Table of Contents

Predicate in Java with Examples

只有一個抽象方法的接口，定義了 test 方法，可以在一個元素上測試或應用一些操作，常用於過濾條件，如：過濾數量介於 3 – 6 之間，取代傳統實作接口的方法，讓程式碼更加簡潔和易讀， Predicate 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 Predicate<T> {
    boolean test(T t);
}

檔案目錄

./
   +- src
       +- test
       |   +- org
       |       +- ruoxue
       |           +- java_147
       |               +- functional
       |                   +- predicate
       |                       +- PredicateWithExamplesTest.java

單元測試

Predicate Java 提供 test 、 and 、 or 條件或組合成鏈式判斷等操作 Predicate 。

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();
		}
	}

test

建立 Predicate 物件，傳入 1 個參數，判斷是否為真。

	@Test
	public void test() {
		Predicate<Food> lessThan = o -> o.quantity < 3;
		Food food = new Food("Bacon", 1, 1);
		boolean result = lessThan.test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Pork", 3, 1);
		result = lessThan.test(food);
		System.out.println(result);
		assertFalse(result);
	}

true
false

negate

Predicate Java 建立 Predicate 物件，傳入 1 個參數，否定判斷。

	@Test
	public void negate() {
		Predicate<Food> lessThan = o -> o.quantity < 3;
		Food food = new Food("Bacon", 1, 1);
		boolean result = lessThan.negate().test(food);
		System.out.println(result);
		assertFalse(result);
		food = new Food("Pork", 3, 1);
		result = lessThan.negate().test(food);
		System.out.println(result);
		assertTrue(result);
	}

false
true

and

Predicate Java 建立 2 個 Predicate 物件，各傳入 1 個參數，使用 and 組合判斷是否為真，並傳回 Predicate 是一個短路邏輯運算，其中當第一個 Predicate 為 False 時將不會評估第二個 Predicate 。

	@Test
	public void and() {
		Predicate<Food> nonNull = Objects::nonNull;
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Bacon", 1, 1);
		boolean result = nonNull.and(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		result = nonNull.and(contains).test(null);
		System.out.println(result);
		assertFalse(result);
	}

true
false

or

Predicate Java 建立 2 個 Predicate 物件，各傳入 1 個參數，使用 or 組合判斷是否為真，並傳回 Predicate 是一個短路邏輯運算，其中當第一個 Predicate 為 True 時將不會評估第二個 Predicate 。

	@Test
	public void or() {
		Predicate<Food> isNull = Objects::isNull;
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Pork", 3, 1);
		boolean result = isNull.or(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		result = isNull.or(contains).test(null);
		System.out.println(result);
		assertTrue(result);
	}

true
true

chaining

Java Predicate 建立多個 Predicate 物件，各傳入 1 個參數，使用 and 、 or 組合判斷是否為真。

	@Test
	public void chaining() {
		Predicate<Food> nonNull = Objects::nonNull;
		Predicate<Food> startsWith = o -> o.name.startsWith("B");
		Predicate<Food> endsWith = o -> o.name.endsWith("n");
		Food food = new Food("Bacon", 1, 1);
		boolean result = nonNull.and(startsWith).or(endsWith).test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = nonNull.and(startsWith).or(endsWith).test(food);
		System.out.println(result);
		assertFalse(result);
	}

true
false

isEqual

Java Predicate 建立 Predicate 物件，傳入 1 個參數，比較是否相等。

	@Test
	public void isEqual() {
		Food food = new Food("Bacon", 1, 1);
		Predicate<Food> isEqual = Predicate.isEqual(food);
		boolean result = isEqual.test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = isEqual.test(food);
		System.out.println(result);
		assertFalse(result);
	}

true
false

traditional

Java Predicate 使用傳統方式，實作 Predicate 接口，判斷是否為真。

	public static class LengthGreaterThan<E> implements Predicate<Food> {
		@Override
		public boolean test(Food t) {
			return t.name.length() > 3;
		}
	}

	@Test
	public void traditional() {
		Predicate<Food> lengthGreaterThan = new LengthGreaterThan<Food>();
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Bacon", 1, 1);
		boolean result = lengthGreaterThan.and(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = lengthGreaterThan.and(contains).test(food);
		System.out.println(result);
		assertFalse(result);
	}

true
false

PredicateWithExamplesTest.java

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

package org.ruoxue.java_147.functional.predicate;

import static org.junit.Assert.*;

import java.util.function.Predicate;
import java.util.Objects;
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 PredicateWithExamplesTest {

	@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 test() {
		Predicate<Food> lessThan = o -> o.quantity < 3;
		Food food = new Food("Bacon", 1, 1);
		boolean result = lessThan.test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Pork", 3, 1);
		result = lessThan.test(food);
		System.out.println(result);
		assertFalse(result);
	}

	@Test
	public void negate() {
		Predicate<Food> lessThan = o -> o.quantity < 3;
		Food food = new Food("Bacon", 1, 1);
		boolean result = lessThan.negate().test(food);
		System.out.println(result);
		assertFalse(result);
		food = new Food("Pork", 3, 1);
		result = lessThan.negate().test(food);
		System.out.println(result);
		assertTrue(result);
	}

	@Test
	public void and() {
		Predicate<Food> nonNull = Objects::nonNull;
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Bacon", 1, 1);
		boolean result = nonNull.and(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		result = nonNull.and(contains).test(null);
		System.out.println(result);
		assertFalse(result);
	}

	@Test
	public void or() {
		Predicate<Food> isNull = Objects::isNull;
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Pork", 3, 1);
		boolean result = isNull.or(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		result = isNull.or(contains).test(null);
		System.out.println(result);
		assertTrue(result);
	}

	@Test
	public void chaining() {
		Predicate<Food> nonNull = Objects::nonNull;
		Predicate<Food> startsWith = o -> o.name.startsWith("B");
		Predicate<Food> endsWith = o -> o.name.endsWith("n");
		Food food = new Food("Bacon", 1, 1);
		boolean result = nonNull.and(startsWith).or(endsWith).test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = nonNull.and(startsWith).or(endsWith).test(food);
		System.out.println(result);
		assertFalse(result);
	}

	@Test
	public void isEqual() {
		Food food = new Food("Bacon", 1, 1);
		Predicate<Food> isEqual = Predicate.isEqual(food);
		boolean result = isEqual.test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = isEqual.test(food);
		System.out.println(result);
		assertFalse(result);
	}

	public static class LengthGreaterThan<E> implements Predicate<Food> {
		@Override
		public boolean test(Food t) {
			return t.name.length() > 3;
		}
	}

	@Test
	public void traditional() {
		Predicate<Food> lengthGreaterThan = new LengthGreaterThan<Food>();
		Predicate<Food> contains = o -> o.name.contains("o");
		Food food = new Food("Bacon", 1, 1);
		boolean result = lengthGreaterThan.and(contains).test(food);
		System.out.println(result);
		assertTrue(result);
		food = new Food("Ham", 2, 1);
		result = lengthGreaterThan.and(contains).test(food);
		System.out.println(result);
		assertFalse(result);
	}
}