FULL PRODUCT VERSION :
openjdk version "1.8.0_71"
OpenJDK Runtime Environment (build 1.8.0_71-b15)
OpenJDK 64-Bit Server VM (build 25.71-b15, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux <removed> 4.3.4-300.fc23.x86_64 #1 SMP Mon Jan 25 13:39:23 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux

A DESCRIPTION OF THE PROBLEM :
The java.net.URI documentation explain on its normalize method javadoc the treatment of path components that include a colon character. First trying to create a new URI with only a path component with a colon on result in an absolute URL with an scheme based on the path string

    new URI(null,null,"a:b",null).isAbsolute()

return true.

    new URI(null,null,"a:b",null).getScheme()

return "a", it should return null and be a relative URI

If try to use relativize method to get a"a:b" as a relative URI

    URI root = new URI("file", null, "/", null);
    URI withPath = new URI("file",null,"/./a:b",null);
    URI rel = root.relativize(withPath);

You get a relative URI with a path component with a first segment that contains a colon character

    rel.isAbsolute()

return false, but normalize()

    rel.normalize()

doesn't prepend the "." segment as the Javadoc say

So in summary,

1) Instatiating an URI with only a path component parses an scheme when the first segment of the path contains a colo character
2) Normalize does not prepend the segment "." as the third step of the javadoc say

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the attached test case with assertions enabled

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No assertion should fail
ACTUAL -
Assertion are triggered, the colon character is being detected as a scheme separator when is is used on a path component

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class Test {
    public static void main(String[] args) throws Exception {
        URI withColonURI = new URI(null, null, "a:b", null);

        assert !(withColonURI.isAbsolute()) : "Should be relative";
        assert withColonURI.getScheme() == null : "Should not have a scheme";

        URI root = new URI("file", null, "/", null);
        URI withPath = new URI("file", null, "/a:b", null);
        URI rel = root.relativize(withPath);

        assert !rel.isAbsolute() : "Should be relative";
        assert rel.normalize().toASCIIString().startsWith("./") : "Normalization should prepend \".\" segment";
    }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
For calling

    new URI(null, null, "a:b", null)

always prepend the "." segment

    new URI(null, null, "./a:b", null)