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I want to throw an exception in order to let the user know that the age value input should be between 1 and 120. Find below the example.

Is there a better way to throw an exception?

public class Lesson18Encapsulation3 {

    private int age;
    private String name;
    private int salary;

    public int getAge() {
        return age;
    }
    public void setAge(int age) {

        if ( age < 0 && age >= 120) 

            this.age = age;
        throw new IllegalArgumentException (" age can not be negative or more than 120");

//getters and setters

    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

public class Lesson18Encapsulation4 {

    public static void main(String[] args) {
        Lesson18Encapsulation3 emp = new Lesson18Encapsulation3();

        emp.setAge(220);
        emp.setName("None");

        System.out.println( "age : " + emp.getAge());
        System.out.println( "name : " + emp.getName());
    }
}
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3
  • 1
    Note that if the age is not valid, you shouldn't do this.age = age;. You may also consider using Java Validation Api, an example here : baeldung.com/javax-validation Commented Sep 25, 2018 at 8:03
  • 1
    Are you sure about the if clause for the age? I doubt there are people with age of -8. :) Commented Sep 25, 2018 at 9:03
  • This is just to protect Data from misuse. Someone can change the age via setter method. Lesson18Encapsulation3 emp = new Lesson18Encapsulation3(); emp.setAge(220); // Error: Throw new IllegalArgumentException emp.setAge(-8); // Erro: Throw new IllegalArgumentException Commented Sep 25, 2018 at 15:09

2 Answers 2

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4

In principle, what you are doing here is an instance of design by contract: You are defining a pre-condition for your method.

A) Wrong Condition

Your condition is wrong. You want to throw the exception in exactly the opposite case:

if(age < 0 || age >= 120) {
   // Throw exception
}

(This was also pointed out by @Nikolas in an earlier post)

B) Make Design by Contract Explicit

I'd make this explicit by providing an Assertion-method:

private void setAge(int age) {
    assertCorrectAge(age);
    this.age = age;
}

private void assertCorrectAge(int age) {
    if(age < 0 || age >= 120) {
        throw new IllegalArgumentException(" ... ");
    }
}

Some people will suggest to use Java's build in assert keyword. I suggest to not do that, because it is disabled in most JVM versions (so it doesn't fire).

C) Use a library

Having many assertion method for checking pre-conditions, adds up to a lot of boilerplate code. You could have a library take care of this (for example Google's Guava):

public void setAge(int age) {
    Preconditions.checkArgument(age >= 0, "negative age: %s", age);
    Preconditions.checkArgument(age < 120, "age too high: %s", age);
    this.age = age;
}
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4 Comments

A brings nothing new. B is not necessary - much better would be a use of Java Validation API. Agreed that avoiding the assert keyword is a good way. C I recommend again the Java Validation API rather than Guava.
Thanks for the comment. I disagree particularly regarding B: It is an improvement towards readibility and makes the intent clearer. Also, it encapsulates the check of the pre-condition. Regarding A: I was not aware that you had pointed out this as well. I apologize and will edit my post to reflect your contribution.
Well A brings the correct if-clause (which I assume is wrong in the question). C is a suggestion. I thought Java Validation API could be a little bit too much.
Your comment on C makes me think a bit more: It might be true for a Java EE context. But in general Java, I have not seen it used for the checks of invariants, pre- and post conditions. Can you provide an example application/ reference?
2

The condition is wrong. You want to set the age only if the range is between 0 and 120:

if (age >= 0 && age < 120) {
    this.age = age;
} else throw new IllegalArgumentException("Age can not be negative or more than 120");

Alternatively (note the changed || operator which means or):

if (age < 0 || age >= 120) {
    throw new IllegalArgumentException("Age can not be negative or more than 120");  
}
this.age = age;

Throwing an exception itself upon wrong input is perfectly valid.

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