I would agree that this is a bad habbit althoug I always try to find alternatives to an if-else-statement.

My first attempt would be:

class Person {
    private final int age;

    /**
     *  Creates a new person with age 0.
     */
    Person() {
        age = 0;
    }

    /**
     * Creates a new person with age
     * @param age the age of the person. Must be <code>>0</code>
     */
    Person(int age) {
        assert age>0;
        this.age = age;
    }
}

My second attempt would constist of an interface and two classes:

/**
 * Anything with an age.
 */
interface HasAge {
    /**
     * @return the age
     */
    int getAge();
}



/**
 * A Person has name, gender, address etc
 */
class Person {
}


/**
 * A Person with a fixed age.
 */
class PersonWithAge extends Person implements HasAge{
    private int age;

    PersonWithAge(int age) {
        this.age = age;
    }

    @Override
    public int getAge() {
        return age;
    }
}

I would agree that this is a bad habbit althoug I always try to find alternatives to an if-else-statement.

My first attempt would be:

class Person {
    private final int age;

    /**
     *  Creates a new person with age 0.
     */
    Person() {
        age = 0;
    }

    /**
     * Creates a new person with age
     * @param age the age of the person. Must be <code>>0</code>
     */
    Person(int age) {
        assert age>0;
        this.age = age;
    }
}

My second attempt would constist of an interface and two classes:

/**
 * Anything with an age.
 */
interface HasAge {
    /**
     * @return the age
     */
    int getAge();
}



/**
 * A Person has name, gender, address etc
 */
class Person {
}


/**
 * A Person with a fixed age.
 */
class PersonWithAge extends Person implements HasAge{
    private final int age;

    PersonWithAge(int age) {
        this.age = age;
    }

    @Override
    public int getAge() {
        return age;
    }
}