对于如何从 if 语句中提取值感到非常困惑

Question

所以基本上我有这个 if 语句：

int md; // md = marriage deduction
    if (married == 'M'){
    md = 750;
    System.out.println("Deduction for Being Married: " + md);
    }
    else if (married == 'S'){
    md = 500;
    System.out.println("Deduction for Being Single: " + md);
    }

我真的很困惑如何从该 if 语句中提取 md 的值。我必须在 if 语句之后根据 md 的值计算另一个整数，但是当我尝试这样做时，md 未定义并显示为错误。像这样：

int total = balance - md - ad

Balance 和 ad 工作正常，因为我不必为它们使用 if 语句，但 md 不会有值。该错误表明它是未定义的，因为我从未在 if 语句之外初始化它，我只是想知道如何从 if 语句中获取 md 的值。非常感谢您的帮助。

Answer 1

md 未定义，因为当您声明它时，您没有设置它的值。如果 married 不是 M 或 S，则 md 未定义。否则，md 应保留您分配给它的任何值，因为它超出了 if 语句的范围。

只需添加一个 else 子句即可将 md 设置为零：

if (married == 'M')
{
    md = 750;
    System.out.println("Deduction for Being Married: " + md);
}
else if (married == 'S')
{
    md = 500;
    System.out.println("Deduction for Being Single: " + md);
}
else
{
    md = 0;
    // TODO: System.out.println("Oops. You are neither married nor single.");
}

或者，您也可以将 md 初始化为零。我建议两者都做：

int md = 0;

Answer 2

我会为这些答案添加一个附录。

if (married == 'M') {
    md = 750;
    System.out.println("Deduction for Being Married: " + md);
} else if (married == 'S') {
    md = 500;
    System.out.println("Deduction for Being Single: " + md);
} else {
    md = 0;
    // what if we later add functionality to add divorced or widowed?
    // would this really be the desired functionality to set md to 0
    // better to throw an exception saying we don't recognise the value of married
    // eg. 
    throw new IllegalStateException("unrecognised married state: "+married);
}

或者，您可能想探索使用枚举

public enum Married {

    M("married", 750),
    S("single", 500);

    private String name;
    private int points;

    Married(String name, int points) {
        this.name = name;
        this.points = points;
    }

    public String getName() { return name; }
    public int getPoints() { return points; }

}

String input = // get input somehow

Married status = Married.valueOf(input);
// status is either M or S now. It cannot be null as valueOf throws an exception if it 
// cannot match the input to string to any of the possible values of the enum

int total = balance - status.getPoints() - ad;

Answer 3

你对已婚的价值几乎肯定不是M或S，因此你没有达到你的陈述的任何一个分支，并且md永远不会被定义。

Answer 4

如果已婚不是“M”或“S”，则 md 将是未定义的。

Answer 5

您遇到的情况是 married 既不是 S 也不是 M...因此添加最后一个 else 来设置它为零。

if (married == 'M'){
md = 750;
System.out.println("Deduction for Being Married: " + md);
}
else if (married == 'S'){
md = 500;
System.out.println("Deduction for Being Single: " + md);
}
else {
md = 0;
}

或者在整个 if 语句之前执行此操作...

或者您有小写的 s 或 m ，它们不符合区分大小写的条件

Answer 6

假设这是我们正在讨论的 JAVA，并且 ma​​rried 是一个 String 对象，您应该使用 .equalsTo 来比较它们：

if (married.equalsTo("M")) {...