BAEL-2582 Building DSLs in Kotlin

* added basic SQL DSL
* basic SQL DSL tests

core-kotlin/src/main/kotlin/com/baeldung/kotlin/dsl/SqlDsl.kt

@@ -0,0 +1,114 @@
+package com.baeldung.kotlin.dsl
+
+abstract class Condition {
+
+    fun and(initializer: Condition.() -> Unit) {
+        addCondition(And().apply(initializer))
+    }
+
+    fun or(initializer: Condition.() -> Unit) {
+        addCondition(Or().apply(initializer))
+    }
+
+    infix fun String.eq(value: Any?) {
+        addCondition(Eq(this, value))
+    }
+
+    protected abstract fun addCondition(condition: Condition)
+}
+
+open class CompositeCondition(private val sqlOperator: String) : Condition() {
+    private val conditions = mutableListOf<Condition>()
+
+    override fun addCondition(condition: Condition) {
+        conditions += condition
+    }
+
+    override fun toString(): String {
+        return if (conditions.size == 1) {
+            conditions.first().toString()
+        } else {
+            conditions.joinToString(prefix = "(", postfix = ")", separator = " $sqlOperator ") {
+                "$it"
+            }
+        }
+    }
+}
+
+class And : CompositeCondition("and")
+
+class Or : CompositeCondition("or")
+
+class Eq(private val column: String, private val value: Any?) : Condition() {
+
+    init {
+        if (value != null && value !is Number && value !is String) {
+            throw IllegalArgumentException("Only <null>, numbers and strings values can be used in the 'where' clause")
+        }
+    }
+
+    override fun addCondition(condition: Condition) {
+        throw IllegalStateException("Can't add a nested condition to the sql 'eq'")
+    }
+
+    override fun toString(): String {
+        return when (value) {
+            null -> "$column is null"
+            is String -> "$column = '$value'"
+            else -> "$column = $value"
+        }
+    }
+}
+
+class SqlSelectBuilder {
+
+    private val columns = mutableListOf<String>()
+    private lateinit var table: String
+    private var condition: Condition? = null
+
+    fun select(vararg columns: String) {
+        if (columns.isEmpty()) {
+            throw IllegalArgumentException("At least one column should be defined")
+        }
+        if (this.columns.isNotEmpty()) {
+            throw IllegalStateException("Detected an attempt to re-define columns to fetch. Current columns list: "
+                                        + "${this.columns}, new columns list: $columns")
+        }
+        this.columns.addAll(columns)
+    }
+
+    fun from(table: String) {
+        this.table = table
+    }
+
+    fun where(initializer: Condition.() -> Unit) {
+        condition = And().apply(initializer)
+    }
+
+    fun build(): String {
+        if (!::table.isInitialized) {
+            throw IllegalStateException("Failed to build an sql select - target table is undefined")
+        }
+        return toString()
+    }
+
+    override fun toString(): String {
+        val columnsToFetch =
+                if (columns.isEmpty()) {
+                    "*"
+                } else {
+                    columns.joinToString(", ")
+                }
+        val conditionString =
+                if (condition == null) {
+                    ""
+                } else {
+                    " where $condition"
+                }
+        return "select $columnsToFetch from $table$conditionString"
+    }
+}
+
+fun query(initializer: SqlSelectBuilder.() -> Unit): SqlSelectBuilder {
+    return SqlSelectBuilder().apply(initializer)
+}

core-kotlin/src/test/kotlin/com/baeldung/kotlin/dsl/SqlDslTest.kt

@@ -0,0 +1,74 @@
+package com.baeldung.kotlin.dsl
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.Test
+import java.lang.Exception
+
+class SqlDslTest {
+
+    @Test
+    fun `when no columns are specified then star is used`() {
+        doTest("select * from table1") {
+            from ("table1")
+        }
+    }
+
+    @Test
+    fun `when no condition is specified then correct query is built`() {
+        doTest("select column1, column2 from table1") {
+            select("column1", "column2")
+            from ("table1")
+        }
+    }
+
+    @Test(expected = Exception::class)
+    fun `when no table is specified then an exception is thrown`() {
+        query {
+            select("column1")
+        }.build()
+    }
+
+    @Test
+    fun `when a list of conditions is specified then it's respected`() {
+        doTest("select * from table1 where (column3 = 4 and column4 is null)") {
+            from ("table1")
+            where {
+                "column3" eq 4
+                "column4" eq null
+            }
+        }
+    }
+
+    @Test
+    fun `when 'or' conditions are specified then they are respected`() {
+        doTest("select * from table1 where (column3 = 4 or column4 is null)") {
+            from ("table1")
+            where {
+                or {
+                    "column3" eq 4
+                    "column4" eq null
+                }
+            }
+        }
+    }
+
+    @Test
+    fun `when either 'and' or 'or' conditions are specified then they are respected`() {
+        doTest("select * from table1 where ((column3 = 4 or column4 is null) and column5 = 42)") {
+            from ("table1")
+            where {
+                and {
+                    or {
+                        "column3" eq 4
+                        "column4" eq null
+                    }
+                    "column5" eq 42
+                }
+            }
+        }
+    }
+
+    private fun doTest(expected: String, sql: SqlSelectBuilder.() -> Unit) {
+        assertThat(query(sql).build()).isEqualTo(expected)
+    }
+}