This is meant to be a canonical question/answer that can be used as a duplicate target. These requirements are based on the most common questions posted every day and may be added to as needed. They all require the same basic code structure to get to each of the scenarios and they are generally dependent on one another.

You will probably never actually see Scanner used in professional/commercial line of business apps because everything it does is done better by something else. Real world software has to be more resilient and maintainable than Scanner allows you to write code. Real world software uses standardized file format parsers and documented file formats, not the adhoc input formats that you are given in stand alone assignments.

最佳答案

惯用例:

以下是如何正确使用java.util.Scanner类从System.in(有时称为stdin，以交互方式)中正确读取用户输入的信息，尤其是在C，C++和其他语言以及Unix和Linux中。它惯用地演示了要求完成的最常见的事情。

package com.stackoverflow.scanner;

import javax.annotation.Nonnull;
import java.math.BigInteger;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.*;
import java.util.regex.Pattern;

import static java.lang.String.format;

public class ScannerExample
{
    private static final Set<String> EXIT_COMMANDS;
    private static final Set<String> HELP_COMMANDS;
    private static final Pattern DATE_PATTERN;
    private static final String HELP_MESSAGE;

    static
    {
        final SortedSet<String> ecmds = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
        ecmds.addAll(Arrays.asList("exit", "done", "quit", "end", "fino"));
        EXIT_COMMANDS = Collections.unmodifiableSortedSet(ecmds);
        final SortedSet<String> hcmds = new TreeSet<String>(String.CASE_INSENSITIVE_ORDER);
        hcmds.addAll(Arrays.asList("help", "helpi", "?"));
        HELP_COMMANDS = Collections.unmodifiableSet(hcmds);
        DATE_PATTERN = Pattern.compile("\\d{4}([-\\/])\\d{2}\\1\\d{2}"); // http://regex101.com/r/xB8dR3/1
        HELP_MESSAGE = format("Please enter some data or enter one of the following commands to exit %s", EXIT_COMMANDS);
    }

    /**
     * Using exceptions to control execution flow is always bad.
     * That is why this is encapsulated in a method, this is done this
     * way specifically so as not to introduce any external libraries
     * so that this is a completely self contained example.
     * @param s possible url
     * @return true if s represents a valid url, false otherwise
     */
    private static boolean isValidURL(@Nonnull final String s)
    {
        try { new URL(s); return true; }
        catch (final MalformedURLException e) { return false; }
    }

    private static void output(@Nonnull final String format, @Nonnull final Object... args)
    {
        System.out.println(format(format, args));
    }

    public static void main(final String[] args)
    {
        final Scanner sis = new Scanner(System.in);
        output(HELP_MESSAGE);
        while (sis.hasNext())
        {
            if (sis.hasNextInt())
            {
                final int next = sis.nextInt();
                output("You entered an Integer = %d", next);
            }
            else if (sis.hasNextLong())
            {
                final long next = sis.nextLong();
                output("You entered a Long = %d", next);
            }
            else if (sis.hasNextDouble())
            {
                final double next = sis.nextDouble();
                output("You entered a Double = %f", next);
            }
            else if (sis.hasNext("\\d+"))
            {
                final BigInteger next = sis.nextBigInteger();
                output("You entered a BigInteger = %s", next);
            }
            else if (sis.hasNextBoolean())
            {
                final boolean next = sis.nextBoolean();
                output("You entered a Boolean representation = %s", next);
            }
            else if (sis.hasNext(DATE_PATTERN))
            {
                final String next = sis.next(DATE_PATTERN);
                output("You entered a Date representation = %s", next);
            }
            else // unclassified
            {
                final String next = sis.next();
                if (isValidURL(next))
                {
                    output("You entered a valid URL = %s", next);
                }
                else
                {
                    if (EXIT_COMMANDS.contains(next))
                    {
                        output("Exit command %s issued, exiting!", next);
                        break;
                    }
                    else if (HELP_COMMANDS.contains(next)) { output(HELP_MESSAGE); }
                    else { output("You entered an unclassified String = %s", next); }
                }
            }
        }
        /*
           This will close the underlying InputStream, in this case System.in, and free those resources.
           WARNING: You will not be able to read from System.in anymore after you call .close().
           If you wanted to use System.in for something else, then don't close the Scanner.
        */
        sis.close();
        System.exit(0);
    }
}

This may look like a lot of code, but it illustrates the minimum effort needed to use the Scanner class correctly and not have to deal with subtle bugs and side effects that plague those new to programming and this terribly implemented class called java.util.Scanner. It tries to illustrate what idiomatic Java code should look like and behave like.

关于java - 如何使用java.util.Scanner从System.in正确读取用户输入并对其进行操作？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/26446599/