Base PGPainlessCLI on new sop-java module

* Rename pgpainless-sop -> pgpainless-cli
* Introduce sop-java (implementation-independent SOP API)
* Introduce sop-java-picocli (CLI frontend for sop-java)
* Introduce pgpainless-sop (implementation of sop-java using PGPainless)
* Rework pgpainless-cli (plugs pgpainless-sop into sop-java-picocli)

sop-java/src/test/java/sop/util/OptionalTest.java

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2021 Paul Schaub.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package sop.util;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class OptionalTest {
+
+    @Test
+    public void testEmpty() {
+        Optional<String> optional = new Optional<>();
+        assertEmpty(optional);
+    }
+
+    @Test
+    public void testArg() {
+        String string = "foo";
+        Optional<String> optional = new Optional<>(string);
+        assertFalse(optional.isEmpty());
+        assertTrue(optional.isPresent());
+        assertEquals(string, optional.get());
+    }
+
+    @Test
+    public void testOfEmpty() {
+        Optional<String> optional = Optional.ofEmpty();
+        assertEmpty(optional);
+    }
+
+    @Test
+    public void testNullArg() {
+        Optional<String> optional = new Optional<>(null);
+        assertEmpty(optional);
+    }
+
+    @Test
+    public void testOfWithNullArgThrows() {
+        assertThrows(NullPointerException.class, () -> Optional.of(null));
+    }
+
+    @Test
+    public void testOf() {
+        String string = "Hello, World!";
+        Optional<String> optional = Optional.of(string);
+        assertFalse(optional.isEmpty());
+        assertTrue(optional.isPresent());
+        assertEquals(string, optional.get());
+    }
+
+    @Test
+    public void testOfNullableWithNull() {
+        Optional<String> optional = Optional.ofNullable(null);
+        assertEmpty(optional);
+    }
+
+    @Test
+    public void testOfNullableWithArg() {
+        Optional<String> optional = Optional.ofNullable("bar");
+        assertEquals("bar", optional.get());
+        assertFalse(optional.isEmpty());
+        assertTrue(optional.isPresent());
+    }
+
+    private <T> void assertEmpty(Optional<T> optional) {
+        assertTrue(optional.isEmpty());
+        assertFalse(optional.isPresent());
+
+        assertNull(optional.get());
+    }
+}