[Java] Exception in thread "main" java.lang.IllegalArgumentException: Comparison method violates its general contract!

 

예를 들어

import java.util.Date;

public class person {
    Date birthDt;
    int age;
    String name;
    String address_country;
    String address_city;
    String address_detail;
    String hobby;
    char gender;
    double weight;
    double height;

    public Date getBirthDt() {
        return birthDt;
    }

    public void setBirthDt(Date birthDt) {
        this.birthDt = birthDt;
    }

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

    public String getAddress_country() {
        return address_country;
    }

    public void setAddress_country(String address_country) {
        this.address_country = address_country;
    }

    public String getAddress_city() {
        return address_city;
    }

    public void setAddress_city(String address_city) {
        this.address_city = address_city;
    }

    public String getAddress_detail() {
        return address_detail;
    }

    public void setAddress_detail(String address_detail) {
        this.address_detail = address_detail;
    }

    public String getHobby() {
        return hobby;
    }

    public void setHobby(String hobby) {
        this.hobby = hobby;
    }

    public char getGender() {
        return gender;
    }

    public void setGender(char gender) {
        this.gender = gender;
    }

    public double getWeight() {
        return weight;
    }

    public void setWeight(double weight) {
        this.weight = weight;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }
}

person class가 있고, 

import java.util.Comparator;

public class PeopleCompareUtil implements Comparator<Object> {
    /**
     * Compares its two arguments for order.  Returns a negative integer,
     * zero, or a positive integer as the first argument is less than, equal
     * to, or greater than the second.<p>
     * <p>
     * The implementor must ensure that {@link Integer#signum
     * signum}{@code (compare(x, y)) == -signum(compare(y, x))} for
     * all {@code x} and {@code y}.  (This implies that {@code
     * compare(x, y)} must throw an exception if and only if {@code
     * compare(y, x)} throws an exception.)<p>
     * <p>
     * The implementor must also ensure that the relation is transitive:
     * {@code ((compare(x, y)>0) && (compare(y, z)>0))} implies
     * {@code compare(x, z)>0}.<p>
     * <p>
     * Finally, the implementor must ensure that {@code compare(x,
     * y)==0} implies that {@code signum(compare(x,
     * z))==signum(compare(y, z))} for all {@code z}.
     *
     * @param o1 the first object to be compared.
     * @param o2 the second object to be compared.
     * @return a negative integer, zero, or a positive integer as the
     * first argument is less than, equal to, or greater than the
     * second.
     * @throws NullPointerException if an argument is null and this
     *                              comparator does not permit null arguments
     * @throws ClassCastException   if the arguments' types prevent them from
     *                              being compared by this comparator.
     * @apiNote It is generally the case, but <i>not</i> strictly required that
     * {@code (compare(x, y)==0) == (x.equals(y))}.  Generally speaking,
     * any comparator that violates this condition should clearly indicate
     * this fact.  The recommended language is "Note: this comparator
     * imposes orderings that are inconsistent with equals."
     */
    @Override
    public int compare(Object o1, Object o2) {
        Person obj1 = (Person)o1;
        Person obj2 = (Person)o2;
        int result = 0;
        if(obj1.getAge() > obj2.getAge()) {
            result = 1;
        } else {
            result = -1;
        }
        if(obj1.getAge() == obj2.getAge()) {
            if(obj1.getHeight() > obj2.getHeight()){
                result = 1;
            }
        }
        return result;
    }
}

이런 경우에

 

height 가 같은 경우에는 다루고 있지 않아서 에러를 내는 줄 알았는데..

 

Person에 age와 height 만 빼서 돌린 경우에는 에러안난다..

뭘까