Merge pull request #5449 from kcacademic/kotlin-interfaces

Adding files for the article BAEL-2206

core-kotlin/src/main/kotlin/com/baeldung/interfaces/ConflictingInterfaces.kt

@@ -0,0 +1,23 @@
+package com.baeldung.interfaces
+
+interface BaseInterface {
+    fun someMethod(): String
+}
+
+interface FirstChildInterface : BaseInterface {
+    override fun someMethod(): String {
+        return("Hello, from someMethod in FirstChildInterface")
+    }
+}
+
+interface SecondChildInterface : BaseInterface {
+    override fun someMethod(): String {
+        return("Hello, from someMethod in SecondChildInterface")
+    }
+}
+
+class ChildClass : FirstChildInterface, SecondChildInterface {
+    override fun someMethod(): String {
+        return super<SecondChildInterface>.someMethod()
+    }
+}

core-kotlin/src/main/kotlin/com/baeldung/interfaces/InterfaceDelegation.kt

@@ -0,0 +1,13 @@
+package com.baeldung.interfaces
+
+interface MyInterface {
+    fun someMethod(): String
+}
+
+class MyClass() : MyInterface {
+    override fun someMethod(): String {
+        return("Hello, World!")
+    }
+}
+
+class MyDerivedClass(myInterface: MyInterface) : MyInterface by myInterface

core-kotlin/src/main/kotlin/com/baeldung/interfaces/MultipleInterfaces.kt

@@ -0,0 +1,29 @@
+package com.baeldung.interfaces
+
+interface FirstInterface {	
+    fun someMethod(): String
+	
+    fun anotherMethod(): String {
+        return("Hello, from anotherMethod in FirstInterface")
+    }
+}
+
+interface SecondInterface {
+    fun someMethod(): String {
+        return("Hello, from someMethod in SecondInterface")
+    }
+	
+    fun anotherMethod(): String {
+        return("Hello, from anotherMethod in SecondInterface")
+    }
+}
+
+class SomeClass: FirstInterface, SecondInterface {	
+    override fun someMethod(): String {
+        return("Hello, from someMethod in SomeClass")
+    }
+	
+    override fun anotherMethod(): String {
+        return("Hello, from anotherMethod in SomeClass")
+    }
+}

core-kotlin/src/main/kotlin/com/baeldung/interfaces/SimpleInterface.kt

@@ -0,0 +1,24 @@
+package com.baeldung.interfaces
+
+interface SimpleInterface {
+    val firstProp: String
+    val secondProp: String
+        get() = "Second Property"
+    fun firstMethod(): String
+    fun secondMethod(): String {
+        println("Hello, from: " + secondProp)
+        return ""
+    }
+}
+
+class SimpleClass: SimpleInterface {
+    override val firstProp: String = "First Property"
+    override val secondProp: String
+        get() = "Second Property, Overridden!"
+    override fun firstMethod(): String {
+        return("Hello, from: " + firstProp)
+    }
+    override fun secondMethod(): String {
+        return("Hello, from: " + secondProp + firstProp)
+    }
+}

core-kotlin/src/test/kotlin/com/baeldung/interfaces/InterfaceExamplesUnitTest.kt

@@ -0,0 +1,32 @@
+package com.baeldung.interfaces
+
+import org.junit.Test
+import kotlin.test.assertEquals
+
+class InterfaceExamplesUnitTest {
+    @Test
+    fun givenAnInterface_whenImplemented_thenBehavesAsOverridden() {
+        val simpleClass = SimpleClass()
+        assertEquals("Hello, from: First Property", simpleClass.firstMethod())
+        assertEquals("Hello, from: Second Property, Overridden!First Property", simpleClass.secondMethod())
+    }
+
+    @Test
+    fun givenMultipleInterfaces_whenImplemented_thenBehavesAsOverridden() {
+        val someClass = SomeClass()
+        assertEquals("Hello, from someMethod in SomeClass", someClass.someMethod())
+        assertEquals("Hello, from anotherMethod in SomeClass", someClass.anotherMethod())
+    }
+
+    @Test
+    fun givenConflictingInterfaces_whenImplemented_thenBehavesAsOverridden() {
+        val childClass = ChildClass()
+        assertEquals("Hello, from someMethod in SecondChildInterface", childClass.someMethod())
+    }
+
+    @Test
+    fun givenAnInterface_whenImplemented_thenBehavesAsDelegated() {
+        val myClass = MyClass()
+        assertEquals("Hello, World!", MyDerivedClass(myClass).someMethod())
+    }
+}