文件名里的 + 号，为什么解码后变成了空格？

import java.net.URI;
import java.net.URLDecoder;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;

public class Main {
    static void check(boolean condition, String message) {
        if (!condition) throw new AssertionError(message);
    }

    static String formDecode(String value) {
        return URLDecoder.decode(value, StandardCharsets.UTF_8);
    }

    static void decode(String mode, String raw, String expectedPath, String expectedForm) {
        URI uri = URI.create(raw);
        String path = uri.getPath();
        String form = formDecode(uri.getRawPath());
        check(path.equals(expectedPath), "unexpected URI path");
        check(form.equals(expectedForm), "unexpected form decoding");
        System.out.printf("{\"type\":\"decode\",\"mode\":\"%s\",\"rawPath\":\"%s\","
                + "\"uriPath\":\"%s\",\"formDecoded\":\"%s\"}%n",
                mode, uri.getRawPath(), path, form);
    }

    static void construct(String mode, String original, URI uri) {
        System.out.printf("{\"type\":\"construct\",\"mode\":\"%s\",\"original\":\"%s\","
                + "\"rawPath\":\"%s\",\"uriPath\":\"%s\",\"formDecoded\":\"%s\","
                + "\"uriMatchesOriginal\":%s}%n",
                mode, original, uri.getRawPath(), uri.getPath(),
                formDecode(uri.getRawPath()), uri.getPath().equals(original));
    }

    public static void main(String[] args) throws Exception {
        decode("literal-plus", "/files/A+B.txt", "/files/A+B.txt", "/files/A B.txt");
        decode("escaped-space", "/files/A%20B.txt", "/files/A B.txt", "/files/A B.txt");
        decode("escaped-plus", "/files/A%2BB.txt", "/files/A+B.txt", "/files/A+B.txt");
        String name = "A B+C.txt";
        String original = "/files/" + name;
        String formEncoded = URLEncoder.encode(name, StandardCharsets.UTF_8);
        check(formEncoded.equals("A+B%2BC.txt"), "unexpected form encoding");
        check(formDecode(formEncoded).equals(name), "form round trip must work");
        URI wrong = URI.create("/files/" + formEncoded);
        URI right = new URI(null, null, original, null, null);
        check(!wrong.getPath().equals(original), "form output is not path encoding");
        check(right.getPath().equals(original), "path constructor round trip must work");
        construct("form-as-path", original, wrong);
        construct("path-constructor", original, right);
    }
}