From 2d94269f1b8baacacd2f30a26306425abc438fcb Mon Sep 17 00:00:00 2001
From: Josh Cummings <josh.cummings@gmail.com>
Date: Mon, 29 Jul 2019 07:07:57 -0600
Subject: [PATCH] Add Converter.andThen

Fixes: gh-22381
---
 .../core/convert/converter/Converter.java     | 18 ++++++++++++
 .../convert/converter/ConverterTests.java     | 28 +++++++++++++++++++
 2 files changed, 46 insertions(+)
 create mode 100644 spring-core/src/test/java/org/springframework/core/convert/converter/ConverterTests.java

diff --git a/spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java b/spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java
index d7cf51432d6e..a9c893b12d57 100644
--- a/spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java
+++ b/spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java
@@ -17,6 +17,7 @@
 package org.springframework.core.convert.converter;
 
 import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
 
 /**
  * A converter converts a source object of type {@code S} to a target of type {@code T}.
@@ -26,6 +27,7 @@
  * <p>Implementations may additionally implement {@link ConditionalConverter}.
  *
  * @author Keith Donald
+ * @author Josh Cummings
  * @since 3.0
  * @param <S> the source type
  * @param <T> the target type
@@ -42,4 +44,20 @@ public interface Converter<S, T> {
 	@Nullable
 	T convert(S source);
 
+	/**
+	 * Construct a composed {@link Converter} that first applies this {@link Converter} to
+	 * its input, and then applies the {@code after} {@link Converter} to the result.
+	 *
+	 * @since 5.2
+	 * @param <U> the type of output of both the {@code after} {@link Converter} and the
+	 * composed {@link Converter}
+	 * @param after the {@link Converter} to apply after this {@link Converter}
+	 * is applied
+	 * @return a composed {@link Converter} that first applies this {@link Converter} and then
+	 * applies the {@code after} {@link Converter}
+	 */
+	default <U> Converter<S, U> andThen(Converter<? super T, ? extends U> after) {
+		Assert.notNull(after, "after cannot be null");
+		return (S s) -> after.convert(convert(s));
+	}
 }
diff --git a/spring-core/src/test/java/org/springframework/core/convert/converter/ConverterTests.java b/spring-core/src/test/java/org/springframework/core/convert/converter/ConverterTests.java
new file mode 100644
index 000000000000..076ef507a9e5
--- /dev/null
+++ b/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.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);
+	}
+}
\ No newline at end of file