java - 类型不匹配 : Cannot Convert From Void to String in Java

Question

我正在为类(class)创建一个程序，该程序使用公式、ifelse、变量以及用于消息和输入的对话框来复制河流流量的曼宁计算器。我已经创建了大部分程序，即公式、变量、一些 ifelse 语句，但我遇到了与将 resultMessage 变量显示为 JOptionPane 消息框相关的问题。我将多个 Double 类型变量与一些字符串混合的输出连接在一起，如下所示:

resultMessage=JOptionPane.showMessageDialog(null, "At a depth of "+enteredDepth+"ft the flow is "+ calculatedFlow+" cubic ft per second."+"\n"+ "Difference:"+difference);

这给了我标题中所述的错误消息。我不确定是什么会导致这个问题，这是我的代码中唯一的串联部分，我想快速输入它，这样以后就不必担心它。连接中的一些变量尚未制定，但在我看来，仍然不应该引发错误。我的整个程序如下:

import java.text.DecimalFormat;
import javax.swing.JOptionPane;

public class ManningCalculator {

    public static void main(String[] args) {

        DecimalFormat df=new DecimalFormat("##.00");

        //Assigned Constants

        final double MANNING_COEFFICIENT = 1.486;
        final double COEFFICIENT_BRICK = 0.015;
        final double COEFFICIENT_CONCRETE=0.012;
        final double COEFFICIENT_ASPHALT= 0.016;
        final double COEFFICIENT_GRAVEL= 0.023;
        final String MATERIAL_BRICK= "Brick";
        final String MATERIAL_CONCRETE="Concrete";
        final String MATERIAL_ASPHALT="Asphalt";
        final String MATERIAL_GRAVEL="Gravel";
        final double TOLERANCE= 0.01;

        //input Variables

        int attemptNumber;
        double desiredFlow;
        double maxDepth;
        double width;
        double roughnessCoefficient;
        double deltaHeight=0;
        double deltaLength=0;
        double enteredDepth;
        double calculatedFlow;
        double difference;
        double minError;
        String resultMessage="";

        //Math variables

        double channelVelocity=0;
        double hydraulicRadius=0;
        double channelArea=0;
        double channelSlope=0;
        double wettedPerimeter=0;

        //JPane for input variables

        desiredFlow=Double.parseDouble(JOptionPane.showInputDialog("Enter desired flow"));
        maxDepth= Double.parseDouble(JOptionPane.showInputDialog("Enter the maximum depth"));
        width= Double.parseDouble(JOptionPane.showInputDialog("Enter the width"));
        roughnessCoefficient=Double.parseDouble(JOptionPane.showInputDialog("Enter choice for roughness coefficient:"+
            "\n"+"1 Brick"+"\n"+"2 Concrete"+"\n"+"3 Asphalt"+"\n"+"4 Gravel"));
        deltaHeight=Double.parseDouble(JOptionPane.showInputDialog("Enter the change in channel height"));
        deltaLength=Double.parseDouble(JOptionPane.showInputDialog("Enter the change in channel length"));
        enteredDepth=Double.parseDouble(JOptionPane.showInputDialog("Attempt1"+"\n"+"Enter a depth:"));


        //If else if statements

        if(roughnessCoefficient==1) {
            roughnessCoefficient=COEFFICIENT_BRICK;
        }else if(roughnessCoefficient==2) {
            roughnessCoefficient=COEFFICIENT_CONCRETE;
        }else if(roughnessCoefficient==3) {
            roughnessCoefficient=COEFFICIENT_ASPHALT;
        }else {
            roughnessCoefficient=COEFFICIENT_GRAVEL;
        };

        if(desiredFlow<0 ) {
            JOptionPane.showInputDialog("Desired flow must be greater than 0");
        }else {
            JOptionPane.showMessageDialog(null, "Desired flow not valid."+"\n"+"The program terminates");
        };

        if(enteredDepth<0.0) {
            if(enteredDepth>maxDepth) {
                JOptionPane.showMessageDialog(null, "Entered depth is not valid."+"\n"+"Desired depth is set to 10.0");
                enteredDepth=maxDepth;
            }
        }

        //Math Formulas
        desiredFlow=channelVelocity * channelArea;
        channelVelocity= (MANNING_COEFFICIENT /roughnessCoefficient) *
                Math.pow(hydraulicRadius, (2/3)) *Math.pow(channelSlope, (1/2));
        hydraulicRadius =channelArea/wettedPerimeter;
        channelSlope=deltaHeight/deltaLength;


        resultMessage=JOptionPane.showMessageDialog(null, "At a depth of "+enteredDepth+"ft the flow is "+
                calculatedFlow+" cubic ft per second."+"\n"+ "Difference:"+difference);

    System.exit(0); 

    }
}

任何有关此问题的帮助将不胜感激。

Answer 1

改变

resultMessage = JOptionPane.showMessageDialog(null, "At a depth of + enteredDepth + "ft the flow is " + calculatedFlow + " cubic ft per second." + "\n" + "Difference:" + difference);

至

JOptionPane.showMessageDialog(null, "At a depth of " + enteredDepth + "ft the flow is " + calculatedFlow+" cubic ft per second." + "\n" + "Difference:" + difference);

问题是 JOptionPane.showMessageDialog 没有返回值(换句话说，它有一个 void 返回类型)。因此，您无法将此类调用的值分配给变量。