《Java核心技术II》消费Optional值

消费Optional值

可选值存在的情况下消费该值

ifPresent，接受一个函数，存在则传递，否则不发生事情

optionalValue.ifPresent(v -> Process v);

值存在，添加到集合中：

optionalValue.ifPresent(v->results.add(v));

或者直接调用:optionalValue.ifPresent(result::add);

ifPresentOrElse,存在与否执行两套动作

optionalValue.ifPresentOrElse(v->System.out.println("发现"+v),()->logger.warning("不匹配"));

综合案例ifPresent

package streams.optional;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
import java.util.logging.Logger;

public class OptionalIfPresent {

    public static void main(String[] args) throws IOException {
        // Path相对路径是指JavaCore2
        var contents = Files.readString(Path.of("./resources/alice.txt"));
        List<String> words = List.of(contents.split("\\PL+"));
        List results = new ArrayList<String>();
        // ifPresent
        Optional<String> startsWithQ = words.stream().filter(s -> s.startsWith("Q")).findFirst();
        startsWithQ.ifPresent(s -> System.out.println(s));
        // ifPresent(lambda)
        startsWithQ.ifPresent(s -> results.add(s));
        System.out.println(results);
        // ifPresentOrElse
        startsWithQ.ifPresentOrElse(s -> System.out.println("找到: " + s),
                () -> Logger.getAnonymousLogger().warning("不匹配"));
    }

}