为什么扫描仪在输出之前先读取？答案

【问题标题】：Why is scanner reading first before the output?为什么扫描仪在输出之前先读取？
【发布时间】：2020-02-02 22:44:54
【问题描述】：

我正在用 Hackerrank 做地图练习。目标是输入要输入的条目总数，然后输入键和值，直到达到条目数。还要寻找密钥是否存在。但是，我的代码运行不正常。即使我在扫描仪输入之前打印出“请输入姓名以查找电话号码：”，但扫描仪输入首先读取。这是为什么呢？

Scanner scanner = new Scanner(System.in);
Map<String, Integer> phone = new HashMap<>();
System.out.print("Please enter total number of entries: ");
int n = scanner.nextInt();
scanner.nextLine();

for(int x=0;x<n;x++) {
    System.out.print("Please enter name: ");
    String name = scanner.nextLine();
    System.out.print("Please enter phone number: ");
    int num = scanner.nextInt();
    scanner.nextLine();
    phone.put(name,num);
}

while(scanner.hasNext()) {
    System.out.println("Please enter name to find phone number: "); //the problem is here
    String findName = scanner.nextLine(); // this will read first before above line
    if(phone.containsKey(findName)) {
        System.out.println(findName + "=" + phone.get(findName));
    } else {
        System.out.println("not found");
    }
}

【问题讨论】：

标签： java dictionary java.util.scanner

【解决方案1】：

问题出在 while 循环上——直到 Scanner 检测到输入可用时循环才会开始，因此程序会阻塞等待输入。或许可以暂时做一下，这样就可以开始了。

你仍然需要一些机制来让你退出最后一个循环，也许检查输入的字符串是否有"exit" 或类似的字符串

事实上，我会完全摆脱scanner.hasNext()，因为它真的没有任何用处，因为scanner.nextLine() 会为你做必要的阻止。

相反，我会创建一个标记值，例如：

private static final String SENTINEL = "exit";

然后循环直到输入该值：

String findName = "";
while (!findName.equalsIgnoreCase(SENTINEL)) {
    System.out.print("Please enter name to find phone number or \"" + SENTINEL + "\" to exit: " );
    findName = scanner.nextLine();

    if (phone.containsKey(findName)) {
        System.out.println(findName + "=" + phone.get(findName));
    } else if (!findName.equalsIgnoreCase(SENTINEL)){
        System.out.println("not found");
    }
}

例如：

import java.util.*;

public class Foo {
    private static final String SENTINEL = "exit";

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        Map<String, Integer> phone = new HashMap<>();
        System.out.print("Please enter total number of entries: ");
        int n = scanner.nextInt();
        scanner.nextLine();

        for (int x = 0; x < n; x++) {
            System.out.print("Please enter name: ");
            String name = scanner.nextLine();
            System.out.print("Please enter phone number: ");
            int num = scanner.nextInt();
            scanner.nextLine();
            phone.put(name, num);
        }

        String findName = "";
        while (!findName.equalsIgnoreCase(SENTINEL)) {
            System.out.print("Please enter name to find phone number or \"" + SENTINEL + "\" to exit: " );
            findName = scanner.nextLine();
            if (phone.containsKey(findName)) {
                System.out.println(findName + "=" + phone.get(findName));
            } else if (!findName.equalsIgnoreCase(SENTINEL)){
                System.out.println("not found");
            }
        } 
    }
}

【讨论】：