Add method to get a list of alternate certificates

acme4j-client/src/main/java/org/shredzone/acme4j/Certificate.java

@@ -24,6 +24,7 @@ import java.security.cert.X509Certificate;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.stream.Collectors;
 
 import edu.umd.cs.findbugs.annotations.Nullable;
 import org.shredzone.acme4j.connector.Connection;
@@ -110,6 +111,19 @@ public class Certificate extends AcmeResource {
         }
     }
 
+    /**
+     * Returns alternate certificate chains, if available.
+     *
+     * @return Alternate certificate chains, or empty if there are none.
+     * @since 2.11
+     */
+    public List<Certificate> getAlternateCertificates() {
+        Login login = getLogin();
+        return getAlternates().stream()
+                .map(login::bindCertificate)
+                .collect(Collectors.toList());
+    }
+
     /**
      * Writes the certificate to the given writer. It is written in PEM format, with the
      * end-entity cert coming first, followed by the intermediate ceritificates.

acme4j-client/src/test/java/org/shredzone/acme4j/CertificateTest.java

@@ -113,6 +113,11 @@ public class CertificateTest {
         assertThat(cert.getAlternates().get(0), is(url("https://example.com/acme/alt-cert/1")));
         assertThat(cert.getAlternates().get(1), is(url("https://example.com/acme/alt-cert/2")));
 
+        assertThat(cert.getAlternateCertificates(), is(notNullValue()));
+        assertThat(cert.getAlternateCertificates().size(), is(2));
+        assertThat(cert.getAlternateCertificates().get(0).getLocation(), is(url("https://example.com/acme/alt-cert/1")));
+        assertThat(cert.getAlternateCertificates().get(1).getLocation(), is(url("https://example.com/acme/alt-cert/2")));
+
         provider.close();
     }