功能接口，使用 Lambda 語法，可當作其他方法的傳入參數或是引用其他方法為實例，常用於物件轉換或數字運算，傳入 2 個同型別物件參數，結果傳回同型別物件，可以使用 andThen 組合成鏈式判斷，提高了程式碼的可管理性，有助於分別進行單元測試，BinaryOperator Java Methods 介紹常見的 apply 、 andThen 、 minBy 、 maxBy 等方法，了解 BinaryOperator 的不同操作和方法，本篇增加了範例，並透過單元測試來驗證產出結果。
@FunctionalInterface
public interface BinaryOperator<T> extends BiFunction<T,T,T> {
public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {
Objects.requireNonNull(comparator);
return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
}
public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {
Objects.requireNonNull(comparator);
return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
}
}檔案目錄
./
+- src
+- test
| +- org
| +- ruoxue
| +- java_147
| +- functional
| +- binaryoperator
| +- BinaryOperatorMethodsTest.java 單元測試
BinaryOperator Java Methods 提供 apply 、 andThen 條件或組合成鏈式判斷等操作 BinaryOperator 。
apply
建立 BinaryOperator 物件，傳入 2 個參數，執行程式邏輯，傳回物件。
@Test
public void apply() {
BinaryOperator<String> concact = (s, s2) -> s.concat(s2);
String result = concact.apply("Bacon", "Ham");
System.out.println(result);
assertNotNull(result);
result = concact.apply("Ham", "Bacon");
System.out.println(result);
assertNotNull(result);
BinaryOperator<Integer> addition = (i, i2) -> i + i2;
Integer intResult = addition.apply(1, 1);
System.out.println(intResult);
assertEquals(2, intResult.intValue());
intResult = addition.apply(1, 10);
System.out.println(intResult);
assertEquals(11, intResult.intValue());
}BaconHam
HamBacon
2
11andThen
BinaryOperator Java Methods 建立 2 個 BinaryOperator 物件，傳入 2 個參數，使用 andThen 組合執行程式邏輯，傳回物件，其中若有例外拋出，將會中斷執行。
@Test
public void andThen() {
BinaryOperator<Double> addition = (d, d1) -> d + d1;
Function<Double, Double> twice = d -> d * d;
double result = addition.andThen(twice).apply(1d, 2d);
System.out.println(result);
assertEquals(9d, result, 2);
BinaryOperator<String> concact = (s, s2) -> s.concat(s2);
Function<String, String> toUpperCase = s -> s.toUpperCase();
String stringResult = concact.andThen(toUpperCase).apply("Bacon", "Ham");
System.out.println(stringResult);
assertEquals("BACONHAM", stringResult);
}
9.0
BACONHAMandThenThrowException
BinaryOperator Java Methods 建立 BinaryOperator 物件，傳入參數 null ，會拋出例外 。
@Test(expected = NullPointerException.class)
public void andThenThrowException() {
BinaryOperator<Double> addition = (d, d1) -> d + d1;
BiFunction<Double, Double, Double> biFunction = addition.andThen(null);
assertNotNull(biFunction);
}java.lang.NullPointerException
at java.util.Objects.requireNonNull(Objects.java:203)
at java.util.function.BiFunction.andThen(BiFunction.java:69)
at org.ruoxue.java_147.functional.binaryoperator.BinaryOperatorMethodsTest.andThenThrowException(BinaryOperatorMethodsTest.java:51)
maxBy
BinaryOperator Methods Java 建立 BinaryOperator 物件，傳入 2 個參數，比對後傳回最大值。
@Test
public void maxBy() {
BinaryOperator<String> maxBy = BinaryOperator.maxBy(Comparator.comparingInt(String::length));
String result = maxBy.apply("Bacon", "Ham");
System.out.println(result);
assertEquals("Bacon", result);
result = maxBy.apply("Ham", "Port");
System.out.println(result);
assertEquals("Port", result);
BinaryOperator<Integer> intMaxBy = BinaryOperator.maxBy(Comparator.comparing(Integer::intValue));
Integer intResult = intMaxBy.apply(1, 1);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
intResult = intMaxBy.apply(1, 10);
System.out.println(intResult);
assertEquals(10, intResult.intValue());
}Bacon
Port
1
10minBy
BinaryOperator Methods Java 建立 BinaryOperator 物件，傳入 2 個參數，比對後傳回最小值。
@Test
public void minBy() {
BinaryOperator<String> minBy = BinaryOperator.minBy(Comparator.comparingInt(String::length));
String result = minBy.apply("Bacon", "Ham");
System.out.println(result);
assertEquals("Ham", result);
result = minBy.apply("Ham", "Port");
System.out.println(result);
assertEquals("Ham", result);
BinaryOperator<Integer> intMinBy = BinaryOperator.minBy(Comparator.comparing(Integer::intValue));
Integer intResult = intMinBy.apply(1, 1);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
intResult = intMinBy.apply(1, 10);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
}Ham
Ham
1
1traditional
BinaryOperator Methods in Java 使用傳統方式，實作 BinaryOperator 接口，傳回物件。
public static class Addition<E> implements BinaryOperator<Double> {
@Override
public Double apply(Double t, Double u) {
return t + u;
}
}
@Test
public void traditional() {
BinaryOperator<Double> addition = new Addition<Double>();
Function<Double, Double> multiply = i -> i * 2;
double result = addition.andThen(multiply).apply(1d, 2d);
System.out.println(result);
assertEquals(6.0, result, 2);
result = addition.andThen(multiply).apply(5d, 6d);
System.out.println(result);
assertEquals(22.0, result, 2);
}6.0
22.0BinaryOperatorMethodsTest.java
BinaryOperator Methods in Java 新增單元測試，驗證 BinaryOperator Functions in Java 是否符合預期。
package org.ruoxue.java_147.functional.binaryoperator;
import static org.junit.Assert.*;
import java.util.Comparator;
import java.util.function.BiFunction;
import java.util.function.BinaryOperator;
import java.util.function.Function;
import org.junit.Test;
public class BinaryOperatorMethodsTest {
@Test
public void apply() {
BinaryOperator<String> concact = (s, s2) -> s.concat(s2);
String result = concact.apply("Bacon", "Ham");
System.out.println(result);
assertNotNull(result);
result = concact.apply("Ham", "Bacon");
System.out.println(result);
assertNotNull(result);
BinaryOperator<Integer> addition = (i, i2) -> i + i2;
Integer intResult = addition.apply(1, 1);
System.out.println(intResult);
assertEquals(2, intResult.intValue());
intResult = addition.apply(1, 10);
System.out.println(intResult);
assertEquals(11, intResult.intValue());
}
@Test
public void andThen() {
BinaryOperator<Double> addition = (d, d1) -> d + d1;
Function<Double, Double> twice = d -> d * d;
double result = addition.andThen(twice).apply(1d, 2d);
System.out.println(result);
assertEquals(9d, result, 2);
BinaryOperator<String> concact = (s, s2) -> s.concat(s2);
Function<String, String> toUpperCase = s -> s.toUpperCase();
String stringResult = concact.andThen(toUpperCase).apply("Bacon", "Ham");
System.out.println(stringResult);
assertEquals("BACONHAM", stringResult);
}
@Test(expected = NullPointerException.class)
public void andThenThrowException() {
BinaryOperator<Double> addition = (d, d1) -> d + d1;
BiFunction<Double, Double, Double> biFunction = addition.andThen(null);
assertNotNull(biFunction);
}
@Test
public void maxBy() {
BinaryOperator<String> maxBy = BinaryOperator.maxBy(Comparator.comparingInt(String::length));
String result = maxBy.apply("Bacon", "Ham");
System.out.println(result);
assertEquals("Bacon", result);
result = maxBy.apply("Ham", "Port");
System.out.println(result);
assertEquals("Port", result);
BinaryOperator<Integer> intMaxBy = BinaryOperator.maxBy(Comparator.comparing(Integer::intValue));
Integer intResult = intMaxBy.apply(1, 1);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
intResult = intMaxBy.apply(1, 10);
System.out.println(intResult);
assertEquals(10, intResult.intValue());
}
@Test
public void minBy() {
BinaryOperator<String> minBy = BinaryOperator.minBy(Comparator.comparingInt(String::length));
String result = minBy.apply("Bacon", "Ham");
System.out.println(result);
assertEquals("Ham", result);
result = minBy.apply("Ham", "Port");
System.out.println(result);
assertEquals("Ham", result);
BinaryOperator<Integer> intMinBy = BinaryOperator.minBy(Comparator.comparing(Integer::intValue));
Integer intResult = intMinBy.apply(1, 1);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
intResult = intMinBy.apply(1, 10);
System.out.println(intResult);
assertEquals(1, intResult.intValue());
}
public static class Addition<E> implements BinaryOperator<Double> {
@Override
public Double apply(Double t, Double u) {
return t + u;
}
}
@Test
public void traditional() {
BinaryOperator<Double> addition = new Addition<Double>();
Function<Double, Double> multiply = i -> i * 2;
double result = addition.andThen(multiply).apply(1d, 2d);
System.out.println(result);
assertEquals(6.0, result, 2);
result = addition.andThen(multiply).apply(5d, 6d);
System.out.println(result);
assertEquals(22.0, result, 2);
}
}心得分享
BinaryOperator Functions in Java 屬於 java.util.function ，常用於物件轉換或數字運算，例如：取得字串長度、數字加減乘除運算， 使用 Lambda 表達式能讓程式碼更加簡潔與直接，取代傳統實作接口的方法，減少了很多程式碼，大幅提高可讀性， BinaryOperator Methods in Java 提供更清晰、更易讀且更靈活的方式來組合多個 BinaryOperator 條件，來表示非常複雜的邏輯條件，如： apply 、 andThen 等，提供了幾種 BinaryOperator 常見範例。
