Cleanup JavaVersion

This commit includes a few minor cleanups to o/e/b/JavaVersion.java:
 - Stronger argument checking in JavaVersion#parse
 - Use JDK 8 string joiner
 - Keep an immutable copy of the version sequence

core/src/main/java/org/elasticsearch/bootstrap/JavaVersion.java

@@ -19,28 +19,26 @@
 
 package org.elasticsearch.bootstrap;
 
-import org.elasticsearch.common.Strings;
-
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
+import java.util.stream.Collectors;
 
 class JavaVersion implements Comparable<JavaVersion> {
     private final List<Integer> version;
 
     public List<Integer> getVersion() {
-        return Collections.unmodifiableList(version);
+        return version;
     }
 
     private JavaVersion(List<Integer> version) {
-        this.version = version;
+        this.version = Collections.unmodifiableList(version);
     }
 
     public static JavaVersion parse(String value) {
-        if (value == null) {
-            throw new NullPointerException("value");
-        }
-        if ("".equals(value)) {
+        Objects.requireNonNull(value);
+        if (!isValid(value)) {
             throw new IllegalArgumentException("value");
         }
 
@@ -79,6 +77,6 @@ class JavaVersion implements Comparable<JavaVersion> {
 
     @Override
     public String toString() {
-        return Strings.collectionToDelimitedString(version, ".");
+        return version.stream().map(v -> Integer.toString(v)).collect(Collectors.joining("."));
     }
 }

core/src/test/java/org/elasticsearch/bootstrap/JavaVersionTests.java

@@ -36,8 +36,10 @@ public class JavaVersionTests extends ESTestCase {
     }
 
     public void testToString() {
-        JavaVersion javaVersion = JavaVersion.parse("1.7.0");
-        assertThat("1.7.0", is(javaVersion.toString()));
+        JavaVersion javaVersion170 = JavaVersion.parse("1.7.0");
+        assertThat(javaVersion170.toString(), is("1.7.0"));
+        JavaVersion javaVersion9 = JavaVersion.parse("9");
+        assertThat(javaVersion9.toString(), is("9"));
     }
 
     public void testCompare() {