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()); }
100
- 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):
原文地址:https://www.cnblogs.com/920913cheng/p/10268175.html
时间: 2024-10-10 03:51:59