diff --git a/kmath-nd4j/src/main/kotlin/scientifik.kmath.nd4j/INDArrayStructures.kt b/kmath-nd4j/src/main/kotlin/scientifik.kmath.nd4j/INDArrayStructures.kt
index f39d84716..66aa00fac 100644
--- a/kmath-nd4j/src/main/kotlin/scientifik.kmath.nd4j/INDArrayStructures.kt
+++ b/kmath-nd4j/src/main/kotlin/scientifik.kmath.nd4j/INDArrayStructures.kt
@@ -14,25 +14,30 @@ interface INDArrayStructure<T> : NDStructure<T> {
     override fun elements(): Sequence<Pair<IntArray, T>> = Sequence(::elementsIterator)
 }
 
-data class INDArrayIntStructure(override val ndArray: INDArray) : INDArrayStructure<Int>, MutableNDStructure<Int> {
+inline class INDArrayIntStructure(override val ndArray: INDArray) : INDArrayStructure<Int>, MutableNDStructure<Int> {
     override fun elementsIterator(): Iterator<Pair<IntArray, Int>> = INDArrayIntIterator(ndArray)
     override fun get(index: IntArray): Int = ndArray.getInt(*index)
     override fun set(index: IntArray, value: Int): Unit = run { ndArray.putScalar(index, value) }
+    override fun toString(): String = "INDArrayIntStructure(ndArray=$ndArray)"
 }
 
-data class INDArrayLongStructure(override val ndArray: INDArray) : INDArrayStructure<Long> {
+inline class INDArrayLongStructure(override val ndArray: INDArray) : INDArrayStructure<Long> {
     override fun elementsIterator(): Iterator<Pair<IntArray, Long>> = INDArrayLongIterator(ndArray)
     override fun get(index: IntArray): Long = ndArray.getLong(*widenToLongArray(index))
+    override fun toString(): String = "INDArrayLongStructure(ndArray=$ndArray)"
+
 }
 
-data class INDArrayDoubleStructure(override val ndArray: INDArray) : INDArrayStructure<Double>, MutableNDStructure<Double> {
+inline class INDArrayDoubleStructure(override val ndArray: INDArray) : INDArrayStructure<Double>, MutableNDStructure<Double> {
     override fun elementsIterator(): Iterator<Pair<IntArray, Double>> = INDArrayDoubleIterator(ndArray)
     override fun get(index: IntArray): Double = ndArray.getDouble(*index)
     override fun set(index: IntArray, value: Double): Unit = run { ndArray.putScalar(index, value) }
+    override fun toString(): String = "INDArrayDoubleStructure(ndArray=$ndArray)"
 }
 
-data class INDArrayFloatStructure(override val ndArray: INDArray) : INDArrayStructure<Float>, MutableNDStructure<Float> {
+inline class INDArrayFloatStructure(override val ndArray: INDArray) : INDArrayStructure<Float>, MutableNDStructure<Float> {
     override fun elementsIterator(): Iterator<Pair<IntArray, Float>> = INDArrayFloatIterator(ndArray)
     override fun get(index: IntArray): Float = ndArray.getFloat(*index)
     override fun set(index: IntArray, value: Float): Unit = run { ndArray.putScalar(index, value) }
+    override fun toString(): String = "INDArrayFloatStructure(ndArray=$ndArray)"
 }
diff --git a/kmath-nd4j/src/test/kotlin/scientifik/kmath/nd4j/INDArrayStructureTest.kt b/kmath-nd4j/src/test/kotlin/scientifik/kmath/nd4j/INDArrayStructureTest.kt
index 77565856a..ad1cbb585 100644
--- a/kmath-nd4j/src/test/kotlin/scientifik/kmath/nd4j/INDArrayStructureTest.kt
+++ b/kmath-nd4j/src/test/kotlin/scientifik/kmath/nd4j/INDArrayStructureTest.kt
@@ -4,6 +4,7 @@ import org.nd4j.linalg.factory.Nd4j
 import scientifik.kmath.structures.get
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertNotEquals
 
 internal class INDArrayStructureTest {
     @Test
@@ -25,9 +26,25 @@ internal class INDArrayStructureTest {
     fun testEquals() {
         val nd1 = Nd4j.create(doubleArrayOf(1.0, 2.0, 3.0))!!
         val struct1 = INDArrayDoubleStructure(nd1)
+        assertEquals(struct1, struct1)
+        assertNotEquals(struct1, null as INDArrayDoubleStructure?)
         val nd2 = Nd4j.create(doubleArrayOf(1.0, 2.0, 3.0))!!
         val struct2 = INDArrayDoubleStructure(nd2)
         assertEquals(struct1, struct2)
+        assertEquals(struct2, struct1)
+        val nd3 = Nd4j.create(doubleArrayOf(1.0, 2.0, 3.0))!!
+        val struct3 = INDArrayDoubleStructure(nd3)
+        assertEquals(struct2, struct3)
+        assertEquals(struct1, struct3)
+    }
+
+    @Test
+    fun testHashCode() {
+        val nd1 = Nd4j.create(doubleArrayOf(1.0, 2.0, 3.0))!!
+        val struct1 = INDArrayDoubleStructure(nd1)
+        val nd2 = Nd4j.create(doubleArrayOf(1.0, 2.0, 3.0))!!
+        val struct2 = INDArrayDoubleStructure(nd2)
+        assertEquals(struct1.hashCode(), struct2.hashCode())
     }
 
     @Test