Java错误：找不到符号，变量是否已声明？

嘿，伙计们，我正试图用Java编写一个简单的程序，找出军事时间中给出的2次时间之间的差异，并给出小时和分钟数的输出。每当我在命令提示符下编译此文件时，它都会在最后一行中为变量hours和变量minutes提供一个错误，并显示“找不到符号”。我本想在if语句之前声明它们，但这也不起作用。很抱歉，我对编程非常陌生，非常感谢您提供的帮助。

当您在

if

块（或任何此类块）中定义对象引用或任何基本变量时，它只能在该块中访问

更改如下

import java.util.Scanner;
public class Assignment1Q3
{
    public static void main(String[] args) 
    { 
    Scanner in = new Scanner(System.in);
    System.out.print("Please enter the first time: ");
    int fTime = in.nextInt();
    System.out.print("Please enter second time: ");
    int lTime = in.nextInt();
    int tDifference = Math.abs(fTime - lTime);
    String strTDiff = String.valueOf(tDifference);
    int length = strTDiff.length();
    if (length == 4)
    {
        String hours = strTDiff.substring(0, 2);
        String minutes = strTDiff.substring(3, 5);
    }
    else if (length == 3)
    {
        String hours = strTDiff.substring(0);
        String minutes = strTDiff.substring(2, 4);
    }
    else
    {
        String hours = ("0");
        String minutes = strTDiff.substring(0, 1);
    }
    System.out.println(hours + " hours " + minutes + " minutes");
    }
}

---

通过调用if语句中的字符串，它们只存在于if语句中，您需要在if之外对它们进行贴标，并在if内关联一个值

final String hours, minutes;
if (length == 4)
{
    hours = strTDiff.substring(0, 2);
    minutes = strTDiff.substring(3, 5);
}
else if (length == 3)
{
    hours = strTDiff.substring(0);
    minutes = strTDiff.substring(2, 4);
}
else
{
    hours = ("0");
    minutes = strTDiff.substring(0, 1);
}

因此，您可以将它们初始化为“”或删除不必要的else语句将它们初始化为else值

import java.util.Scanner;
public class Assignment1Q3
{
    public static void main(String[] args) 
    { 
    Scanner in = new Scanner(System.in);
    System.out.print("Please enter the first time: ");
    int fTime = in.nextInt();
    System.out.print("Please enter second time: ");
    int lTime = in.nextInt();
    int tDifference = Math.abs(fTime - lTime);
    String strTDiff = String.valueOf(tDifference);
    int length = strTDiff.length();
    String hours = "";
    String minutes = "";

    if (length == 4)
    {
        hours = strTDiff.substring(0, 2);
        minutes = strTDiff.substring(3, 5);
    }
    else if (length == 3)
    {
        hours = strTDiff.substring(0);
        minutes = strTDiff.substring(2, 4);
    }
    else
    {
        hours = ("0");
        minutes = strTDiff.substring(0, 1);
    }
    System.out.println(hours + " hours " + minutes + " minutes");
    }
}

---

哦，所以我必须为每个子块制作3个单独的输出？无需将它们分配给

null

。让编译器帮助您，并确保所有代码路径都对它们进行初始化。我正要这么说。如果可能的话，让它们成为

final

。@GregKopff C/C++习惯，修复了这一点，非常感谢，但是null命令到底做什么呢？我以前从未被介绍过。

    String hours = "0";
    String minutes = strTDiff.substring(0, 1);

    if (length == 4)
    {
        hours = strTDiff.substring(0, 2);
        minutes = strTDiff.substring(3, 5);
    }
    else if (length == 3)
    {
        hours = strTDiff.substring(0);
        minutes = strTDiff.substring(2, 4);
    }
System.out.println(hours + " hours " + minutes + " minutes");