Help understanding this code:

package dto;

import java.util.Objects;

public class Dto<K> {
    protected K key;

    protected Dto(K key) {
        this.key = key;
    }

    public K getKey() {
        return this.key;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || this.getClass() != o.getClass()) {
            return false;
        }
        Dto<?> dto = (Dto<?>) o;
        return dto.key == this.key;
    }

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

package dto;

import java.util.Objects;

public class Dto<K> {
    protected K key;

    protected Dto(K key) {
        this.key = key;
    }

    public K getKey() {
        return this.key;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || this.getClass() != o.getClass()) {
            return false;
        }
        Dto<?> dto = (Dto<?>) o;
        return dto.key == this.key;
    }

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

package repository;

import dto.Dto;

import java.util.List;

public interface Repository<T, K extends Dto<T>> {
    void add(K item) throws RepositoryException;
    void remove(T key) throws RepositoryException;
    K get(T key);
    List<K> getAll();
    boolean contains(T key);
}

package repository;

import dto.Dto;

import java.util.List;

public interface Repository<T, K extends Dto<T>> {
    void add(K item) throws RepositoryException;
    void remove(T key) throws RepositoryException;
    K get(T key);
    List<K> getAll();
    boolean contains(T key);
}

package repository;

import dto.Dto;

import java.util.List;

public interface Dao<T, K extends Dto<T>> {
    void insert(K item) throws RepositoryException;
    void delete(T key) throws RepositoryException;
    void update(K item) throws RepositoryException;
    K get(T key);
    List<K> getAll();
}

package repository;

import dto.Dto;

import java.util.List;

public interface Dao<T, K extends Dto<T>> {
    void insert(K item) throws RepositoryException;
    void delete(T key) throws RepositoryException;
    void update(K item) throws RepositoryException;
    K get(T key);
    List<K> getAll();
}