Generate a GObject constructor that takes a Java class as first argum…

…ent instead of a GType, followed by varargs for (optionally) specifying property names and values.

A new utility class `VarargsUtil` was created that simplifies splitting the varargs into a "first" and "rest" component like `g_object_new` expects, and also adds a `null` at the end, in case the user forgets it.

generator/src/main/java/io/github/jwharm/javagi/configuration/ClassNames.java

@@ -50,6 +50,7 @@ public final class ClassNames {
     public static final ClassName MEMORY_CLEANER = get(PKG_INTEROP, "MemoryCleaner");
     public static final ClassName INTEROP = get(PKG_INTEROP, "Interop");
     public static final ClassName PLATFORM = get(PKG_INTEROP, "Platform");
+    public static final ClassName VARARGS_UTIL = get(PKG_INTEROP, "VarargsUtil");
 
     public static final ClassName AUTO_CLOSEABLE = get(PKG_GIO, "AutoCloseable");
     public static final ClassName LIST_MODEL_JAVA_LIST = get(PKG_GIO, "ListModelJavaList");

generator/src/main/java/io/github/jwharm/javagi/generators/ClassGenerator.java

@@ -142,10 +142,11 @@ else if (cls.isInstanceOf("GObject", "ParamSpec"))
         }
 
         if ("GObject".equals(cls.cType()))
-            builder.addMethod(gobjectConstructor())
-                    .addMethod(gobjectConstructorVarargs())
-                    .addMethod(gobjectClassConstructor())
-                    .addMethod(gobjectClassConstructorVarargs())
+            builder.addMethod(gobjectClassConstructor())
+                    .addMethod(gobjectFactory())
+                    .addMethod(gobjectFactoryVarargs())
+                    .addMethod(gobjectClassFactory())
+                    .addMethod(gobjectClassFactoryVarargs())
                     .addMethod(gobjectGetProperty())
                     .addMethod(gobjectSetProperty())
                     .addMethod(gobjectBindProperty())
@@ -224,7 +225,7 @@ protected MethodSpec paramSpecGetTypeMethod() {
                 .build();
     }
 
-    private MethodSpec gobjectConstructor() {
+    private MethodSpec gobjectFactory() {
         return MethodSpec.methodBuilder("newInstance")
                 .addJavadoc("""
                     Creates a new GObject instance of the provided GType.
@@ -243,7 +244,7 @@ private MethodSpec gobjectConstructor() {
                 .build();
     }
 
-    private MethodSpec gobjectConstructorVarargs() {
+    private MethodSpec gobjectFactoryVarargs() {
         return MethodSpec.methodBuilder("newInstance")
                 .addJavadoc("""
                     Creates a new GObject instance of the provided GType and with the
@@ -266,7 +267,7 @@ private MethodSpec gobjectConstructorVarargs() {
                 .build();
     }
 
-    private MethodSpec gobjectClassConstructor() {
+    private MethodSpec gobjectClassFactory() {
         var paramType = ParameterizedTypeName.get(
                 ClassName.get(java.lang.Class.class), TypeVariableName.get("T"));
 
@@ -292,7 +293,7 @@ private MethodSpec gobjectClassConstructor() {
                 .build();
     }
 
-    private MethodSpec gobjectClassConstructorVarargs() {
+    private MethodSpec gobjectClassFactoryVarargs() {
         var paramType = ParameterizedTypeName.get(
                 ClassName.get(java.lang.Class.class), TypeVariableName.get("T"));
 
@@ -322,6 +323,35 @@ private MethodSpec gobjectClassConstructorVarargs() {
                 .build();
     }
 
+    private MethodSpec gobjectClassConstructor() {
+        var paramType = ParameterizedTypeName.get(
+                ClassName.get(java.lang.Class.class), TypeVariableName.get("?"));
+
+        return MethodSpec.constructorBuilder()
+                .addJavadoc("""
+                    Creates a new instance of a GObject-derived class with the provided
+                    property values. For your own GObject-derived Java classes, a GType
+                    must have been registered using {@code Types.register(Class<?>)}
+                    
+                    @param  objectClass the Java class of the new GObject
+                    @param  propertyNamesAndValues pairs of property names and values
+                            (Strings and Objects). Does not need to be null-terminated.
+                    @return the newly created GObject instance
+                    @throws ClassCastException invalid property name
+                    """)
+                .addModifiers(Modifier.PUBLIC)
+                .addParameter(paramType, "objectClass")
+                .addParameter(Object[].class, "propertyNamesAndValues")
+                .varargs(true)
+                .addStatement("this(constructNew($1T.getType(objectClass),$W" +
+                                "(String) $2T.first(propertyNamesAndValues),$W" +
+                                "$2T.rest(propertyNamesAndValues)))",
+                        ClassNames.TYPE_CACHE, ClassNames.VARARGS_UTIL)
+                .addStatement("$T.put(handle(), this)",
+                        ClassNames.INSTANCE_CACHE)
+                .build();
+    }
+
     private MethodSpec gobjectGetProperty() {
         return MethodSpec.methodBuilder("getProperty")
                 .addJavadoc("""

modules/glib/src/main/java/io/github/jwharm/javagi/interop/VarargsUtil.java

@@ -0,0 +1,60 @@
+/* Java-GI - Java language bindings for GObject-Introspection-based libraries
+ * Copyright (C) 2022-2025 Jan-Willem Harmannij
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+package io.github.jwharm.javagi.interop;
+
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Arrays;
+
+/**
+ * Utility functions to split an array of variadic arguments into a first
+ * argument and a null-terminated array of remaining arguments.
+ */
+public class VarargsUtil {
+
+    /**
+     * Return the first array element.
+     *
+     * @param  array input array, can be {@code null}
+     * @param  <T>   array element type
+     * @return the first element, or {@code null} if the input array is
+     *         {@code null} or empty
+     */
+    public static <T> @Nullable T first(@Nullable T @Nullable[] array) {
+        return array == null || array.length == 0 ? null : array[0];
+    }
+
+    /**
+     * Return all but the first array elements, terminated with a {@code null}.
+     * For example, {@code [1, 2, 3]} returns {@code [2, 3, null]}.
+     *
+     * @param  array input array, can be {@code null}
+     * @param  <T>   array element type
+     * @return a new array of all elements except the first, terminated with a
+     *         {@code null}, or {@code null} if the input array is {@code null}
+     *         or empty
+     */
+    @SuppressWarnings("unchecked") // cast is safe because the array is empty
+    public static <T> @Nullable T @Nullable[] rest(@Nullable T @Nullable[] array) {
+        return array == null ? null
+                : array.length == 0 ? (T[]) new Object[] {}
+                : Arrays.copyOfRange(array, 1, array.length + 1);
+    }
+}

modules/gobject/src/test/java/io/github/jwharm/javagi/test/gobject/DerivedClassTest.java

@@ -112,6 +112,18 @@ public void writeAndReadBooleanProperty() {
         assertEquals(input2, object.getProperty("bool-property"));
     }
 
+    @Test
+    public void constructWithClassParameter() {
+        TestObject object = new TestObject();
+        assertEquals(object.readGClass().readGType(), TestObject.gtype);
+
+        String input1 = "abc";
+        boolean input2 = true;
+        TestObject object2 = new TestObject(input1, input2);
+        assertEquals(input1, object2.getProperty("string-property"));
+        assertEquals(input2, object2.getProperty("bool-property"));
+    }
+
     /**
      * Simple GObject-derived class used in the above tests
      */
@@ -122,6 +134,16 @@ public TestObject(MemorySegment address) {
             super(address);
         }
 
+        public TestObject() {
+            super(TestObject.class);
+        }
+
+        public TestObject(String stringValue, boolean booleanValue) {
+            super(TestObject.class,
+                    "string-property", stringValue,
+                    "bool-property", booleanValue);
+        }
+
         @ClassInit
         public static void construct(GObject.ObjectClass typeClass) {
             classInitHasRun = true;