Table of Contents

Java BiFunction Methods

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 個泛型物件參數，結果傳回泛型物件，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，BiFunction Java Methods 介紹常見的 apply 、 andThen 等方法，了解 BiFunction 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 BiFunction<T, U, R> {
    R apply(T t, U u);
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- bifunctional
       |                   +- bifunction
       |                       +- BiFunctionMethodsTest.java

單元測試

BiFunction Java Methods 提供 apply 、 andThen 條件或組合成鏈式判斷等操作 BiFunction 。

apply

建立 BiFunction 物件，傳入 2 個參數，執行程式邏輯，傳回物件。

	@Test
	public void apply() {
		BiFunction<String, String, Boolean> compareLength = (s, s2) -> s.length() > s2.length();
		boolean result = compareLength.apply("Bacon", "Ham");
		System.out.println(result);
		assertTrue(result);
		result = compareLength.apply("Ham", "Bacon");
		System.out.println(result);
		assertFalse(result);

		BiFunction<String, Integer, Boolean> greaterThan = (s, i) -> s.length() > i;
		result = greaterThan.apply("Bacon", 3);
		System.out.println(result);
		assertTrue(result);
		result = greaterThan.apply("Ham", 3);
		System.out.println(result);
		assertFalse(result);
	}

true
false
true
false

andThen

BiFunction Java Methods 建立 1 個 BiFunction 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() {
		BiFunction<Double, Double, 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);

		BiFunction<String, String, Integer> length = (s, s1) -> s.length() + s1.length();
		Function<Integer, Integer> multiply = i -> i * 2;
		int intResult = length.andThen(multiply).apply("Bacon", "Ham");
		System.out.println(intResult);
		assertEquals(16, intResult);
	}

9.0
16

andThenThrowException

BiFunction Java Methods 建立 BiFunction 物件，傳入參數 null ，會拋出例外。

	@Test(expected = NullPointerException.class)
	public void andThenThrowException() {
		BiFunction<Double, Double, Double> addition = (d, d1) -> d + d1;
		addition = addition.andThen(null);
	}

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.bifunction.BiFunctionMethodsTest.andThenThrowException(BiFunctionMethodsTest.java:49)

traditional

BiFunction Methods in Java 使用傳統方式，實作 BiFunction 接口，傳回物件。

	public static class Addition<E, F, D> implements BiFunction<Double, Double, Double> {
		@Override
		public Double apply(Double t, Double u) {
			return t + u;
		}
	}

	@Test
	public void traditional() {
		BiFunction<Double, Double, Double> addition = new Addition<Double, Double, 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
22.0

BiFunctionMethodsTest.java

BiFunction Methods in Java 新增單元測試，驗證 BiFunction Functions in Java 是否符合預期。

package org.ruoxue.java_147.functional.bifunction;

import static org.junit.Assert.*;

import java.util.function.BiFunction;
import java.util.function.Function;

import org.junit.Test;

public class BiFunctionMethodsTest {

	@Test
	public void apply() {
		BiFunction<String, String, Boolean> compareLength = (s, s2) -> s.length() > s2.length();
		boolean result = compareLength.apply("Bacon", "Ham");
		System.out.println(result);
		assertTrue(result);
		result = compareLength.apply("Ham", "Bacon");
		System.out.println(result);
		assertFalse(result);

		BiFunction<String, Integer, Boolean> greaterThan = (s, i) -> s.length() > i;
		result = greaterThan.apply("Bacon", 3);
		System.out.println(result);
		assertTrue(result);
		result = greaterThan.apply("Ham", 3);
		System.out.println(result);
		assertFalse(result);
	}

	@Test
	public void andThen() {
		BiFunction<Double, Double, 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);

		BiFunction<String, String, Integer> length = (s, s1) -> s.length() + s1.length();
		Function<Integer, Integer> multiply = i -> i * 2;
		int intResult = length.andThen(multiply).apply("Bacon", "Ham");
		System.out.println(intResult);
		assertEquals(16, intResult);
	}

	@Test(expected = NullPointerException.class)
	public void andThenThrowException() {
		BiFunction<Double, Double, Double> addition = (d, d1) -> d + d1;
		addition = addition.andThen(null);
	}

	public static class Addition<E, F, D> implements BiFunction<Double, Double, Double> {
		@Override
		public Double apply(Double t, Double u) {
			return t + u;
		}
	}

	@Test
	public void traditional() {
		BiFunction<Double, Double, Double> addition = new Addition<Double, Double, 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

BiFunction Functions in Java 屬於 java.util.bifunction ，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， BiFunction Methods in Java 提供更清晰、更易讀且更靈活的方式來組合多個 BiFunction 條件，來表示非常複雜的邏輯條件，如： apply 、 andThen 等，提供了幾種 BiFunction 常見範例。