Optional

java8 的 Optional

所属包:

package java.util;

Optional 就是一个容器

• Optional 无法直接构造因为构造方法是私有的

    /**
     * Constructs an empty instance.
     *
     * @implNote Generally only one empty instance, {@link Optional#EMPTY},
     * should exist per VM.
     */
    private Optional() {
        this.value = null;
    }

• 构造Optional方式有以下

    /**
     * Returns an empty {@code Optional} instance.  No value is present for this
     * Optional.
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

    /**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     */
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }

    /**
     * Returns an {@code Optional} describing the specified value, if non-null,
     * otherwise returns an empty {@code Optional}.
     *
     */
    public static <T> Optional<T> ofNullable(T value) {
        return value == null ? empty() : of(value);
    }

Optional使用

    public static void main(String[] args) {

        Employee employee = new Employee();
        employee.setName("zhangsan");

        Employee employee1 = new Employee();
        employee1.setName("lisi");

        Company company = new Company();
        company.setName("company1");

        List<Employee> employees = Arrays.asList(employee, employee1);
        company.setEmployees(employees);

        

        Optional<Company> optional = Optional.ofNullable(company);
        // 不推荐用法
        if (optional.isPresent()) {
            // ....
        }
        // 推荐用法
        System.out.println(optional.map(theCompany -> theCompany.getEmployees())
                .orElse(Collections.emptyList()));
        
    }