JDK8新特性之Optional

转载自 JDK8新特性之Optional

Optional是什么

java.util.Optional

Jdk8提供 Optional，一个可以包含null值的容器对象，可以用来代替xx != null的判断。

Optional常用方法

of

public static <T> Optional<T> of(T value) {return new Optional<>(value);
} static <T> Optional<T> of(T value) {return new Optional<>(value);
}

为value创建一个Optional对象，如果value为空则会报出NullPointerException异常。

ofNullable

public static <T> Optional<T> ofNullable(T value) {return value == null ? empty() : of(value);
} static <T> Optional<T> ofNullable(T value) {return value == null ? empty() : of(value);
}

为value创建一个Optional对象，但可以允许value为null值。

isPresent

public boolean isPresent() {return value != null;
} boolean isPresent() {return value != null;
}

判断当前value是否为null,如果不为null则返回true，否则false。

ifPresent

如果不为null值就执行函数式接口的内容。

public void ifPresent(Consumer<? super T> consumer) {if (value != null)consumer.accept(value);
} void ifPresent(Consumer<? super T> consumer) {if (value != null)consumer.accept(value);
}

get

public T get() {if (value == null) {throw new NoSuchElementException("No value present");}return value;
} T get() {if (value == null) {throw new NoSuchElementException("No value present");}return value;
}

返回当前的值，如果为空则报异常。

orElse

返回当前值，如果为null则返回other。

public T orElse(T other) {return value != null ? value : other;
} T orElse(T other) {return value != null ? value : other;
}

orElseGet

orElseGet和orElse类似，只是orElseGet支持函数式接口来生成other值。

public T orElseGet(Supplier<? extends T> other) {return value != null ? value : other.get();
} T orElseGet(Supplier<? extends T> other) {return value != null ? value : other.get();
}

orElseThrow

如果有值则返回，没有则用函数式接口抛出生成的异常。

public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {if (value != null) {return value;} else {throw exceptionSupplier.get();}
} <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {if (value != null) {return value;} else {throw exceptionSupplier.get();}
}

示例

public static void main(String[] args) {testOf();testNullable();
}
private static void testNullable() {User user = null;User john = new User("john", 18);User dick = new User("dick", 12);

    System.out.println(Optional.ofNullable(user).orElse(john));System.out.println(Optional.ofNullable(john).get());System.out.println(Optional.ofNullable(dick).orElse(john));System.out.println(Optional.ofNullable(user).orElseGet(() -> john));

    System.out.println();
}

private static void testOf() {try {User user1 = new User();Optional<User> userOptional1 = Optional.of(user1);if (userOptional1.isPresent()) {System.out.println("user is not null");}

        User user2 = null;Optional<User> userOptional2 = Optional.of(user2);//NullPointerExceptionif (userOptional2.isPresent()) {System.out.println("user is not null");}} catch (Exception e) {e.printStackTrace();}System.out.println();
} static void main(String[] args) {testOf();testNullable();
}
private static void testNullable() {User user = null;User john = new User("john", 18);User dick = new User("dick", 12);

    System.out.println(Optional.ofNullable(user).orElse(john));System.out.println(Optional.ofNullable(john).get());System.out.println(Optional.ofNullable(dick).orElse(john));System.out.println(Optional.ofNullable(user).orElseGet(() -> john));

    System.out.println();
}

private static void testOf() {try {User user1 = new User();Optional<User> userOptional1 = Optional.of(user1);if (userOptional1.isPresent()) {System.out.println("user is not null");}

        User user2 = null;Optional<User> userOptional2 = Optional.of(user2);//NullPointerExceptionif (userOptional2.isPresent()) {System.out.println("user is not null");}} catch (Exception e) {e.printStackTrace();}System.out.println();
}