Allow creating a builder from a pre-existing purl

This is useful if you have an existing purl and need to modify one or
more of its components.

This differs from `PackageURL::toBuilder` in that it doesn't require
the direct use of the `PackageURL` constructor which conflicts with the
builder pattern.

Author: dwalluck

src/main/java/com/github/packageurl/PackageURL.java

@@ -147,7 +147,6 @@ public PackageURL(final String type, final String namespace, final String name,
     private String subpath;
 
     public PackageURLBuilder toBuilder() {
-
         PackageURLBuilder builder = PackageURLBuilder.aPackageURL()
                 .withType(getType())
                 .withNamespace(getNamespace())
@@ -160,7 +159,6 @@ public PackageURLBuilder toBuilder() {
         }
 
         return builder;
-
     }
 
     /**

src/main/java/com/github/packageurl/PackageURLBuilder.java

@@ -47,6 +47,26 @@ public static PackageURLBuilder aPackageURL() {
         return new PackageURLBuilder();
     }
 
+    /**
+     * Obtain a reference to a new builder object initialized with the existing {@link PackageURL} object.
+     *
+     * @param packageURL the existing Package URL object
+     * @return a new builder object.
+     */
+    public static PackageURLBuilder aPackageURL(final PackageURL packageURL) {
+        return packageURL.toBuilder();
+    }
+
+    /**
+     * Obtain a reference to a new builder object initialized with the existing Package URL string.
+     *
+     * @param purl the existing Package URL string
+     * @return a new builder object.
+     */
+    public static PackageURLBuilder aPackageURL(final String purl) throws MalformedPackageURLException {
+        return new PackageURL(purl).toBuilder();
+    }
+
     /**
      * Adds the package URL type.
      *

src/test/java/com/github/packageurl/PackageURLBuilderTest.java

@@ -193,6 +193,16 @@ public void testEditBuilder1() throws MalformedPackageURLException {
 
     }
 
+    @Test
+    public void testFromExistingPurl() throws MalformedPackageURLException {
+        String purl = "pkg:generic/namespace/[email protected]?k=v#s";
+        PackageURL p = new PackageURL(purl);
+        PackageURL purl2 = PackageURLBuilder.aPackageURL(p).build();
+        PackageURL purl3 = PackageURLBuilder.aPackageURL(purl).build();
+        assertEquals(p, purl2);
+        assertEquals(purl2, purl3);
+    }
+
     private void assertBuilderMatch(PackageURL expected, PackageURLBuilder actual) throws MalformedPackageURLException {
 
         Assert.assertEquals(expected.toString(), actual.build().toString());
@@ -216,4 +226,4 @@ private void assertBuilderMatch(PackageURL expected, PackageURLBuilder actual) t
 
     }
 
-}
+}