Table of Contents

Consumer in Java with Examples

只有一個抽象方法的接口，定義了 accept 方法，可以在一個元素上測試或應用一些操作，常用於判斷是否存在，才會執行程式邏輯，如： Optional ifPresent 等方法，取代傳統實作接口的方法，讓程式碼更加簡潔和易讀， Consumer in Java 本篇增加了範例，並透過單元測試來驗證產出結果。

@FunctionalInterface
public interface Consumer<T> {
    void accept(T t);
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- functional
       |                   +- consumer
       |                       +- ConsumerWithExamplesTest.java

單元測試

Consumer Java 提供 accept 、 andThen 條件或組合成鏈式判斷等操作 Consumer 。

Food

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();
		}
	}

accept

建立 Consumer 物件，傳入 1 個參數，執行程式邏輯。

	@Test
	public void accept() {
		Consumer<Food> lessThan = o -> System.out.println(o.quantity < 3);
		Food food = new Food("Bacon", 1, 1);
		lessThan.accept(food);
		food = new Food("Pork", 3, 1);
		lessThan.accept(food);
	}

true
false

andThen

Consumer Java 建立 2 個 Consumer 物件，各傳入 1 個參數，使用 andThen 組合執行程式邏輯，並傳回 Consumer 繼續往下執行，其中若有例外拋出，將會中斷執行。

	@Test
	public void andThen() {
		Consumer<Food> nonNull = o -> System.out.println(Objects.nonNull(o));
		Consumer<Food> contains = o -> {
			Optional<Food> opt = Optional.ofNullable(o);
			opt.ifPresent(e -> System.out.println(e.name.contains("o")));
		};
		Food food = new Food("Bacon", 1, 1);
		nonNull.andThen(contains).accept(food);
		nonNull.andThen(contains).accept(null);
	}

true
true
false

chaining

Java Consumer 建立多個 Consumer 物件，各傳入 1 個參數，使用 andThen 組合執行程式邏輯。

	@Test
	public void chaining() {
		Consumer<Food> nonNull = o -> System.out.println(Objects.nonNull(o));
		Consumer<Food> startsWith = o -> System.out.println(o.name.startsWith("B"));
		Consumer<Food> endsWith = o -> System.out.println(o.name.endsWith("n"));
		Food food = new Food("Bacon", 1, 1);
		nonNull.andThen(startsWith).andThen(endsWith).accept(food);
		food = new Food("Ham", 2, 1);
		nonNull.andThen(startsWith).andThen(endsWith).accept(food);
	}

true
true
true
true
false
false

traditional

Java Consumer 使用傳統方式，實作 Consumer 接口，執行程式邏輯。

	public static class LengthGreaterThan<E> implements Consumer<Food> {
		@Override
		public void accept(Food t) {
			System.out.println(t.name.length() > 3);
		}
	}

	@Test
	public void traditional() {
		Consumer<Food> lengthGreaterThan = new LengthGreaterThan<Food>();
		Consumer<Food> contains = o -> System.out.println(o.name.contains("o"));
		Food food = new Food("Bacon", 1, 1);
		lengthGreaterThan.andThen(contains).accept(food);
		food = new Food("Ham", 2, 1);
		lengthGreaterThan.andThen(contains).accept(food);
	}

true
true
false
false

ConsumerWithExamplesTest.java

Consumer in Java 新增單元測試，驗證 Java Consumer Example 是否符合預期。

package org.ruoxue.java_147.functional.consumer;

import java.util.function.Consumer;
import java.util.Objects;
import java.util.Optional;

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 ConsumerWithExamplesTest {

	@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 accept() {
		Consumer<Food> lessThan = o -> System.out.println(o.quantity < 3);
		Food food = new Food("Bacon", 1, 1);
		lessThan.accept(food);
		food = new Food("Pork", 3, 1);
		lessThan.accept(food);
	}

	@Test
	public void andThen() {
		Consumer<Food> nonNull = o -> System.out.println(Objects.nonNull(o));
		Consumer<Food> contains = o -> {
			Optional<Food> opt = Optional.ofNullable(o);
			opt.ifPresent(e -> System.out.println(e.name.contains("o")));
		};
		Food food = new Food("Bacon", 1, 1);
		nonNull.andThen(contains).accept(food);
		nonNull.andThen(contains).accept(null);
	}

	@Test
	public void chaining() {
		Consumer<Food> nonNull = o -> System.out.println(Objects.nonNull(o));
		Consumer<Food> startsWith = o -> System.out.println(o.name.startsWith("B"));
		Consumer<Food> endsWith = o -> System.out.println(o.name.endsWith("n"));
		Food food = new Food("Bacon", 1, 1);
		nonNull.andThen(startsWith).andThen(endsWith).accept(food);
		food = new Food("Ham", 2, 1);
		nonNull.andThen(startsWith).andThen(endsWith).accept(food);
	}

	public static class LengthGreaterThan<E> implements Consumer<Food> {
		@Override
		public void accept(Food t) {
			System.out.println(t.name.length() > 3);
		}
	}

	@Test
	public void traditional() {
		Consumer<Food> lengthGreaterThan = new LengthGreaterThan<Food>();
		Consumer<Food> contains = o -> System.out.println(o.name.contains("o"));
		Food food = new Food("Bacon", 1, 1);
		lengthGreaterThan.andThen(contains).accept(food);
		food = new Food("Ham", 2, 1);
		lengthGreaterThan.andThen(contains).accept(food);
	}
}