java1.8新特性（optional 使用）

经常在程序中出现 java.lang.NullPointerException 为了避免报错，总是要进行一些是否为null 的if else 判断，1.8 可以使用optional 类来简化处置
optional :A container object which may or may not contain a non-null value.:可能包含也可能不包含非空值的容器对象。

既然optional 是一个容器对象，那就应该先创建该对象才能调用该对象的一些方法创建optional的方式：

调用Optional静态方法.empty() 来创建一个optional 容器对象

/**
     * Returns an empty {@code Optional} instance.  No value is present for this
     * Optional.
     *
     * @apiNote Though it may be tempting to do so, avoid testing if an object
     * is empty by comparing with {@code ==} against instances returned by
     * {@code Option.empty()}. There is no guarantee that it is a singleton.
     * Instead, use {@link #isPresent()}.
     *
     * @param <T> Type of the non-existent value
     * @return an empty {@code Optional}
     */
    public static<T> Optional<T> empty() {
        @SuppressWarnings("unchecked")
        Optional<T> t = (Optional<T>) EMPTY;
        return t;
    }

public static void testOptional() {
        Optional<Object> empty = Optional.empty();
    }

optional 的构造函数权限是private 的所以不能直接通过构造函数的方法生成该对象，调用静态方法of（T t）可以创建一个指定参数的optional

/**
     * Returns an {@code Optional} with the specified present non-null value.
     *
     * @param <T> the class of the value
     * @param value the value to be present, which must be non-null
     * @return an {@code Optional} with the value present
     * @throws NullPointerException if value is null
     */
    public static <T> Optional<T> of(T value) {
        return new Optional<>(value);
    }

public static void testOptional() {
        // 使用 Optional.empty() 创建 Optional 对象
        Optional<Object> empty = Optional.empty();
        // 私有化构造函数 不能直接 创建
        // Optional<User> optional = new Optional(new User());
        // 使用 Optional.of() 创建 Optional 对象
        Optional<User> optional = Optional.of(new User());
    }

Opional 有一个静态方法 ofNullable（T t）可以根据你传入的值来来判断调用 Optional的无参构造方法还是调用 optional 的有参构造方法如果是null 则调用 empty（）为空则调用 of（T t）方法

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

public static void testOptional() {
        // 使用 Optional.empty() 创建 Optional 对象
        Optional<Object> empty = Optional.empty();
        // 私有化构造函数 不能直接 创建
        // Optional<User> optional = new Optional(new User());
        // 使用 Optional.of() 创建 Optional 对象
        Optional<User> optional = Optional.of(new User());
        //使用 Optional.ofNullable() 创建 Optional 对象
        Optional<Object> ofNullable = Optional.ofNullable(null);
    }

以上三种方法都可以创建一个Optional 对象，如何使用该对象的方法

获取 optional 的值调用 get()方法如果为null 则抛出异常

    /**
     * If a value is present in this {@code Optional}, returns the value,
     * otherwise throws {@code NoSuchElementException}.
     *
     * @return the non-null value held by this {@code Optional}
     * @throws NoSuchElementException if there is no value present
     *
     * @see Optional#isPresent()
     */
    public T get() {
        if (value == null) {
            throw new NoSuchElementException("No value present");
        }
        return value;
    }

检测 optional的值是否为空如果为空则 false

    /**
     * Return {@code true} if there is a value present, otherwise {@code false}.
     *
     * @return {@code true} if there is a value present, otherwise {@code false}
     */
    public boolean isPresent() {
        return value != null;
    }

package lambda.stream;

import java.util.Optional;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testIsPresent();
    }

    public static void testIsPresent() {
        Optional<User> optional = Optional.ofNullable(new User());
        boolean flag = optional.isPresent();
        System.out.println(flag);
        optional = Optional.ofNullable(null);
        flag = optional.isPresent();
        System.out.println(flag);
    }
}

true
false

optional 可以根据 ifPresent 来对值T 进行自身的处理

    /**
     * If a value is present, invoke the specified consumer with the value,
     * otherwise do nothing.
     *
     * @param consumer block to be executed if a value is present
     * @throws NullPointerException if value is present and {@code consumer} is
     * null
     */
    public void ifPresent(Consumer<? super T> consumer) {
        if (value != null)
            consumer.accept(value);
    }

public class OptionalDemo {
    public static void main(String[] args) {
        testIfPresent();
        // testIsPresent();
    }

    public static void testIfPresent() {
        Optional<User> optional = Optional.ofNullable(new User());
        optional.ifPresent((age) -> age.setAge(100));
        System.out.println(optional.get().getAge());

    }

检测一个用户的名称是否满足特定条件的时候，optional 的filter 方法

    /**
     * If a value is present, and the value matches the given predicate,
     * return an {@code Optional} describing the value, otherwise return an
     * empty {@code Optional}.
     *
     * @param predicate a predicate to apply to the value, if present
     * @return an {@code Optional} describing the value of this {@code Optional}
     * if a value is present and the value matches the given predicate,
     * otherwise an empty {@code Optional}
     * @throws NullPointerException if the predicate is null
     */
    public Optional<T> filter(Predicate<? super T> predicate) {
        Objects.requireNonNull(predicate);
        if (!isPresent())
            return this;
        else
            return predicate.test(value) ? this : empty();
    }

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testFilter();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testFilter() {
        Optional<User> ofNullable = Optional.ofNullable(new User("Tom", 15));
        Optional<User> filter = ofNullable.filter(user -> user.getAge() == 15);
    }

User [name=Tom, age=15]

如果没有满足条件是 15的返回值打印报错

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testFilter();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testFilter() {
        Optional<User> ofNullable = Optional.ofNullable(new User("Tom", 25));
        Optional<User> filter = ofNullable.filter(user -> user.getAge() == 15);

Exception in thread "main" java.util.NoSuchElementException: No value present
    at java.util.Optional.get(Optional.java:135)
    at lambda.stream.OptionalDemo.testFilter(OptionalDemo.java:25)
    at lambda.stream.OptionalDemo.main(OptionalDemo.java:16)

Optional 对象中map的使用：返回 User对象的 name属性

    /**
     * If a value is present, apply the provided mapping function to it,
     * and if the result is non-null, return an {@code Optional} describing the
     * result.  Otherwise return an empty {@code Optional}.
     *
     * @apiNote This method supports post-processing on optional values, without
     * the need to explicitly check for a return status.  For example, the
     * following code traverses a stream of file names, selects one that has
     * not yet been processed, and then opens that file, returning an
     * {@code Optional<FileInputStream>}:
     *
     * <pre>{@code
     *     Optional<FileInputStream> fis =
     *         names.stream().filter(name -> !isProcessedYet(name))
     *                       .findFirst()
     *                       .map(name -> new FileInputStream(name));
     * }</pre>
     *
     * Here, {@code findFirst} returns an {@code Optional<String>}, and then
     * {@code map} returns an {@code Optional<FileInputStream>} for the desired
     * file if one exists.
     *
     * @param <U> The type of the result of the mapping function
     * @param mapper a mapping function to apply to the value, if present
     * @return an {@code Optional} describing the result of applying a mapping
     * function to the value of this {@code Optional}, if a value is present,
     * otherwise an empty {@code Optional}
     * @throws NullPointerException if the mapping function is null
     */
    public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
        Objects.requireNonNull(mapper);
        if (!isPresent())
            return empty();
        else {
            return Optional.ofNullable(mapper.apply(value));
        }
    }

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testMap();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testMap() {
        Optional<User> ofNullable = Optional.ofNullable(new User("Tom", 25));
        Optional<String> map = ofNullable.map(user -> user.getName());
        System.out.println(map.get());
    }

结果：

Tom

Optional 的orElse 方法

    /**
     * Return the value if present, otherwise return {@code other}.
     *
     * @param other the value to be returned if there is no value present, may
     * be null
     * @return the value, if present, otherwise {@code other}
     */
    public T orElse(T other) {
        return value != null ? value : other;
    }

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testOrelse();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testOrelse() {
        Optional<User> ofNullable = Optional.ofNullable(null);
        User orElse = ofNullable.orElse(new User("Tom", 35));
        System.out.println(orElse);//结果：User [name=Tom, age=35] 
    }

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testOrelse();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testOrelse() {
        Optional<User> ofNullable = Optional.ofNullable(new User("jack", 45));
        User orElse = ofNullable.orElse(new User("Tom", 35));
        System.out.println(orElse);//结果：User [name=jack, age=45]

    }

Optional 的orElseGet方法方法用处和orElseGet 一样只是传递的参数类型不一样一个是T 一个 function类型的接口

    /**
     * Return the value if present, otherwise invoke {@code other} and return
     * the result of that invocation.
     *
     * @param other a {@code Supplier} whose result is returned if no value
     * is present
     * @return the value if present otherwise the result of {@code other.get()}
     * @throws NullPointerException if value is not present and {@code other} is
     * null
     */
    public T orElseGet(Supplier<? extends T> other) {
        return value != null ? value : other.get();
    }

package lambda.stream;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * @author 作者:cb
 * @version 创建时间：2019年1月14日 上午11:12:12
 * 
 */
public class OptionalDemo {
    public static void main(String[] args) {
        testOrElseGet();
        // testIfPresent();
        // testIsPresent();
    }

    public static void testOrElseGet() {
        Optional<User> ofNullable = Optional.ofNullable(new User("jack", 45));
        User orElseGet = ofNullable.orElseGet(() -> new User("Tom", 35));
        System.out.println(orElseGet);// 结果：User [name=Tom, age=35]

    }

optional 的flatMap使用（扁平化）：

posted on 2019-01-14 17:49 年少不知愁阅读(2565) 评论(0) 编辑收藏举报