== Why is this an issue?

The `Object#equals(Object obj)` method is used to compare two objects to see if they are equal.

The `obj` parameter's type is `Object`, this means that an object of any type can be passed as a parameter to this method.

Any class overriding `Object#equals(Object obj)` should respect this contract, accept any object as an argument, and return `false` when the argument's type differs
from the expected type. The `obj` parameter's type can be checked using `instanceof` or by comparing the `getClass()` value:

[source,java]
----
@Override
public boolean equals(Object obj) {
  // ...
  if (this.getClass() != obj.getClass()) {
    return false;
  }
  // ...
}
----

However, it is an issue to assume that the `equals` method will only be used to compare objects of the same type. Casting the `obj` parameter without a prior test will throw a `ClassCastException` instead of returning false.
[source,java]
----
public class MyClass {
  @Override
  public boolean equals(Object obj) {
    MyClass that = (MyClass) obj; // may throw a ClassCastException
    // ...
  }
  // ...
}
----

This rule raises an issue when `obj` parameter's type has not been tested before a cast operation.

== How to fix it

Ensure the `obj` parameter's type is checked by comparing `this.getClass()` and `obj.getClass()`, or use the `instanceof` operator to test `obj`'s type.

=== Code examples

==== Noncompliant code example
[source,java,diff-id=1,diff-type=noncompliant]
----
public class MyClass {
  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    MyClass that = (MyClass) obj; // Noncompliant, may throw a ClassCastException
    // ...
  }
  // ...
}
----

==== Compliant solution
[source,java,diff-id=1,diff-type=compliant]
----
public class MyClass {
  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null || this.getClass() != obj.getClass()) {
      return false;
    }
    MyClass that = (MyClass) obj; // Compliant, obj's type is MyClass
    // ...
  }
  // ...
}
----

== Resources

=== Documentation

* https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Object.html#equals(java.lang.Object)[Oracle SDK - java.lang.Object#equals(Object obj)]

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Add a type test to this method.


'''
== Comments And Links
(visible only on this page)

=== on 3 Oct 2014, 20:09:10 Ann Campbell wrote:
\[~nicolas.peru] this will be covered by RSPEC-1944 if it includes downcasts.

=== on 12 Oct 2014, 19:00:05 Freddy Mallet wrote:
This rule should be linked to Findbugs rule BC_EQUALS_METHOD_SHOULD_WORK_FOR_ALL_OBJECTS

endif::env-github,rspecator-view[]