Fix #2019: Introduce InteractionObjectBuilder (#2020)

* introduce interaction object helper class

* nit change

* format fix

domain/src/test/java/org/oppia/android/domain/classify/InteractionObjectTestBuilder.kt

@@ -0,0 +1,91 @@
+package org.oppia.android.domain.classify
+
+import org.oppia.android.app.model.Fraction
+import org.oppia.android.app.model.InteractionObject
+import org.oppia.android.app.model.ListOfSetsOfHtmlStrings
+import org.oppia.android.app.model.StringList
+import javax.inject.Inject
+
+/**
+ * Helper class for test cases which can provide the [InteractionObject]
+ * of a particular ObjectTypeCase.
+ */
+class InteractionObjectTestBuilder @Inject constructor() {
+
+  fun createNonNegativeInt(value: Int): InteractionObject {
+    return InteractionObject.newBuilder().setNonNegativeInt(value).build()
+  }
+
+  fun createString(value: String): InteractionObject {
+    return InteractionObject.newBuilder().setNormalizedString(value).build()
+  }
+
+  fun createListOfSetsOfHtmlStrings(listOfStringList: List<StringList>): InteractionObject {
+    val listOfSetsOfHtmlStrings = ListOfSetsOfHtmlStrings.newBuilder()
+      .addAllSetOfHtmlStrings(
+        listOfStringList
+      )
+      .build()
+
+    return InteractionObject.newBuilder().setListOfSetsOfHtmlString(listOfSetsOfHtmlStrings).build()
+  }
+
+  fun createHtmlStringList(vararg items: String): StringList {
+    return StringList.newBuilder().addAllHtml(items.toList()).build()
+  }
+
+  fun createWholeNumber(isNegative: Boolean, value: Int): InteractionObject {
+    // Whole number fractions imply '0/1' fractional parts.
+    return InteractionObject.newBuilder().setFraction(
+      Fraction.newBuilder()
+        .setIsNegative(isNegative)
+        .setWholeNumber(value)
+        .setNumerator(0)
+        .setDenominator(1)
+        .build()
+    ).build()
+  }
+
+  fun createFraction(
+    isNegative: Boolean,
+    numerator: Int,
+    denominator: Int
+  ): InteractionObject {
+    // Fraction-only numbers imply no whole number.
+    return InteractionObject.newBuilder().setFraction(
+      Fraction.newBuilder()
+        .setIsNegative(isNegative)
+        .setNumerator(numerator)
+        .setDenominator(denominator)
+        .build()
+    ).build()
+  }
+
+  fun createMixedNumber(
+    isNegative: Boolean,
+    wholeNumber: Int,
+    numerator: Int,
+    denominator: Int
+  ): InteractionObject {
+    return InteractionObject.newBuilder().setFraction(
+      Fraction.newBuilder()
+        .setIsNegative(isNegative)
+        .setWholeNumber(wholeNumber)
+        .setNumerator(numerator)
+        .setDenominator(denominator)
+        .build()
+    ).build()
+  }
+
+  fun createSignedInt(value: Int): InteractionObject {
+    return InteractionObject.newBuilder().setSignedInt(value).build()
+  }
+
+  fun createReal(value: Double): InteractionObject {
+    return InteractionObject.newBuilder().setReal(value).build()
+  }
+
+  fun createInt(value: Int): InteractionObject {
+    return InteractionObject.newBuilder().setReal(value.toDouble()).build()
+  }
+}