Why is this an issue?

While not mandatory, using the @Override annotation on compliant methods improves readability by making it explicit that methods are overriden.

A compliant method either overrides a parent method or implements an interface or abstract method.

Noncompliant code example

class ParentClass {
  public boolean doSomething(){/*...*/}
}
class FirstChildClass extends ParentClass {
  public boolean doSomething(){/*...*/}  // Noncompliant
}

Compliant solution

class ParentClass {
  public boolean doSomething(){/*...*/}
}
class FirstChildClass extends ParentClass {
  @Override
  public boolean doSomething(){/*...*/}  // Compliant
}

Exceptions

This rule does not raise issues when overriding methods from Object (eg: equals(), hashCode(), toString(), …​).