FULL PRODUCT VERSION :
java version "9.0.1"
Java(TM) SE Runtime Environment (build 9.0.1+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.1+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Linux 123 4.13.11-1-ARCH #1 SMP PREEMPT Thu Nov 2 10:25:56 CET 2017 x86_64 GNU/Linux

EXTRA RELEVANT SYSTEM CONFIGURATION :
hostname is a number

A DESCRIPTION OF THE PROBLEM :
new java.net.Socket(java.net.InetAddress.getLocalHost(), 8080);

If the hostname is a number which is allowed by RFC 1123 the following exception will be raised :

java.net.SocketException thrown: Invalid argument or cannot assign requested address
| at PlainSocketImpl.socketConnect (Native Method)
| at AbstractPlainSocketImpl.doConnect (AbstractPlainSocketImpl.java:400)
| at AbstractPlainSocketImpl.connectToAddress (AbstractPlainSocketImpl.java:243)
| at AbstractPlainSocketImpl.connect (AbstractPlainSocketImpl.java:225)
| at SocksSocketImpl.connect (SocksSocketImpl.java:402)
| at Socket.connect (Socket.java:591)
| at Socket.connect (Socket.java:540)
| at Socket.<init> (Socket.java:436)
| at Socket.<init> (Socket.java:246)
| at (#3:1)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1) Set a number as a hostname :
#hostname 123

2)Open a port on the machine :
python -m http.server 8080

2)in jshell :
new java.net.Socket(java.net.InetAddress.getLocalHost(), 8080);
#################################
I got the same result with the following class on :
Java(TM) SE Runtime Environment (build 1.8.0_144-b01)

import java.net.*;
import java.io.*;

public class TestHostName {

public static void main(String[] args) {
try {
System.out.println("hostname: " + java.net.InetAddress.getLocalHost());
Socket socket = new Socket(java.net.InetAddress.getLocalHost(), 8080);
} catch (IOException e) {
System.err.println(e);
}
}

}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The connection should be established even with a number as a hostname.
ACTUAL -
From jshell :

java.net.SocketException thrown: Invalid argument or cannot assign requested address
| at PlainSocketImpl.socketConnect (Native Method)
| at AbstractPlainSocketImpl.doConnect (AbstractPlainSocketImpl.java:400)
| at AbstractPlainSocketImpl.connectToAddress (AbstractPlainSocketImpl.java:243)
| at AbstractPlainSocketImpl.connect (AbstractPlainSocketImpl.java:225)
| at SocksSocketImpl.connect (SocksSocketImpl.java:402)
| at Socket.connect (Socket.java:591)
| at Socket.connect (Socket.java:540)
| at Socket.<init> (Socket.java:436)
| at Socket.<init> (Socket.java:246)
| at (#3:1)

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.net.SocketException thrown: Invalid argument or cannot assign requested address
| at PlainSocketImpl.socketConnect (Native Method)
| at AbstractPlainSocketImpl.doConnect (AbstractPlainSocketImpl.java:400)
| at AbstractPlainSocketImpl.connectToAddress (AbstractPlainSocketImpl.java:243)
| at AbstractPlainSocketImpl.connect (AbstractPlainSocketImpl.java:225)
| at SocksSocketImpl.connect (SocksSocketImpl.java:402)
| at Socket.connect (Socket.java:591)
| at Socket.connect (Socket.java:540)
| at Socket.<init> (Socket.java:436)
| at Socket.<init> (Socket.java:246)
| at (#3:1)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.net.*;
import java.io.*;

public class TestHostName {

public static void main(String[] args) {
try {
System.out.println("hostname: " + java.net.InetAddress.getLocalHost());
Socket socket = new Socket(java.net.InetAddress.getLocalHost(), 8080);
} catch (IOException e) {
System.err.println(e);
}
}

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Set a hostname with only letters