java对如何从if语句中提取值感到困惑

Question

所以基本上我有一个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if语句中提取md的值。在这个if语句之后，我必须根据md的值计算另一个整数，但是当我尝试这样做时，md是未定义的，并且显示为错误。像这样：

int total = balance - md - ad

Balance和ad工作得很好，因为我不必为它们使用if语句，但md不会有值。这个错误表示它是未定义的，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。非常感谢你的帮助

Answer 1

如果已婚不是“M”或“S”，那么md将是未定义的

Answer 2

我想在这些答案上加上一句话

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，您可能希望使用枚举进行探索

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

md未定义，因为在声明它时，没有设置它的值。如果married不是M或S，则md未定义。否则，md应该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，您可以将md初始化为零。我建议两方面都做：

int md = 0;

Answer 4

你的已婚价值几乎肯定不是M或S，因此你没有触及你陈述的任何一个分支，md也从未被定义

Answer 5

假设这是我们正在讨论的JAVA，并且是一个字符串对象，您应该使用来比较它们。相等于：

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

Answer 6

您有一个条件，其中married既不是S也不是M。。。因此，添加最后一个else将其设置为零

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;
}

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，它不满足区分大小写的条件