我有3个班级宠物，猫和狗课程。猫和狗类是Pet类的孩子，我已经超越了toString方法。我在Dog类中重写的toString方法打印了Dog类的名称字段和权重字段。我创建了一个名为mainList的ArrayList和我需要一个计算ArrayList中权重的最小值，最大值和平均值的方法。 你能告诉我如何创建一个计算最小值的方法， ArrayList中狗对象权重的最大值和平均值？
我的班级低于......
宠物等级：
public class Pet
{
private String name;
public String getName()
{
return name;
}
protected void setName(String newName)
{
name = newName;
}
public Pet(String petName) {
name = petName;
}
}
狗类：
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课程：
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
答案 0 :(得分：1)
在Java 8中，可以使用Stream s：
mainList.stream()
.filter (animal -> animal instanceof Dog)
.mapToDouble(animal -> ((Dog)animal).getWeight())
.min()
//.max()
//.average()
.get()
你可能会得到与Java 7相同的结果：
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();
答案 1 :(得分：0)
我会这样做（底部更短）：
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);
}
或者这样没有排序：
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);
}
也可以让你的宠物课：
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;
}
}
超类中没有重量。