble(animal -> ((Dog)animal).getWeight())
    .min()
    //.max()
    //.average()
    .get()

使用Java 7可能會得到相同的結果：

List<Dog> dogs = new ArrayList<>();
for (Animal animal : mainList)
    if (animal instanceof Dog)
        dogs.add((Dog)animal);

double min = dogs.get(0).getWeight();
double max = dogs.get(0).getWeight();
double sum = 0;
for (Dog dog : dogs) {
    min = Math.min(min, dog.getWeight());
    max = Math.max(max, dog.getWeight());
    sum += dog.getWeight();
}
System.out.format("min = %i, max = %i, average = %f%n", min, max, sum * 1.0 / dogs.size());
// or by implementing comparator
Comparator <Dog> dogComparator = new Comparator<>() {
    public int compare (Dog dog1, Dog dog2) {
        return Double.compare(dog1.getWeight(), dog2.getWeight());
    }
};
double min = Collections.min(dogs, dogComparator).getWeight();
double max = Collections.max(dogs, dogComparator).getWeight();

Answer 2

我會這樣做（下一個較短）：

public static void calculateDogValues(){
    List<Double> dogWeightList = new ArrayList<>();
    double max = 0, min = 0, average = 0;
       for(Pet pet: mainList){
          if (pet instanceof Dog){    //add all dog weights to list
              dogWeightList.add(pet.getWeight());
          }
       }

       Collections.sort(dogWeightList);   //sort the list
       min = dogWeightList.get(0);   //get smallest value
       max = dogWeightList.get(dogWeightList.size()-1);   //get highest value

       for (double weight : dogWeightList) {   //make sum of all weights
          average += weight;
       }
       average = average/dogWeightList.size();   //sum/number of weights

       System.out.println("Minimum weight is " + min);
       System.out.println("Maximum weight is " + max);
       System.out.println("Average weight is " + average);

    }

或者像這樣沒有排序：

    double max = 0, min = 0, average = 0, amountOfDogs = 0;
       for(Pet dog: mainList){
          if (dog instanceof Dog){   
              min = Math.min(min, dog.getWeight()); 
              max = Math.max(max, dog.getWeight());
              average += dog.getWeight();
              amountOfDogs++;
          }
       }
       average = average/amountOfDogs;

       System.out.println("Minimum weight is " + min);
       System.out.println("Maximum weight is " + max);
       System.out.println("Average weight is " + average);

    }

也讓您的寵物班級：

public class Pet{

    private String name;
    private double weight;

    public String getName()
    {
        return name;
    }

    protected void setName(String newName)
    {
        name = newName;
    }

    public Pet(String petName) {
        name = petName;
    }

    public void setWeight(double newWeight){
    weight = newWeight;
    }

    public double getWeight(){
    return weight;
    }
} 

超類沒有權重。