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《Java核心技术II》管道化Optional值

article 2025/1/7 4:09:27

管道化Optional值

map来转换Optional内部的值

Optional transformed = optionalString.map(String::toUpperCase);

将结果添加到列表中，如果存在则添加，为空则什么也不会发生

optionalValue.map(result::add);

类似的使用filter方法来进行过滤

Optional transformed = optionalString.filter(s->s.length()>=8).map(String::toUpperCase)

用or将Option替换为另一个，替换值将以惰性方式计算

Optional result = optionalString.or(()->alternatives.stream().findFirst());

管道化Optional案例

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 OptionalMapFilterOr {

    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>();
        // map
        Optional<String> startsWithQ = words.stream().filter(s -> s.startsWith("Q")).findFirst();
        Optional<String> transformed = startsWithQ.map(String::toUpperCase);
        System.out.println("transformed:"+transformed.orElse(""));
        //filter
        Optional<String> filter = startsWithQ.filter(s -> s.length()>=8).map(String::toUpperCase);
        System.out.println("filter:"+filter);
        //or
        Optional<String> result = startsWithQ.filter(s -> s.length()>=8).or(()->words.stream().max(String::compareTo));
        System.out.println("result:"+result.orElse(""));
    }

}