Java：三个字符串，字典顺序

初学者 Java 程序员在这里。我试图将三个字符串相互比较，并让系统按字典顺序吐出第二个/中间的单词。

import java.util.*;

public class Ordered2
{
public static void main(String[] args)
{
    String firstString, secondString, thirdString;

    Scanner keyboard = new Scanner(System.in);

    System.out.println("Enter three different strings.");
    System.out.println("The string in the middle order lexicographically will be displayed.");
    firstString = keyboard.nextLine();
    secondString = keyboard.nextLine();
    thirdString = keyboard.nextLine();

    String topString, middleString, bottomString;

    if (firstString.compareTo(secondString) > 0 && (firstString.compareTo(thirdString) > 0)) 
    { topString = firstString; }
    else if (firstString.compareTo(secondString) < 0 && (firstString.compareTo(thirdString) > 0)) {
    middleString = firstString; }
    else { bottomString = firstString; }

    if (secondString.compareTo(firstString) > 0 && (secondString.compareTo(thirdString) > 0)) {
    topString = secondString; }
    else if (secondString.compareTo(firstString) < 0 && (secondString.compareTo(thirdString) > 0)) {
    middleString = secondString; }
    else { bottomString = secondString; }

    if (thirdString.compareTo(secondString) > 0 && (thirdString.compareTo(firstString) > 0)) {
    topString = thirdString; }
    else if (thirdString.compareTo(secondString) < 0 && (thirdString.compareTo(firstString) > 0)) {
    middleString = thirdString; }
    else { bottomString = thirdString; }

    System.out.println("The second string in lexicographic order: " + middleString);
    }
}

这不会编译，并告诉我 middleString 尚未初始化。任何帮助，将不胜感激。

Java 编译器不知道哪个分支if语句将被执行。这意味着，如果您在一个分支中初始化变量而不是在另一个分支中初始化，则不能保证为该变量分配值。在您的代码中，所有变量当然都会被初始化，但编译器无法知道这一点，因此会出现错误。您可以将这三个初始化为null或空字符串。代替String topString, middleString, bottomString; with

String topString = null;
String middleString = null;
String bottomString = null;

此外，您可能希望使用 Java 的一些内置排序功能来为您进行排序：

import java.util.*;

public class Ordered2
{
public static void main(String[] args)
{
    String firstString, secondString, thirdString;

    Scanner keyboard = new Scanner(System.in);

    System.out.println("Enter three different strings.");
    System.out.println("The string in the middle order lexicographically will be displayed.");
    firstString = keyboard.nextLine();
    secondString = keyboard.nextLine();
    thirdString = keyboard.nextLine();

    String[] array = new String[] {firstString, secondString, thirdString};

    Arrays.sort(array);

    System.out.println("The second string in lexicographic order: " + array[1]);
}
}

Arrays.sort() http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#sort%28java.lang.Object%5B%5D%29为您对字符串进行排序。从排序数组中取出第二个（索引 1）字符串即可得到中间字符串。如果您想使用不区分大小写的顺序进行排序，您可以使用Arrays.sort(array, String.CASE_INSENSITIVE_ORDER).