Introduce Converter.andThen(...)

Closes gh-23379

spring-core/src/test/java/org/springframework/core/convert/converter/ConverterTests.java

@@ -0,0 +1,28 @@
+package org.springframework.core.convert.converter;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+
+/**
+ * Tests for {@link Converter}
+ *
+ * @author Josh Cummings
+ */
+public class ConverterTests {
+	Converter<Integer, Integer> moduloTwo = number -> number % 2;
+
+	@Test
+	public void andThenWhenGivenANullConverterThenThrowsException() {
+		assertThatExceptionOfType(IllegalArgumentException.class)
+				.isThrownBy(() -> this.moduloTwo.andThen(null));
+	}
+
+	@Test
+	public void andThenWhenGivenConverterThenComposesInOrder() {
+		Converter<Integer, Integer> addOne = number-> number + 1;
+		assertThat(this.moduloTwo.andThen(addOne).convert(13)).isEqualTo(2);
+		assertThat(addOne.andThen(this.moduloTwo).convert(13)).isEqualTo(0);
+	}
+}