有没有办法在另一种方法中访问变量？

要使变量在类的所有函数中均可访问，您可以在当前类中静态相关变量

软件包 javaapplication6;

导入 java.util.Scanner;

@author武伦贝雷

公共类 JavaApplication6 {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    mass();
    Concentration();
    if (w >= 0.5) {
        System.out.println("You cannot drive!");
    } else {
        System.out.println("You can drive");

    }
}

static double w;

public static void mass() {
    double a, e, p, v;
    Scanner scanner = new Scanner(System.in);
    System.out.println("Enter the volume: ");
    v = scanner.nextDouble();
    p = 0.8;
    System.out.println("Enter the alcohol volume:(in percents) ");
    e = scanner.nextDouble();
    e = e / 100;
    a = v * e * p;

    System.out.println("mass is: " + a);
}

public static void Concentration() {
    double r, m;
    String person;
    System.out.println("Enter the person: m for male, f for female, j for teenager ");
    Scanner sc = new Scanner(System.in);
    person = sc.nextLine();
    switch (person) {
        case "m":
            r = 0.7;
            break;
        case "f":
            r = 0.6;
            break;
        case "j":
            r = 0.5;
            break;
    }
    System.out.println("Enter person's weight: ");
    m = sc.nextDouble();
    //w=a/(m*r); //a from the method mass

}

}

有两种类型的变量：

• 局部变量（块范围）
• 成员变量（类/实例级别范围）

请参见：https://www.geeksforgeeks.org/variable-scope-in-java/

因此，如果要在多个方法中重用变量，则需要将其从局部变量转换为成员变量。在你的例子中，你只使用方法，所以在方法之外会声明为statica

static double a;

并避免在方法中声明它，因此声明行将更改为mass

double e,p,v;

请注意，缺少 以避免变量阴影。a

您可能希望将方法更改为实例级方法，在这种情况下，您可以根据计划和需求不使用关键字进行声明。astatic

此外，一种众所周知的方法是实现 getter 和 setter，以确保每当您获取或设置值时，如果存在常见操作，则它们只实现一次，而不是重复代码。

下面你可以看到为实现指定目标而对代码进行的最简单的更改：

import java.util.Scanner;

public class program{
    
    static double a;
    
    public static void mass(){
        double e,p,v;
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the volume: ");
        v=scanner.nextDouble();
        p=0.8;
        System.out.println("Enter the alcohol volume:(in percents) ");
        e=scanner.nextDouble();
        e=e/100;
        a=v*e*p;
        
        System.out.println("mass is: "+a);
    }
    public static void Concentration(){
        double w,r,m;
        String person;
        System.out.println("Enter the person: m for male, f for female, j for teenager ");
        Scanner sc=new Scanner(System.in);
        person=sc.nextLine();
        switch(person){
            case "m": r=0.7;
            break;
            case "f": r=0.6;
            break;
            case "j": r=0.5;
            break;
        }
        System.out.println("Enter person's weight: ");
        m=sc.nextDouble();
        w=a/(m*r); //a from the method mass

    }
    public static void main(String[]args){
       mass();
       Concentration();
        /*If(w>=0.5){ //w from the method concentration
            System.out.println("You cannot drive!");
            Else{
                System.out.println("You can drive");
            }
        } */

    }
}

最后，您还可以用作返回值，例如：a

import java.util.Scanner;

public class program{
    
    public static double mass(){
        double a,e,p,v;
        Scanner scanner = new Scanner(System.in);
        System.out.println("Enter the volume: ");
        v=scanner.nextDouble();
        p=0.8;
        System.out.println("Enter the alcohol volume:(in percents) ");
        e=scanner.nextDouble();
        e=e/100;
        a=v*e*p;
        
        System.out.println("mass is: "+a);
        return a;
    }
    public static void Concentration(double a){
        double w,r,m;
        String person;
        System.out.println("Enter the person: m for male, f for female, j for teenager ");
        Scanner sc=new Scanner(System.in);
        person=sc.nextLine();
        switch(person){
            case "m": r=0.7;
            break;
            case "f": r=0.6;
            break;
            case "j": r=0.5;
            break;
        }
        System.out.println("Enter person's weight: ");
        m=sc.nextDouble();
        //w=a/(m*r); //a from the method mass

    }
    public static void main(String[]args){
       Concentration(mass());
        /*If(w>=0.5){ //w from the method concentration
            System.out.println("You cannot drive!");
            Else{
                System.out.println("You can drive");
            }
        } */

    }
}