Question

I’m implementing a TreeSet which will sort by person’s age, but a person will not be saved in set if person’s name is equals. I implement equals and hashcode, but this set will save all persons even if they have the same name. I don’t know why.

public class Person implements Comparable<Person>{
    private String name;
    private int age;

    public Person(String name, int age){
        this.name = name;
        this.age = age;
    }

    @Override
    public int compareTo(Person o) {
        if(this.getAge()<o.getAge()){
            return -1;
        }
        return this.getAge() == o.getAge()?0:1;
    }

    @Override
    public boolean equals(Object object){
        return name.equals(((Person)object).getName());
    }

    @Override
    public int hashCode(){
        return name.hashCode();
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }

    public static void main(String[] args){
        Set<Person> set = new TreeSet<Person>();
        set.add(new Person("Jack",30));
        set.add(new Person("Jack",20));
        System.out.println(set);
    }
}

Answer 1

You can add

if(name.equals(o.getName())) {
    return 0;
}

in the beginning of your compareTo() function. That way TreeSet can assume a.equals(b) is equivalent to a.compareTo(b) == 0 and you still get the ordering by age as all names in the set are different.