Polish contribution

See gh-23379

spring-core/src/main/java/org/springframework/core/convert/converter/Converter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,19 +45,20 @@ public interface Converter<S, T> {
 	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}
+	 * 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.
 	 * @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}
+	 * @param <U> the type of output of both the {@code after} {@link Converter}
+	 * and the composed {@link Converter}
+	 * @return a composed {@link Converter} that first applies this {@link Converter}
+	 * and then applies the {@code after} {@link Converter}
+	 * @since 5.3
 	 */
 	default <U> Converter<S, U> andThen(Converter<? super T, ? extends U> after) {
-		Assert.notNull(after, "after cannot be null");
+		Assert.notNull(after, "after must not be null");
 		return (S s) -> after.convert(convert(s));
 	}
+
 }

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

@@ -1,28 +1,63 @@
+/*
+ * Copyright 2002-2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 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;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Tests for {@link Converter}
  *
  * @author Josh Cummings
+ * @author Sam Brannen
+ * @since 5.3
  */
-public class ConverterTests {
-	Converter<Integer, Integer> moduloTwo = number -> number % 2;
+class ConverterTests {
+
+	private final Converter<Integer, Integer> moduloTwo = number -> number % 2;
+	private final Converter<Integer, Integer> addOne = number -> number + 1;
+
 
 	@Test
-	public void andThenWhenGivenANullConverterThenThrowsException() {
-		assertThatExceptionOfType(IllegalArgumentException.class)
-				.isThrownBy(() -> this.moduloTwo.andThen(null));
+	void andThenWhenGivenANullConverterThenThrowsException() {
+		assertThatIllegalArgumentException().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);
+	void andThenWhenGivenConverterThenComposesInOrder() {
+		assertThat(this.moduloTwo.andThen(this.addOne).convert(13)).isEqualTo(2);
+		assertThat(this.addOne.andThen(this.moduloTwo).convert(13)).isEqualTo(0);
 	}
-}
+
+	@Test
+	void andThenCanConvertfromDifferentSourceType() {
+		Converter<String, Integer> length = String::length;
+		assertThat(length.andThen(this.moduloTwo).convert("example")).isEqualTo(1);
+		assertThat(length.andThen(this.addOne).convert("example")).isEqualTo(8);
+	}
+
+	@Test
+	void andThenCanConvertToDifferentTargetType() {
+		Converter<String, Integer> length = String::length;
+		Converter<Integer, String> toString = Object::toString;
+		assertThat(length.andThen(toString).convert("example")).isEqualTo("7");
+		assertThat(toString.andThen(length).convert(1_000)).isEqualTo(4);
+	}
+
+}