Why is this an issue?

Superfluous exceptions within throws clauses have negative effects on the readability and maintainability of the code. An exception in a throws clause is superfluous if it is:

Noncompliant code example

void foo() throws MyException, MyException {}  // Noncompliant; should be listed once
void bar() throws Throwable, Exception {}  // Noncompliant; Exception is a subclass of Throwable
void boo() throws IOException { // Noncompliant; IOException cannot be thrown
  System.out.println("Hi!");
}

Compliant solution

void foo() throws MyException {}
void bar() throws Throwable {}
void boo() {
  System.out.println("Hi!");
}

Exceptions

The rule will not raise any issue for exceptions that cannot be thrown from the method body:

interface MyInterface {
  default void defaultMethod() throws IOException {
    System.out.println("Hi!");
  }
  void doSomething() throws IOException;
}

class A implements MyInterface {
  @Override
  void doSomething() throws IOException {
    System.out.println("Hi!");
  }

  public void emptyBody() throws IOException {}

  protected void singleThrowStatement() throws IOException {
    throw new UnsupportedOperationException("This method should be implemented in subclasses");
  }

  Object singleReturnStatement() throws IOException {
    return null;
  }

  /**
   * @throws IOException Overriding classes may throw this exception if they print values into a file
   */
  protected void overridable() throws IOException { // no issue, method is overridable and the exception has proper javadoc
    System.out.println("foo");
  }
}

Also, the rule will not raise issues on RuntimeException, or one of its sub-classes, because documenting runtime exceptions which could be thrown can ultimately help users of the method understand its behavior.

class B {
  int possibleDivisionByZero(int a, int b) throws ArithmeticException {
      return a / b;
  }
}