如何从IF语句的结果声明String？

我一直在玩并测试我学到的一些东西,但由于某种原因,这对我不起作用.它只是mid-app,但是我在开发过程中继续运行它,以便在我完成时不会有一千个问题堆积起来.它应该能够按原样运行.

import java.util.Scanner;

public class Speed {
    public void speedAsker(){
        Scanner scan = new Scanner(System.in);

        System.out.println("Should we use: 1.KMPH or 2. MPH");
        int s1 = scan.nextInt();
        if(s1==1){
            String j1 = "KMPH";
            System.out.println("We will be using Kilometres for this calculation.");
        }if(s1 ==2){
            String j1 = "MPH";
            System.out.println("We will be using Miles for this calculation.");
        }else{
            System.out.println("That is an invalid input, you must choose between 1 or 2.");
        }
        System.out.println("What speed is your vehicle going in?");
        int d1 = scan.nextInt();
        System.out.println("Your vehicle was going at " + d1 + j1 + ".");
    }
}

这就是我得到的输出.启动器类只是字面上启动这个类,我只是为了良好的实践.我遇到的问题是尝试根据答案标记j1,然后在我的输出中使用它.

线程"main"中的异常java.lang.Error:未解决的编译问题:
j1无法解析为 Launcher.main
中的Speed.speedAsker(Speed.java:28)
中的变量(Launcher.java:7)

提前致谢.

你在外面声明它,然后在if/else中定义它

String j1;
if(s1==1){
    j1 = "KMPH";
    System.out.println("We will be using Kilometres for this calculation.");
}if(s1 ==2){
    j1 = "MPH";
    System.out.println("We will be using Miles for this calculation.");
}else{
    j1 = null;
    System.out.println("That is an invalid input, you must choose between 1 or 2.");
}

在for循环外声明你的字符串,并在里面分配它.

例如:

String j1;

if(s1==1){
    j1 = "KMPH";
    System.out.println("We will be using Kilometres for this calculation.");
}if(s1 ==2){
    j1 = "MPH";
    System.out.println("We will be using Miles for this calculation.");
}else{
    j1 = "";
    System.out.println("That is an invalid input, you must choose between 1 or 2.");
}

...

System.out.println("Your vehicle was going at " + d1 + j1 + ".");

请注意,Java要求局部变量在使用之前具有明确的赋值.上面的声明"String j1"不提供默认值,因此else子句必须提供一个或异常退出.

您还可以在声明中提供默认值:

 String j1 = "";