Add toBuilder method to create a builder from a properties object

configlib-core/src/main/java/de/exlll/configlib/ConfigurationProperties.java

@@ -40,7 +40,20 @@ class ConfigurationProperties {
         return new BuilderImpl();
     }
 
+    /**
+     * Creates a new builder and initializes it with values taken from this properties object.
+     *
+     * @return new builder
+     */
+    public Builder<?> toBuilder() {
+        return new BuilderImpl(this);
+    }
+
     private static final class BuilderImpl extends Builder<BuilderImpl> {
+        private BuilderImpl() {}
+
+        private BuilderImpl(ConfigurationProperties properties) {super(properties);}
+
         @Override
         protected BuilderImpl getThis() {return this;}
 
@@ -64,6 +77,15 @@ class ConfigurationProperties {
 
         protected Builder() {}
 
+        protected Builder(ConfigurationProperties properties) {
+            this.serializersByType.putAll(properties.serializersByType);
+            this.formatter = properties.formatter;
+            this.filter = properties.filter;
+            this.outputNulls = properties.outputNulls;
+            this.inputNulls = properties.inputNulls;
+            this.serializeSetsAsLists = properties.serializeSetsAsLists;
+        }
+
         /**
          * Sets the field filter. The given filter is applied in addition to and
          * after the default filter.

configlib-core/src/main/java/de/exlll/configlib/FileConfigurationProperties.java

@@ -31,7 +31,15 @@ public class FileConfigurationProperties extends ConfigurationProperties {
         return new BuilderImpl();
     }
 
+    public Builder<?> toBuilder() {
+        return new BuilderImpl(this);
+    }
+
     private static final class BuilderImpl extends Builder<BuilderImpl> {
+        private BuilderImpl() {}
+
+        private BuilderImpl(FileConfigurationProperties properties) {super(properties);}
+
         @Override
         protected BuilderImpl getThis() {return this;}
 
@@ -51,10 +59,23 @@ public class FileConfigurationProperties extends ConfigurationProperties {
         private boolean createParentDirectories = true;
 
         /**
-         * A constructor that can be overridden by subclasses.
+         * The default constructor.
          */
         protected Builder() {}
 
+        /**
+         * A constructor that initializes this builder with values taken from the properties object.
+         *
+         * @param properties the properties object the values are taken from
+         * @throws NullPointerException if {@code properties} is null
+         */
+        protected Builder(FileConfigurationProperties properties) {
+            super(properties);
+            this.header = properties.header;
+            this.footer = properties.footer;
+            this.createParentDirectories = properties.createParentDirectories;
+        }
+
         /**
          * Sets the header. The header is written as a comment before the actual configuration.
          * If the header is set to null (the default), nothing is written.

configlib-core/src/main/java/de/exlll/configlib/YamlConfigurationProperties.java

@@ -24,7 +24,15 @@ public final class YamlConfigurationProperties extends FileConfigurationProperti
         return new BuilderImpl();
     }
 
+    public Builder<?> toBuilder() {
+        return new BuilderImpl(this);
+    }
+
     private static final class BuilderImpl extends Builder<BuilderImpl> {
+        private BuilderImpl() {}
+
+        private BuilderImpl(YamlConfigurationProperties properties) {super(properties);}
+
         @Override
         protected BuilderImpl getThis() {return this;}
 
@@ -41,10 +49,20 @@ public final class YamlConfigurationProperties extends FileConfigurationProperti
             extends FileConfigurationProperties.Builder<B> {
 
         /**
-         * A constructor that can be overridden by subclasses.
+         * The default constructor.
          */
         protected Builder() {}
 
+        /**
+         * A constructor that initializes this builder with values taken from the properties object.
+         *
+         * @param properties the properties object the values are taken from
+         * @throws NullPointerException if {@code properties} is null
+         */
+        protected Builder(YamlConfigurationProperties properties) {
+            super(properties);
+        }
+
         /**
          * Builds a {@code ConfigurationProperties} instance.
          *

configlib-core/src/test/java/de/exlll/configlib/ConfigurationPropertiesTest.java

@@ -48,6 +48,31 @@ class ConfigurationPropertiesTest {
         assertThat(properties.getFieldFilter(), sameInstance(filter));
     }
 
+    @Test
+    void builderCtorCopiesValues() {
+        FieldFormatter formatter = field -> field.getName().toLowerCase(Locale.ROOT);
+        FieldFilter filter = field -> field.getName().startsWith("f");
+        TestUtils.PointSerializer serializer = new TestUtils.PointSerializer();
+
+        ConfigurationProperties properties = ConfigurationProperties.newBuilder()
+                .addSerializer(Point.class, serializer)
+                .setFieldFormatter(formatter)
+                .setFieldFilter(filter)
+                .outputNulls(true)
+                .inputNulls(true)
+                .serializeSetsAsLists(false)
+                .build()
+                .toBuilder()
+                .build();
+
+        assertThat(properties.getSerializers(), is(Map.of(Point.class, serializer)));
+        assertThat(properties.outputNulls(), is(true));
+        assertThat(properties.inputNulls(), is(true));
+        assertThat(properties.serializeSetsAsLists(), is(false));
+        assertThat(properties.getFieldFormatter(), sameInstance(formatter));
+        assertThat(properties.getFieldFilter(), sameInstance(filter));
+    }
+
     @Test
     void builderSerializersUnmodifiable() {
         ConfigurationProperties properties = ConfigurationProperties.newBuilder().build();

configlib-core/src/test/java/de/exlll/configlib/FileConfigurationPropertiesTest.java

@@ -2,6 +2,8 @@ package de.exlll.configlib;
 
 import org.junit.jupiter.api.Test;
 
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
 import static org.junit.jupiter.api.Assertions.*;
 
 class FileConfigurationPropertiesTest {
@@ -24,4 +26,21 @@ class FileConfigurationPropertiesTest {
         assertEquals("THE FOOTER", properties.getFooter());
         assertFalse(properties.createParentDirectories());
     }
+
+    @Test
+    void builderCtorCopiesValues() {
+        FileConfigurationProperties properties = FileConfigurationProperties.newBuilder()
+                .outputNulls(true)
+                .header("A")
+                .footer("B")
+                .createParentDirectories(false)
+                .build()
+                .toBuilder()
+                .build();
+
+        assertThat(properties.outputNulls(), is(true));
+        assertThat(properties.getHeader(), is("A"));
+        assertThat(properties.getFooter(), is("B"));
+        assertThat(properties.createParentDirectories(), is(false));
+    }
 }

configlib-core/src/test/java/de/exlll/configlib/YamlConfigurationPropertiesTest.java

@@ -0,0 +1,22 @@
+package de.exlll.configlib;
+
+import org.junit.jupiter.api.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+import static org.junit.jupiter.api.Assertions.*;
+
+class YamlConfigurationPropertiesTest {
+    @Test
+    void builderCtorCopiesValues() {
+        YamlConfigurationProperties properties = YamlConfigurationProperties.newBuilder()
+                .outputNulls(true)
+                .header("A")
+                .build()
+                .toBuilder()
+                .build();
+
+        assertThat(properties.outputNulls(), is(true));
+        assertThat(properties.getHeader(), is("A"));
+    }
+}