Why is this an issue?

The java.util.function package provides a large array of functional interface definitions for use in lambda expressions and method references. In general it is recommended to use the more specialised form to avoid auto-boxing. For instance IntFunction<Foo> should be preferred over Function<Integer, Foo>.

This rule raises an issue when any of the following substitution is possible:

Current Interface	Preferred Interface
`Function<Integer, R>`	`IntFunction<R>`
`Function<Long, R>`	`LongFunction<R>`
`Function<Double, R>`	`DoubleFunction<R>`
`Function<Double,Integer>`	`DoubleToIntFunction`
`Function<Double,Long>`	`DoubleToLongFunction`
`Function<Long,Double>`	`LongToDoubleFunction`
`Function<Long,Integer>`	`LongToIntFunction`
`Function<R,Integer>`	`ToIntFunction<R>`
`Function<R,Long>`	`ToLongFunction<R>`
`Function<R,Double>`	`ToDoubleFunction<R>`
`Function<T,T>`	`UnaryOperator<T>`
`BiFunction<T,T,T>`	`BinaryOperator<T>`
`Consumer<Integer>`	`IntConsumer`
`Consumer<Double>`	`DoubleConsumer`
`Consumer<Long>`	`LongConsumer`
`BiConsumer<T,Integer>`	`ObjIntConsumer<T>`
`BiConsumer<T,Long>`	`ObjLongConsumer<T>`
`BiConsumer<T,Double>`	`ObjDoubleConsumer<T>`
`Predicate<Integer>`	`IntPredicate`
`Predicate<Double>`	`DoublePredicate`
`Predicate<Long>`	`LongPredicate`
`Supplier<Integer>`	`IntSupplier`
`Supplier<Double>`	`DoubleSupplier`
`Supplier<Long>`	`LongSupplier`
`Supplier<Boolean>`	`BooleanSupplier`
`UnaryOperator<Integer>`	`IntUnaryOperator`
`UnaryOperator<Double>`	`DoubleUnaryOperator`
`UnaryOperator<Long>`	`LongUnaryOperator`
`BinaryOperator<Integer>`	`IntBinaryOperator`
`BinaryOperator<Long>`	`LongBinaryOperator`
`BinaryOperator<Double>`	`DoubleBinaryOperator`
`Function<T, Boolean>`	`Predicate<T>`
`BiFunction<T,U,Boolean>`	`BiPredicate<T,U>`

Noncompliant code example

public class Foo implements Supplier<Integer> {  // Noncompliant
    @Override
    public Integer get() {
      // ...
    }
}

Compliant solution

public class Foo implements IntSupplier {

  @Override
  public int getAsInt() {
    // ...
  }
}