Nhảy tới nội dung

Q & A

  • equals() and ==

Equality can be reference based or valued based:

  • == can be used for primitive types, because equality and reference checking work identically for primitives.
  • equals for object types (== performs reference check only)
public class Animal {
    private String name;

    public Animal (String name) { this.name = name; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public static void main(String args[]) {
        List<Animal> animals = Arrays.asList(
                new Animal("cat"),
                new Animal("dog"),
                new Animal("chicken"));

        // these will print out false, false, false if you not implement `equals` and `hashCode`
        System.out.println(animals.contains(new Animal("cat")));
        System.out.println(animals.contains(new Animal("dog")));
        System.out.println(animals.contains(new Animal("foo")));
    }
}

Add these 2, then it will print out true, true, false.

    @Override
    public boolean equals(Object object) {
        if(this == object)
            return true;
        if(object == null || getClass() != object.getClass())
            return false;
        Animal animal = (Animal) object;
        return Objects.equals(name, animal.name);
    }
    @Override
    public int hashCode() {
        return Objects.hash(name);
    }