chapter12-equals_hashCode_toString.md

Implementing equals()/hashCode() and toString()

Why equals()/hashCode() and toString() are important

Those methods are used by the data structures (list, map) to implement operations like add(), contains(), etc. So most of the data structure doesn't work correctly if equals()/hashCode() and toString() are not correctly written on the element.

By example, ArraysList.contains(value) uses value.equals(), HashMap.get(key) uses both key.hashCode() and key.equals().

Default implementations from java.lang.Object

Object defines several methods and provide a default implementation for them

• boolean equals(Object) test if two objects are equals (same type and same values), but the default implementation do an ==, so only check if the two objects are at the same address in memory
• int hashCode() return a summary of the content of the object as an int the default implementation choose a random number when the object is created
• String toString() return a textual representation of the object the default implementation return a concatenation of the

So the default implementations only ensure that an object is equals to itself.

Writing your own equals()/hashCode()

• equals() must be valid for any object and returns false if it's not the right type so it starts with an instanceof and calls equals() if the field value is a reference.
• hashCode() delegates to the hashCode of the field value

class User {
  private final String name;
  public User(String name) {
    this.name = Objects.requireNonNull(name);
  }
  public boolean equals(Object o) {
    return o instanceof User user &&
      name.equals(user.name);
  }
  public int hashCode() {
    return name.hashCode();
  }
  public String toString() {
    return "User " + name;
  }
}
var user1 = new User("Bob");
var user2 = new User("Bob");
System.out.println(user1.equals(user2));
System.out.println(user1.hashCode() == user2.hashCode());
System.out.println(user1);

With two fields

• equals(), it's better to first check the primitive fields because a primitive check is usually faster than a call to equals().
• hashCode() can use the exclusive or ^ to mix the hash code.

class User {
  private final String name;
  private final int age;
  public User(String name, int age) {
    this.name = Objects.requireNonNull(name);
    this.age = age;
  }
  public boolean equals(Object o) {
    return o instanceof User user &&
      age == user.age && name.equals(user.name);
  }
  public int hashCode() {
    return name.hashCode() ^ age;
  }
  public String toString() {
    return "User " + name + " " + age;
  }
}
var user1 = new User("Bob", 31);
var user2 = new User("Bob", 31);
System.out.println(user1.equals(user2));
System.out.println(user1.hashCode() == user2.hashCode());
System.out.println(user1);

With several fields

• equals(), as said in chapter 'basic_types', array.equals() doesn't work, Arrays.equals() should be used instead
• hashCode(), Object.hash compute the hash of several values separated by commas.

class User {
  private final String name;
  private final int age;
  private final String login;
  private final char[] password;
  public User(String name, int age, String login, char[] password) {
    this.name = Objects.requireNonNull(name);
    this.age = age;
    this.login = Objects.requireNonNull(login);
    this.password = password.clone();
  }
  public boolean equals(Object o) {
    return o instanceof User user &&
      age == user.age && name.equals(user.name) &&
      login.equals(user.login) && Arrays.equals(password, user.password);
  }
  public int hashCode() {
    return Objects.hash(name, age, login, Arrays.hashCode(password));
  }
  public String toString() {
    return "User " + name + " " + age + " " + login + " " + "*".repeat(password.length);
  }
}
var user1 = new User("Bob", 31, "bob", "df15cb4e019ec2eac654fb2e486c56df285c8c7b".toCharArray());
var user2 = new User("Bob", 31, "bob", "df15cb4e019ec2eac654fb2e486c56df285c8c7b".toCharArray());
System.out.println(user1.equals(user2));
System.out.println(user1.hashCode() == user2.hashCode());
System.out.println(user1);

Record implementation

For a record, the methods equals()/hashCode() and toString() are already provided so usually you don't have to provide a new implementation.

record User(String name, int age) {
  public User {
    Objects.requireNonNull(name);
  }
  // the compiler automatically adds equals/hashCode/toString !
}
var user1 = new User("Bob", 31);
var user2 = new User("Bob", 31);
System.out.println(user1.equals(user2));
System.out.println(user1.hashCode() == user2.hashCode());
System.out.println(user1);