ChainOfResponsibility.kt

import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.Test

interface HeadersChain {
    fun addHeader(inputHeader: String): String
}

class AuthenticationHeader(val token: String?, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String {
        token ?: throw IllegalStateException("Token should be not null")
        return inputHeader + "Authorization: Bearer $token\n"
            .let { next?.addHeader(it) ?: it }
    }
}

class ContentTypeHeader(val contentType: String, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String =
        inputHeader + "ContentType: $contentType\n"
            .let { next?.addHeader(it) ?: it }
}

class BodyPayload(val body: String, var next: HeadersChain? = null) : HeadersChain {

    override fun addHeader(inputHeader: String): String =
        inputHeader + "$body"
            .let { next?.addHeader(it) ?: it }
}

class ChainOfResponsibilityTest {

    @Test
    fun `Chain Of Responsibility`() {
        //create chain elements
        val authenticationHeader = AuthenticationHeader("123456")
        val contentTypeHeader = ContentTypeHeader("json")
        val messageBody = BodyPayload("Body:\n{\n\"username\"=\"dbacinski\"\n}")

        //construct chain
        authenticationHeader.next = contentTypeHeader
        contentTypeHeader.next = messageBody

        //execute chain
        val messageWithAuthentication =
            authenticationHeader.addHeader("Headers with Authentication:\n")

        println(messageWithAuthentication)

        val messageWithoutAuth =
            contentTypeHeader.addHeader("Headers:\n")
        println(messageWithoutAuth)

        assertThat(messageWithAuthentication).isEqualTo(
            """
                Headers with Authentication:
                Authorization: Bearer 123456
                ContentType: json
                Body:
                {
                "username"="dbacinski"
                }
            """.trimIndent()
        )

        assertThat(messageWithoutAuth).isEqualTo(
            """
                Headers:
                ContentType: json
                Body:
                {
                "username"="dbacinski"
                }
            """.trimIndent()
        )
    }
}