java - 尝试纠正简单的数组程序

Question

在开始新的编程类(class)之前，我正在尝试进行一些 Java 练习。为此，我决定重新制作我在早期类(class)中制作的旧 Perl 程序。原始 Perl 文件将用户输入的数字输入到一个数组中，并以四种方式输出该数组:输入的数字、升序、降序以及仅最大和最小的数字。

我查看了这里和其他地方的在线示例来排除故障，但在程序编译时，输出是错误的。照原样，该数组被输出几十次，仅输出输入的数字。我认为我的四个循环设置错误，但我仍在学习 Java，所以我可能错过了一些东西。这是现在的 Java 代码:

import java.util.ArrayList;
import java.util.Scanner;

public class ArrayOutPut {

    static Scanner input = new Scanner(System.in);

    static String convertToString(ArrayList<Integer> numbers) {

        StringBuilder builder = new StringBuilder();
        for (int i : numbers) {
            builder.append(numbers);
            builder.append(",");
        }
        builder.setLength(builder.length() - 1);
        return builder.toString();

    }

    public static void main(String args[]) {

        ArrayList<Integer> numbers = new ArrayList<Integer>();

        int userInput;
        int Largest = 0, Smallest = 0;

        System.out.print(
                "This program takes a set of integers from the user, and then outputs the results to the screen in four ways:\n");
        System.out.print(
                "the order they were entered seperated by a comma and a space, in ascending order, in descending order, and as Largest:Smallest.\n\n");
        // explain to the user what the program does, needs, etc.
        System.out.print("Please enter a number or -1 to end: ");
        userInput = input.nextInt();
        while (userInput != -1) {
            System.out.print("Please enter a number or -1 to end: ");
            numbers.add(userInput);
            userInput = input.nextInt();
        }

        // Prints the array contents as entered
        String sort = convertToString(numbers);
        System.out.println(sort);
        // prints numbers ascending
        for (int i = 0; i < numbers.size(); i++) {
            System.out.println(sort);
        }
        // prints numbers descending, largest first
        for (int i = numbers.size() - 1; i >= 0; i--) {
            System.out.println(sort);
        }
        for (int i = 0; i < numbers.size(); i++) {
            int number = numbers.get(i);
            if (i < Smallest) {
                Smallest = i;
            }
            if (i > Largest) {
                Largest = i;
            }
            System.out.println(Largest + Smallest);
        }
    }
}

作为其他引用，这里是原始 Perl 程序:

#!/usr/bin/perl    
use Modern::Perl;
my (@numbers, $userInput);# declare the array and more than one variable at a time
print("\nThis program takes a set of integers from the user, and 
\nthen outputs the results to the screen in four ways:
\nthe ordered they were entered seperated by a comma and a space, 
\nin ascending order, in descending order, and as Largest:Smallest.\n\n"); # explain to the user what is does, needs, etc
    print "Please enter a number or -1 to end: ";
    chomp ($userInput = <>); #loop is primed
    if ($userInput == -1) {
    say "\nThere's no numbers to process.\n";
}
else{       
while($userInput != -1) {
push (@numbers, $userInput); 
#Push the variable $userInput into  @numbers. 
        print "Please enter a number or -1 to end: ";
        chomp ($userInput = <>);
  }
}
#Don't declare the @numbers array again, otherwise that'll clear out the entered numbers.
$" = ", ";
print "@numbers\n";
my @upsorted = sort {$a <=> $b} @numbers;
#prints numbers ascending, smallest first. 
#Don't use $a, $b, or any number as a variable because they're reserved.
$" = ", ";
print "@upsorted\n";
my @downsorted = sort {$b <=> $a} @numbers;
#prints numbers descending, largest first
$" = ", ";
print "@downsorted\n";
print "$downsorted[0]:$upsorted[0]\n"; 
#must be set off with a blank line above and below
##End of program

好的，作为一个快速更新，在tutorialspoint.com 上查找比较器后，我终于让程序运行了。这是代码现在的位置:

import java.util.*;

public class ArrayOutPut {

    static String convertToString(ArrayList<Integer> numbers) {
    StringBuilder builder = new StringBuilder();
    for (int i : numbers){
        builder.append(i);
        builder.append(",");
            }
        builder.setLength(builder.length() - 1);
    return builder.toString();
    }
    public static void main(String args[]){
        Scanner input = new Scanner(System.in);
        ArrayList<Integer> numbers = new ArrayList<Integer>();
        int userInput, largest = Integer.MIN_VALUE, smallest = Integer.MAX_VALUE;

    System.out.println("This program takes a set of integers from the user, and then "
        + "outputs the results to the screen in four ways:");
    System.out.println("the order they were entered seperated by a comma and a space, "
        + "in ascending order, in descending order, and as Largest:Smallest.");
    System.out.println();

    System.out.print("This program takes a set of integers from the user, and then outputs the results to the screen in four ways:\n");
    System.out.print("the order they were entered seperated by a comma and a space, in ascending order, in descending order, and as Largest:Smallest.\n\n");
    //explain to the user what the program does, needs, etc.
    do {
        System.out.print("Please enter a number or -1 to end: ");
        userInput = input.nextInt();
        if (userInput != -1) {
            numbers.add(userInput);
            largest = Math.max(largest, userInput);
            smallest = Math.min(smallest, userInput);
        }
    } while (userInput != -1);

    //Prints the array contents as entered
    String sort = convertToString(numbers);
    Comparator cmp = Collections.reverseOrder();
    System.out.println(sort);
    Collections.sort(numbers);
    System.out.println(convertToString(numbers));
    Collections.sort(numbers, Comparator.reverseOrder());
    System.out.println(convertToString(numbers));
    System.out.printf("Smallest = %d, Largest = %d%n", smallest, largest);
        }
    }

我唯一担心的是，当我编译时，我收到一条注释，指出该程序使用未经检查或不安全的操作，并且我应该使用 -Xlint:unchecked 重新编译以了解详细信息。

Answer 1

首先，如果使用 Java 8+，我会在 convertToString¹ 中使用 StringJoiner

>

static String convertToString(List<Integer> numbers) {
    StringJoiner sj = new StringJoiner(",");
    numbers.stream().map(String::valueOf).forEach(sj::add);
    return sj.toString();
}

其次，您需要排序并再次调用convertToString来打印结果。优先使用小写名称的变量。您可以使用内置函数消除循环。在将 userInput 添加到您的 List 之前，请检查它是否不是 -1。我更喜欢do-while。比如，

public static void main(String args[]) {
    Scanner input = new Scanner(System.in);
    List<Integer> numbers = new ArrayList<>();

    int userInput, largest = Integer.MIN_VALUE, smallest = Integer.MAX_VALUE;

    System.out.println("This program takes a set of integers from the user, and then "
            + "outputs the results to the screen in four ways:");
    System.out.println("the order they were entered seperated by a comma and a space, "
            + "in ascending order, in descending order, and as Largest:Smallest.");
    System.out.println();
    // explain to the user what the program does, needs, etc.
    do {
        System.out.print("Please enter a number or -1 to end: ");
        userInput = input.nextInt();
        if (userInput != -1) {
            numbers.add(userInput);
            largest = Math.max(largest, userInput);
            smallest = Math.min(smallest, userInput);
        }
    } while (userInput != -1);

    // Prints the array contents as entered
    String sort = convertToString(numbers);
    System.out.println(sort);
    Collections.sort(numbers);
    System.out.println(convertToString(numbers));
    Collections.sort(numbers, Comparator.reverseOrder());
    System.out.println(convertToString(numbers));
    System.out.printf("Smallest = %d, Largest = %d%n", smallest, largest);
}

更新

根据您的编辑，您使用的是 Java 7(而不是 Java 8+)。在这种情况下，您的代码(很高兴听到它正在工作)可以使用类型化 Comparator 进行“纠正”。比如，

Comparator<Integer> cmp = Collections.<Integer> reverseOrder();

这将为您提供一个基于Integer的Comparator(按相反顺序)。但是，也可以创建自定义比较器。比如，

Comparator<Integer> cmp = new Comparator<Integer>() {
    @Override
    public int compare(Integer o1, Integer o2) {
        if (o1 == null) {
            return (o2 == null) ? 0 : -1;
        }
        return o1.compareTo(o2);
    }
};

然后你可以像这样使用它(并反转它)

System.out.println(sort);
Collections.sort(numbers, cmp);
System.out.println(convertToString(numbers));
Collections.sort(numbers, cmp.reversed());
System.out.println(convertToString(numbers));

^{1_{并且，请优先选择 List 接口(interface)而不是 ArrayList 具体类型。}}