Optional用的比较少,大概看看就好了

package com.baifan.optional;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import java.util.Optional;
import java.util.Set;
import java.util.stream.Stream;
import java.util.*;
import java.nio.file.*;
import java.io.*;

/**
 * @author: baifan
 * @date: 2021/2/5
 */
public class OptionalTest {

    public static void main(String[] args) throws Exception {
        String contents = new String(Files.readAllBytes(Paths.get("D:/sql/java库.sql")), StandardCharsets.UTF_8);
        List<String> wordList = Arrays.asList(contents.split("\\PL+"));

        Optional<String> optionalValue = wordList.stream()
                .filter(s -> s.contains("TABLE"))
                .findFirst();
        System.out.println(optionalValue.orElse("No word") + "contains TABLE");

        Optional<String> optionalString = Optional.empty();
        String result = optionalString.orElse("N/A");
        System.out.println("result:" + result);
        result = optionalString.orElseGet(() -> Locale.getDefault().getDisplayName());
        System.out.println("result:" + result);
        try {
            result = optionalString.orElseThrow(IllegalStateException::new);
            System.out.println("result:" + result);
        } catch (Throwable t) {
            t.printStackTrace();
        }

        optionalValue = wordList.stream()
                .filter(s -> s.contains("SELECT"))
                .findFirst();
        optionalValue.ifPresent(s -> System.out.println(s + "contains SELECT"));

        Set<String> results = new HashSet<>();
        optionalValue.ifPresent(results::add);
        Optional<Boolean> added = optionalValue.map(results::add);
        System.out.println(added);

        System.out.println(inverse(4.0).flatMap(OptionalTest::squareRoot));
        System.out.println(inverse(-1.0).flatMap(OptionalTest::squareRoot));
        System.out.println(inverse(0.0).flatMap(OptionalTest::squareRoot));
        Optional<Double> result2 = Optional.of(-4.0).flatMap(OptionalTest::inverse).flatMap(OptionalTest::squareRoot);
        System.out.println(result2);
    }

    private static Optional<Double> inverse(Double x) {
        return x == 0 ? Optional.empty() : Optional.of(1 / x);
    }

    private static Optional<Double> squareRoot(Double x) {
        return x < 0 ? Optional.empty() : Optional.of(Math.sqrt(x));
    }
}