How to fix checkbox

Question

I have a strange bug in UI part of my pet-app (Java+Spring+Themeleaf+Bootstrap) with Dto edit. Dto looks like (int id, String name, List list). I try checkbox and select to edit List and in both case result the same: if in list only one dto, then it correctly shows in checkbox or selection list,

but if list contain more than one, no one option choosed.

When I choose any options(multiselect allows), and press Save- Dto saves in Data base correctly.

<div class="form-group">
            <label class="col-form-label">Select courses:</label>
            <div class="col-sm-10">
                <input type="hidden" th:field="*{id}"/>
                <div th:each="course : ${allCourses}">
                    <input type="checkbox" th:id="${{course}}" th:value="${{course}}" th:field="*{courses}" /> 
                    <label th:for="${{course}}" th:text="${course.name}">Course</label>
                </div>
                <span class="font-italic text-danger" th:if="${#fields.hasErrors('courses')}"
                      th:errors="*{courses}"></span>
            </div>
        </div>

What I need to change in code to fix this bug?

Answer 1

You need to override the equals & hashCode methods for this DTO strictly like this:

 @Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof DTO)) return false;
    DTO dto = (DTO) o;
    return getId().equals(dto.getId());
}

@Override
public int hashCode() {
    return Objects.hash(getId());
}