Java8_Predicate、Supplier、Optional

Predicate

Predicate接收一个参数，返回一个布尔值，用来进行判断是否符合条件

@FunctionalInterface
public interface Predicate {

boolean test(T t);

default Predicate and(Predicate other) {
    Objects.requireNonNull(other);
    return (t) -> test(t) && other.test(t);
}

来看个例子

//获取集合中大于2的数字
List list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
list.stream().filter(item->item>2).forEach(System.out::println);

这里 filter接收的参数就是Predicate

如果想要获取集合中大于5的偶数该怎么办呢，看下面的例子：

List list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

Predicate p1 = item -> item % 2 == 0;
Predicate p2 = item -> item > 5;

list.stream().filter(p1.and(p2)).collect(Collectors.toList()).forEach(item -> System.out.println(item));

Supplier

Supplier不传入参数，返回一个值,这种结构很适合作为工厂来产生对象

@FunctionalInterface
public interface Supplier {
    T get();
}

例子：

 Supplier supplier = () -> new Student();
Student student = supplier.get(); 
Student student2 = supplier.get();
System.out.println(student); //com.test.Student@6e8cf4c6
System.out.println(student2);//com.test.Student@12edcd21

可以看出每次get都得到一个新的对象

Optional

Optional是为了解决NullPointerException而提出来的

/**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 *
 * @since 1.8
 */
 public final class Optional {
     public void ifPresent(Consumer consumer) {
        if (value != null)
            consumer.accept(value);
    }
 }

of

of方法传入的参数不能为null。如果传入参数为null，则抛出NullPointerException 。

Optional optional = Optional.of("hello");
optional.ifPresent(value -> System.out.println(value)); //hello

ofNullable

ofNullable方法可以传null,如果为null，则返回一个空的Optional。

 Optional optional = Optional.ofNullable(null);
optional.ifPresent(System.out::print);//这里不会打印出东西

map

如果一个对象本身不为null，但是他内部的对象可能为null，这时就可以用map来判断

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

Employee employee2 = new Employee();
employee2.setName("lisi");

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

List employeeList = Arrays.asList(employee, employee2);
company.setList(employeeList);

Optional optional = Optional.ofNullable(company);

System.out.println(optional.map(company1 -> company1.getList()).orElse(Collections.emptyList()));