Equality

When comparing for equality in Java, this may refer to testing for object equality in some cases, and in other cases it may refer to testing for reference equality, which tests whether two variables refer/point to the same underlying object in memory. Every custom type can be compared for equality because it inherits from the Object root class (which provides an equals() method).

For example, when comparing for object and reference equality in Java:

record Point(int x, int y) {}

var a = new Point(1, 2);
var b = new Point(1, 2);
var c = new Point(2, 2);
var d = a;

System.out.println(a == b); // (1) false
System.out.println(a.equals(b)); // (2) true
System.out.println(a.equals(c)); // (2) false
System.out.println(a == d); // (1) true
System.out.println(a.equals(d)); // true

The equality operator == is used to compare for reference equality. In this case, variables a and b are referring/pointing to different objects in memory.
The equals() method is used to compare for object equality, i.e. whether two distinct objects have the same content.

Equivalently in Rust:

#[derive(Copy, Clone)]
struct Point(i32, i32);

fn main() {
    let a = Point(1, 2);
    let b = Point(1, 2);
    let c = Point(2, 2);
    let d = a;
    println!("{}", a == b); // Error: "an implementation of `PartialEq<_>` might be missing for `Point`"
    println!("{}", a.eq(&b));
    println!("{}", a.eq(&c));
}

The compiler error above illustrates that in Rust equality comparisons are always related to a trait implementation. To support a comparison using ==, a type must implement PartialEq.

Fixing the example above means deriving PartialEq for Point. Per default, deriving PartialEq will compare all fields for equality, which therefore have to implement PartialEq themselves.

#[derive(Copy, Clone, PartialEq)]
struct Point(i32, i32);

fn main() {
    let a = Point(1, 2);
    let b = Point(1, 2);
    let c = Point(2, 2);
    let d = a;

    println!("{}", a == b);   // true
    println!("{}", a.eq(&b)); // true
    println!("{}", a.eq(&c)); // false
    println!("{}", a.eq(&d)); // true
}

Rust for Java Developers

Equality