I have a section of code that used to utilize Optional<Department>, but due to some errors I worked out I am now converting it to List<Department>. Obviously this means I now have to change the return types and other method calls. Here are my questions:

• I changed my returns to "new LinkedList<>()" (indicated in the code below) but is that correct?

• There is a red error under ".isPresent" (indicated in the code below) with error message "The method isPresent() is undefined for the type List<Department>". What should I be changing that to?

Below is the updated code with comments indicating where errors are now occurring. Any help and explanations would be GREATLY appreciated!

public List<Department> delete(String department_ID) {
    if ((department_ID == null) || (department_ID.isEmpty())) {
      return new LinkedList<>();      //<<<<<<<<<<<<<<< used to be "return Optional.empty();"
    }

    List<Department> existing = get(department_ID);
    if (existing.isPresent()) {    //<<<<<<<<<<< red error under ".isPresent()"
      String sql = "DELETE employee.*, department.* "   "FROM employee, department "
            "WHERE employee.department_ID = :department_ID AND department.department_ID = :department_ID;";

      MapSqlParameterSource parameters = new MapSqlParameterSource();
      parameters.addValue("department_ID", department_ID);

      int rows = jdbcTemplate.update(sql, parameters);
      if (rows > 0) {
        return existing;
      }
    }
    return new LinkedList<>();     //<<<<<<<<<<<<<<< used to be "return Optional.empty();"
  }

• I changed my returns to "new LinkedList<>()" (indicated in the code below) but is that correct?
• I have googled the error message for my ".isPresent" error and cant find any explanations that fit

CodePudding user response：

Instead of returning new LinkedList<>() you could return List.emptyList().

isPresent() is a method of Optional, but you assign the outcome of method get(department_ID) to an instance of List. You can check the List using

if(!(existing == null || existing.isEmpty())) {

CodePudding user response：

You said:

red error under ".isPresent" (indicated in the code below) with error message "The method isPresent() is undefined for the type List".

The variable existing holds a reference to a List object. If you look at the Javadoc for List, you find no method named isPresent. So of course trying to call a non-existent method generates an error from the compiler.

That isPresent method was from the Optional class. The method checks to see if the optional holds a payload or if the optional is empty.

You seem to be switching to a style where you always expect a List, even if the list is empty (no elements).

If you want to be defensive, you can check that the list object exists.

if ( Objects.nonNull( existing ) ) { … }

But you can omit that null check if you are confident that such a condition cannot exist.

You may want to check if the list is empty, to avoid a needless call to your database. If so, change that code to:

if ( ! list.isEmpty() ) // Perform database work only if list has some elements.

You have other issues. Among them:

• Generally the convention in Java is to avoid underscores in names. And generally best to avoid ALL-CAPS. So departmentId, not department_ID.
• When returning lists, generally best to return an unmodifiable list. If the calling programmer needs a modifiable list, they can easily make from the returned unmodifiable list.
• To get an unmodifiable list, use List.of rather than new LinkedList<>().
• I cannot understand why your delete method would return a list. Your code there mystifies me.