Table of Contents

Function Functional Interface in Java

可當作其他方法的傳入參數或是引用其他方法為實例，使用 Lambda 語法，傳入 1 個泛型物件參數，結果傳回泛型物件，可以使用 andThen 、 compose 組合成鏈式判斷， Functional Interface Function 介紹常見的方法引用、方法參數等操作和方法，本篇增加了範例，並透過單元測試來驗證產出結果。

@FunctionalInterface
public interface Function<T, R> {
    R apply(T t);
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- functional
       |                   +- function
       |                       +- FunctionFunctionalTest.java

單元測試

Function Functional Interface 提供方法引用、參考等操作 Java Function Functional Interface 。

Food

Function Functional Interface 建立 Food 類別，覆寫 equals 、 hashCode ，定義屬性和方法，用來建立一個物件。

	@NoArgsConstructor
	@Getter
	@Setter
	@Builder
	public static class Food {
		private String name;
		private double quantity;
		private int type;

		public Food(String name, double quantity, int type) {
			this.name = name;
			this.quantity = quantity;
			this.type = type;
		}

		public String toString() {
			ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
			builder.appendSuper(super.toString());
			builder.append("name", name);
			builder.append("quantity", quantity);
			builder.append("type", type);
			return builder.toString();
		}

		public boolean equals(Object object) {
			if (!(object instanceof Food)) {
				return false;
			}
			if (this == object) {
				return true;
			}
			Food other = (Food) object;
			return new EqualsBuilder().append(getName(), other.getName()).isEquals();
		}

		public int hashCode() {
			return new HashCodeBuilder().append(getName()).toHashCode();
		}
	}

methodReference

Function Functional Interface 建立一個 Function ，引用其他方法為實例，轉換元素。

	@Test
	public void methodReference() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Bacon", "Ham", "Pork");
		Function<String, String> toUpperCase = String::toUpperCase;
		List<String> result = list.stream().map(toUpperCase).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());

		List<Food> foodList = Arrays.asList(new Food("Bacon", 1, 1), new Food("Ham", 2, 1), new Food("Pork", 3, 1));
		Function<Food, String> getName = Food::getName;
		Function<String, String> toLowerCase = String::toLowerCase;
		result = foodList.stream().map(getName.andThen(toLowerCase)).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[BACON, HAM, PORK]
[bacon, ham, pork]

methodParameter

Functional Interface Function in Java 建立一個 Function ，當作其他方法的傳入參數，轉換元素。

	public static List<String> map(List<String> list, Function<String, String> function) {
		return list.stream().map(function).collect(Collectors.toList());
	}

	public static List<String> foodMap(List<Food> list, Function<Food, String> function) {
		return list.stream().map(function).collect(Collectors.toList());
	}

	@Test
	public void methodParameter() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Bacon", "Ham", "Pork");
		Function<String, String> toUpperCase = s -> s.toUpperCase();
		List<String> result = map(list, toUpperCase);
		System.out.println(result);
		assertEquals(expectedSize, result.size());

		List<Food> foodList = Arrays.asList(new Food("Bacon", 1, 1), new Food("Ham", 2, 1), new Food("Pork", 3, 1));
		Function<Food, String> getName = o -> o.name;
		Function<String, String> toLowerCase = s -> s.toUpperCase();
		result = foodMap(foodList, getName.andThen(toLowerCase));
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

[BACON, HAM, PORK]
[BACON, HAM, PORK]

FunctionFunctionalTest.java

Functional Interface Function in Java 新增單元測試，驗證 Java Function Functional Interface 是否符合預期。

package org.ruoxue.java_147.functional.function;

import static org.junit.Assert.*;

import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

import org.apache.commons.lang3.builder.EqualsBuilder;
import org.apache.commons.lang3.builder.HashCodeBuilder;
import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;
import org.junit.Test;

import lombok.Builder;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;

public class FunctionFunctionalTest {

	@NoArgsConstructor
	@Getter
	@Setter
	@Builder
	public static class Food {
		private String name;
		private double quantity;
		private int type;

		public Food(String name, double quantity, int type) {
			this.name = name;
			this.quantity = quantity;
			this.type = type;
		}

		public String toString() {
			ToStringBuilder builder = new ToStringBuilder(this, ToStringStyle.JSON_STYLE);
			builder.appendSuper(super.toString());
			builder.append("name", name);
			builder.append("quantity", quantity);
			builder.append("type", type);
			return builder.toString();
		}

		public boolean equals(Object object) {
			if (!(object instanceof Food)) {
				return false;
			}
			if (this == object) {
				return true;
			}
			Food other = (Food) object;
			return new EqualsBuilder().append(getName(), other.getName()).isEquals();
		}

		public int hashCode() {
			return new HashCodeBuilder().append(getName()).toHashCode();
		}
	}

	@Test
	public void methodReference() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Bacon", "Ham", "Pork");
		Function<String, String> toUpperCase = String::toUpperCase;
		List<String> result = list.stream().map(toUpperCase).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());

		List<Food> foodList = Arrays.asList(new Food("Bacon", 1, 1), new Food("Ham", 2, 1), new Food("Pork", 3, 1));
		Function<Food, String> getName = Food::getName;
		Function<String, String> toLowerCase = String::toLowerCase;
		result = foodList.stream().map(getName.andThen(toLowerCase)).collect(Collectors.toList());
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}

	public static List<String> map(List<String> list, Function<String, String> function) {
		return list.stream().map(function).collect(Collectors.toList());
	}

	public static List<String> foodMap(List<Food> list, Function<Food, String> function) {
		return list.stream().map(function).collect(Collectors.toList());
	}

	@Test
	public void methodParameter() {
		int expectedSize = 3;
		List<String> list = Arrays.asList("Bacon", "Ham", "Pork");
		Function<String, String> toUpperCase = s -> s.toUpperCase();
		List<String> result = map(list, toUpperCase);
		System.out.println(result);
		assertEquals(expectedSize, result.size());

		List<Food> foodList = Arrays.asList(new Food("Bacon", 1, 1), new Food("Ham", 2, 1), new Food("Pork", 3, 1));
		Function<Food, String> getName = o -> o.name;
		Function<String, String> toLowerCase = s -> s.toUpperCase();
		result = foodMap(foodList, getName.andThen(toLowerCase));
		System.out.println(result);
		assertEquals(expectedSize, result.size());
	}
}

心得分享

Java Function Functional Interface 除了傳統實作接口的方法，使用 Lambda 表達式實作功能，能讓程式碼更加簡潔與直接，大幅提高可讀性， Functional Interface Function in Java 提供了幾種 Function 常見方法的操作範例。