feat: add a checkRequired builder helper for uniform required-field validation

sdk-core/api/sdk-core.api

@@ -56,6 +56,10 @@ public abstract interface class org/dexpace/sdk/core/generics/Builder {
 	public abstract fun build ()Ljava/lang/Object;
 }
 
+public final class org/dexpace/sdk/core/generics/BuilderChecksKt {
+	public static final fun checkRequired (Ljava/lang/String;Ljava/lang/Object;)Ljava/lang/Object;
+}
+
 public final class org/dexpace/sdk/core/http/auth/AuthChallengeParser {
 	public static final field INSTANCE Lorg/dexpace/sdk/core/http/auth/AuthChallengeParser;
 	public static final fun parse (Ljava/lang/String;)Ljava/util/List;

sdk-core/src/main/kotlin/org/dexpace/sdk/core/generics/BuilderChecks.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2026 dexpace and Omar Aljarrah
+ *
+ * Licensed under the MIT License. See LICENSE in the project root.
+ * SPDX-License-Identifier: MIT
+ */
+
+package org.dexpace.sdk.core.generics
+
+/**
+ * Asserts that a required builder field [value] has been set, returning it non-null.
+ *
+ * Builders that materialize an immutable value in [Builder.build] use this to validate
+ * mandatory inputs in one uniform place. When [value] is `null` it throws
+ * [IllegalStateException] with the message `"<name> is required"`, where [name] is the
+ * caller-supplied field name (typically the builder property, e.g. `"method"`); when
+ * [value] is present it is returned as a non-null reference so the result can be assigned
+ * directly:
+ *
+ * ```
+ * override fun build(): Request =
+ *     Request(
+ *         method = checkRequired("method", method),
+ *         url = checkRequired("url", url),
+ *         // ...
+ *     )
+ * ```
+ *
+ * Replacing the ad-hoc `checkNotNull(field) { "Field is required." }` calls scattered
+ * across builders with this helper keeps the missing-field diagnostics identical, which
+ * matters most once builders are emitted by codegen.
+ *
+ * @param name The field name to report in the failure message.
+ * @param value The field value to validate.
+ * @return [value], guaranteed non-null.
+ * @throws IllegalStateException If [value] is `null`.
+ */
+public fun <T : Any> checkRequired(
+    name: String,
+    value: T?,
+): T = checkNotNull(value) { "$name is required" }

sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/request/Request.kt

@@ -8,6 +8,7 @@
 package org.dexpace.sdk.core.http.request
 
 import org.dexpace.sdk.core.generics.Builder
+import org.dexpace.sdk.core.generics.checkRequired
 import org.dexpace.sdk.core.http.common.Headers
 import org.dexpace.sdk.core.http.common.HttpHeaderName
 import java.net.MalformedURLException
@@ -251,8 +252,8 @@ public data class Request private constructor(
          */
         override fun build(): Request =
             Request(
-                method = checkNotNull(method) { "Method is required." },
-                url = checkNotNull(url) { "URL is required." },
+                method = checkRequired("method", method),
+                url = checkRequired("url", url),
                 headers = headersBuilder.build(),
                 body = body,
             )

sdk-core/src/main/kotlin/org/dexpace/sdk/core/http/response/Response.kt

@@ -8,6 +8,7 @@
 package org.dexpace.sdk.core.http.response
 
 import org.dexpace.sdk.core.generics.Builder
+import org.dexpace.sdk.core.generics.checkRequired
 import org.dexpace.sdk.core.http.common.Headers
 import org.dexpace.sdk.core.http.common.HttpHeaderName
 import org.dexpace.sdk.core.http.common.Protocol
@@ -252,9 +253,9 @@ public data class Response private constructor(
          */
         override fun build(): Response =
             Response(
-                request = checkNotNull(request) { "request is required" },
-                protocol = checkNotNull(protocol) { "protocol is required" },
-                status = checkNotNull(status) { "status is required" },
+                request = checkRequired("request", request),
+                protocol = checkRequired("protocol", protocol),
+                status = checkRequired("status", status),
                 message = message,
                 headers = headersBuilder.build(),
                 body = body,

sdk-core/src/test/kotlin/org/dexpace/sdk/core/generics/BuilderChecksTest.kt

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2026 dexpace and Omar Aljarrah
+ *
+ * Licensed under the MIT License. See LICENSE in the project root.
+ * SPDX-License-Identifier: MIT
+ */
+
+package org.dexpace.sdk.core.generics
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertSame
+
+class BuilderChecksTest {
+    @Test
+    fun `returns the value when present`() {
+        val value = "GET"
+        assertEquals(value, checkRequired("method", value))
+    }
+
+    @Test
+    fun `returns the same instance when present`() {
+        val value = listOf(1, 2, 3)
+        assertSame(value, checkRequired("items", value))
+    }
+
+    @Test
+    fun `throws IllegalStateException naming the field when value is null`() {
+        val ex =
+            assertFailsWith<IllegalStateException> {
+                checkRequired<String>("method", null)
+            }
+        assertEquals("method is required", ex.message)
+    }
+
+    @Test
+    fun `message uses the supplied field name verbatim`() {
+        val ex =
+            assertFailsWith<IllegalStateException> {
+                checkRequired<Int>("status", null)
+            }
+        assertEquals("status is required", ex.message)
+    }
+}

sdk-core/src/test/kotlin/org/dexpace/sdk/core/http/request/RequestTest.kt

@@ -207,7 +207,7 @@ class RequestTest {
             assertFailsWith<IllegalStateException> {
                 Request.builder().url("https://example.test").build()
             }
-        assertEquals("Method is required.", ex.message)
+        assertEquals("method is required", ex.message)
     }
 
     @Test
@@ -216,7 +216,7 @@ class RequestTest {
             assertFailsWith<IllegalStateException> {
                 Request.builder().method(Method.GET).build()
             }
-        assertEquals("URL is required.", ex.message)
+        assertEquals("url is required", ex.message)
     }
 
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