如何在Java中中断'if'语句？

Question

In the following code. I want to execute only first if statement. While executing it is displaying output, Valid try again. I want it, to print only 'valid' NOT to display 'try again'. How to do in java.?? I am a beginner.

public class Test {
    public void validate(int age) {
        if (age >= 18) {
            System.out.println("Valid.");
        }
        if (age < 18) {
            System.out.println("Not valid.");
        } else {
            System.out.println("try again.");
        }
    }

    public static void main(String[] args) {
        Test t = new Test();
        t.validate(18);
    }
}

Answer 1

In short:

public void validate(int age) {
    if (age >= 18) {
        System.out.println("Valid.");
    } else {
        System.out.println("Not valid.");
        System.out.println("Try again.");
    }
}

Explanation: The first if only prints a text if the age is at least 18, but that has nothing to do with the following lines. Those act on their own and have two cases: either the user is below 18 or the "try again" text is displayed.

What you want to do is to check "is the age at least 18?" (as you already do) and then display either "valid" or "invalid". Do this by adding the else case to this check, and you are fine.

EDIT: I appended the second line to the else case due to the second comment that I understand as "I need the second message to be displayed but only in case the age is invalid".

Answer 2

    public void validate(int age) {
        if (age >= 18) {
            System.out.println("Valid.");
        } else if (age < 18) {
            System.out.println("Not valid.");
        } else {
            System.out.println("try again.");
        }
    }

    public static void main(String[] args) {
        Test t = new Test();
        t.validate(18);
    }

Problem with your code was even if age is >=18 it will still check if it is also <18, so it will always say try again when age is >=18. Also, please check in comments that it will never say try again with this corrected code because under no circumstances will age be not <18 and >=18 at the same time

EDIT I like Dennis' code better because I think this is what you are trying to achieve.

Answer 3

First of all, lets get some facts about integers sorted out.

1. If an integer is >= 18, that means it is not < 18
2. If an integer is < 18, that means it is not >= 18
3. Any a number is an integer, it is either >= 18 or < 18. It cannot be both. It cannot be neither.

Now to your code. I could write your method like this:

public void validate(int age) {
    if (age >= 18) {
        System.out.println("Valid.");
    }
    if (age < 18) {
        System.out.println("Not valid.");
    }
}

or more simply (relying on the above facts)

public void validate(int age) {
    if (age >= 18) {
        System.out.println("Valid.");
    } else {
        System.out.println("Not valid.");
    }
}

That will write either Valid or Not Valid .

I could also write the code like this:

   public void validate(int age) {
        if (age >= 18) {
            System.out.println("Valid.");
            return;
        }
        if (age < 18) {
            System.out.println("Not valid.");
        } else {
            System.out.println("try again.");
        }
    }

but the else part is redundant because of the above mentioned facts about numbers. Notice how I used a return to "break" the control flow.

You can't use a break statement in this context. The Java Language Specification states in JLS 14.15

For an unlabelled break statement:

"If no switch, while, do, or for statement in the immediately enclosing method, constructor, or initializer contains the break statement, a compile-time error occurs."

We don't have switch , while , do or for statement here.

For a labelled break, the JLS says.

" A break statement must refer to a label within the immediately enclosing method, constructor, initializer, or lambda body. There are no non-local jumps. If no labeled statement with Identifier as its label in the immediately enclosing method, constructor, initializer, or lambda body contains the break statement, a compile-time error occurs. "

... so I suppose it would be possible to write this:

   public void validate(int age) {
       label: { 
           if (age >= 18) {
               System.out.println("Valid.");
               break label;
           }
           if (age < 18) {
               System.out.println("Not valid.");
           } else {
               System.out.println("try again.");
           }
       }
   }

But frankly, that is horrible.