Java 变量-编译器不'；I don’我不在乎我输入的数字

我试着做一个基本的程序，可以做一些数学计算，我想让它使用方法。起初，它要求输入两个数字，然后进行计算，但我不知道为什么无论输入什么数字，它总是返回“0”。我需要在代码中更改什么？这样我的程序就可以进行这些计算了

import java.util.Scanner;

public class Z1k7
{
static double a;
static double b;
static double sum;
static double difference;
static double product;
static double quotient;

public class Operations{

double sum (double a, double b){
    return a+b;
}

double difference (double a, double b){
    return a-b;
}

double product (double a, double b){
    return a*b;
}

double quotient (double a, double b){
    return a/b;
}}


public static void main(String[] args) {
    System.out.println("This programme makes some calculations");
    Scanner scan = new Scanner(System.in);
    System.out.println("Enter the first number ");
    a = scan.nextDouble();
    System.out.println("Enter the second number ");
    b = scan.nextDouble();

    System.out.println("The sum: "+sum);
    System.out.println("The difference: "+difference);
    System.out.println("The product: "+product);
    System.out.println("The quotient: "+quotient);
}

}像您的

double sum (double a, double b){
    return a+b;
}

有两个值，a和b。这些值与您声明为的值无关

static int a;
static int b;

在你们班。它们只是同名而已。这可能会引起混乱

你必须调用你的方法来进行实际的计算，比如

int resultFromSummation = sum(a,b);

这里a和b是您的类变量。这些a和b称为实际参数。它们将被复制到方法内部的a和b（称为*形式参数）

即

• 您必须在接受anb的输入后调用这些方法
• 将结果赋给变量
• 然后输出结果

---

您从不调用方法来执行操作，只需打印变量的值，这些变量的默认值为

0

。在打印变量之前，请设置它们

System.out.println("This programme makes some calculations");
Scanner scan = new Scanner(System.in);
System.out.println("Enter the first number ");
a = scan.nextDouble();
System.out.println("Enter the second number ");
b = scan.nextDouble();

// --- New code ------------

// Since the operations are in a different class you need to 
// create an instance of that class
Operations ops = new Operations();

sum = ops.sum( a, b );
difference = ops.difference( a, b );
product = ops.product( a, b );
quotient = ops.quotient( a, b );

// end new code --------------

System.out.println("The sum: "+sum);
System.out.println("The difference: "+difference);
System.out.println("The product: "+product);
System.out.println("The quotient: "+quotient);

另外，由于

操作

实际上不需要实例来执行这些操作，因此可以创建静态方法：

public class Operations{

    public static double sum (double a, double b){
        return a+b;
    }

    // ...
}

然后在不创建实例的情况下调用它们：

// --- New code ------------

sum = Operations.sum( a, b );
difference = Operations.difference( a, b );
product = Operations.product( a, b );
quotient = Operations.quotient( a, b );

// end new code --------------

教程：