guava/src/com/google/common/base/Suppliers.java
public static <T> Function<Supplier<T>, T> supplierFunction() {
    @SuppressWarnings("unchecked") // implementation is "fully variant"
    SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
    return sf;
  }
public static <T> Function<Supplier<T>, T> supplierFunction() {
    @SuppressWarnings("unchecked") // implementation is "fully variant"
    SupplierFunction<T> sf = (SupplierFunction<T>) SupplierFunctionImpl.INSTANCE;
    return sf;
  }
Returns a function that accepts a supplier and returns the result of invoking {@link
Supplier#get} on that supplier.

<p><b>Java 8 users:</b> use the method reference {@code Supplier::get} instead.

@since 8.0