Question

我试图在默认构造函数中为变量gross赋值，然后让其他方法能够访问它（calc ...方法）。

public class CO2FromWaste
{
CO2FromWaste(int numPeople, boolean paper, boolean plastic, boolean glass, boolean cans)
{
    public double ogGrosss = numPeople*1018;
    public double grosss = ogGrosss;

    if(paper = true)
        gross -= 184*numPeople;
    if(plastic = true)
        gross -= 25.6*numPeople;    
    if(glass = true)
        gross -= 46.6*numPeople;
    if(cans = true)
        gross -= 165.8*numPeople;    
}
private double gross = ogGrosss;
private double ogGross = Grosss;
public void calcGrossWasteEmission()
{
    System.out.printf("%20.2f", gross);
}
public void calcWasteReduction() 
{
    System.out.printf("%20.2f", ogGross - gross);
}   
public void calcNetWasteReduction()
{
   System.out.printf("%20.2f", gross);
}    
}

Answer 1

你想要的是那些变量是这个类的成员 - 你现在已经将它们声明为构造函数的本地变量。

public class CO2FromWaste
{
    // Moved your variables to here, outside of any method.
    // They should be declared at/near the top of the class,
    // *before* the constructor (by convention)
    // I also made them `private` because you don't want code
    // that is *outside* of this class to access them.
    private double ogGrosss;
    private double grosss;

    CO2FromWaste(int numPeople, boolean paper, boolean plastic, boolean glass, boolean cans)
    {
        // Now assign them their values.
        // You can optionally use `this`
        this.ogGrosss = numPeople*1018;

        if(paper == true)
            gross -= 184*numPeople;
        if(plastic == true)
            gross -= 25.6*numPeople;    
        if(glass == true)
            gross -= 46.6*numPeople;
        if(cans == true)
            gross -= 165.8*numPeople;    
    }

    // removed the declarations from here - these were now duplicates.

    public void calcGrossWasteEmission()
    {
        System.out.printf("%20.2f", gross);
    }
    public void calcWasteReduction() 
    {
        System.out.printf("%20.2f", ogGross - gross);
    }   
    public void calcNetWasteReduction()
    {
        System.out.printf("%20.2f", gross);
    }    
}

请注意=中的if(paper = true)运算符是赋值 - 您希望比较这些变量，这是通过{{1}完成的} operator。

（未经测试。我甚至没有尝试编译它，只是修改了你的代码）

另请注意，当您从变量==中减去浮点值时，它会向下舍入结果。

Answer 2

我将添加一些提示，以便改进Stephen P的答案。

首先，没有任何理由与布尔值进行比较。您可以简单地写paper == true而不是paper。所有if语句都需要一个布尔值。如果该值已经是布尔值，则无需执行比较。因此，通过将if语句更改为：

，可以使您的if语句更具可读性

if(paper)
    gross -= 184*numPeople;
if(plastic)
    gross -= 25.6*numPeople;
if(glass)
    gross -= 46.6*numPeople;
if(cans)
    gross -= 165.8*numPeople;

我假设根据您的代码执行的操作，您在构造函数中使用的四个布尔变量中只有一个应该一次为true，其余的应该为false。如果这种情况，我建议使用枚举来表示选项，否则在错误使用时会使您的类容易受到逻辑错误的影响。您可以将以下文件添加到项目中：

WasteMaterial.java

public enum WasteMaterial {
    PAPER, PLASTIC, GLASS, CANS
}

并修改类的构造函数，如下所示：

CO2FromWaste (int numPeople, WasteMaterial material)
{
    // Now assign them their values.
    // You can optionally use `this`
    this.ogGrosss = numPeople*1018;

    switch (material) {
    case PAPER:
        gross -= 184*numPeople;
        break;
    case PLASTIC:
        gross -= 25.6*numPeople;
        break;
    case GLASS:
        gross -= 46.6*numPeople;
        break;
    case CANS:
        gross -= 165.8*numPeople;
        break;
    }
}

这可以通过将多个值设置为true来防止类错误地构造（ if ，当然，您的规范只允许这四个值中的一个为true一次）。

Java - 如何创建一个变量，在对象中赋值，所有对象都可以访问？

2 个答案: