Java 8新特性之Consumer
简介
Java Consumer接口来自Java 8中引入的 java.util.function包。
Consumer是一个功能接口，用来作为lambda表达式或方法引用的任务目标(传递一个参数执行指定的方法)。
Consumer的功能方法是accept(T t)。
@FunctionalInterface
public interface Consumer<T> {
/**
* Performs this operation on the given argument.
*
* @param t the input argument
*/
void accept(T t);
/**
* Returns a composed {@code Consumer} that performs, in sequence, this
* operation followed by the {@code after} operation. If performing either
* operation throws an exception, it is relayed to the caller of the
* composed operation. If performing this operation throws an exception,
* the {@code after} operation will not be performed.
*
* @param after the operation to perform after this operation
* @return a composed {@code Consumer} that performs in sequence this
* operation followed by the {@code after} operation
* @throws NullPointerException if {@code after} is null
*/
default Consumer<T> andThen(Consumer<? super T> after) {
Objects.requireNonNull(after);
return (T t) -> { accept(t); after.accept(t); };
}
}
如上所示
accept : 这是Consumer功能接口的功能方法。accept 方法对给定的参数进行这一操作。
andThen : 此方法返回一个组合的Consumer，该Consumer先执行原始的Consumer操作，然后按照从左到右的顺序执行给定的andThen操作。
使用示例
import java.util.function.Consumer;
/**
* @program: mina
* @description:
* @author: david.he
* @create: 2022-04-20
*/
public class ConsumerTest {
public static void main(String[] args) {
Consumer<String> nameConsumer = s -> System.out.println(s);
// 等同于
// Consumer<String> nameConsumer = new Consumer<String>() {
// @Override
// public void accept(String s) {
// System.out.println(s);
// }
// };
nameConsumer.accept("Mahesh");
nameConsumer.accept("Krishna");
}
}
输出如下
Mahesh
Krishna
使用lambda表达式创建Consumer
- 测试简单的lambda表达式
public class ConsumerTest {
public static void main(String[] args) {
List<Integer> oddList = new ArrayList<>();
List<Integer> evenList = new ArrayList<>();
Consumer<Integer> storeNumber = n -> {
if (n % 2 == 0) {
evenList.add(n);
} else {
oddList.add(n);
}
};
Consumer<List<Integer>> printList = list -> list.forEach(n -> System.out.println(n));
storeNumber.accept(10);
storeNumber.accept(15);
storeNumber.accept(25);
storeNumber.accept(30);
printList.accept(oddList);
printList.accept(evenList);
}
}
- 操作对象类型
public class ConsumerTest {
public static void main(String[] args) {
Consumer<Citizen> electionConsumer = c -> {
if (c.getAge() < 18) {
System.out.println(c.getName() + " is not eligible to vote.");
} else {
System.out.println(c.getName() + " can vote.");
}
};
electionConsumer.accept(new Citizen("Ritesh", 15));
electionConsumer.accept(new Citizen("Shreya", 20));
}
}
class Citizen {
private String name;
private int age;
public Citizen(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
输出结果
Ritesh is not eligible to vote.
Shreya can vote.
使用方法引用的方式创建
public class ConsumerTest {
public static void main(String[] args) {
Map<Integer, String> persons = new HashMap<Integer, String>();
persons.put(101, "Mahesh");
persons.put(102, "Krishna");
Consumer<Map<Integer, String>> updatePersons = Utility::updateData;
Consumer<Map<Integer, String>> displayPersons = Utility::displayData;
updatePersons.accept(persons);
displayPersons.accept(persons);
}
}
class Utility {
static void updateData(Map<Integer, String> persons) {
persons.replaceAll((k, v) -> "Shree ".concat(v));
}
static void displayData(Map<Integer, String> persons) {
for (Map.Entry<Integer, String> entry : persons.entrySet()) {
System.out.println(entry.getKey() + " - " + entry.getValue());
}
}
}
输出结果
101 - Shree Mahesh
102 - Shree Krishna
andThen方法
- 这个方法返回一个组合的Consumer，先执行当前Consumer操作，然后再执行after的Consumer操作。
public class ConsumerTest {
public static void main(String[] args) {
List<Integer> numList = Arrays.asList(3, 4, 5, 6);
Consumer<List<Integer>> squareConsumer = list -> {
for (int i = 0; i < list.size(); i++) {
list.set(i, list.get(i) * list.get(i));
}
};
Consumer<List<Integer>> printConsumer = list -> list.forEach(n -> System.out.println(n));
squareConsumer.andThen(printConsumer).accept(numList);
}
}
输出结果
9
16
25
36
拼接多个addThen
public class ConsumerTest {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(12, 13, 14, 15, 16, 17);
Consumer<List<Integer>> oddNumConsumer = MyNumber::printOddNum;
Consumer<List<Integer>> evenNumConsumer = MyNumber::printEvenNum;
Consumer<List<Integer>> taskFinishConsumer = MyNumber::taskFinishMsg;
oddNumConsumer.andThen(evenNumConsumer).andThen(taskFinishConsumer).accept(list);
}
}
class MyNumber {
static void printOddNum(List<Integer> myNumbers) {
System.out.println("--- odd numbers ---");
myNumbers.forEach(n -> {
if (n % 2 == 1) {
System.out.print(n + " ");
}
});
}
static void printEvenNum(List<Integer> myNumbers) {
System.out.println("\n--- even numbers ---");
myNumbers.forEach(n -> {
if (n % 2 == 0) {
System.out.print(n + " ");
}
});
}
static void taskFinishMsg(List<Integer> myNumbers) {
System.out.println("\nTotal " + myNumbers.size() + " number processed.");
}
}
输出结果
--- odd numbers ---
13 15 17
--- even numbers ---
12 14 16
Total 6 number processed.
总结
主要使用场景是：需要操作一个对象，但是不需要返回任何数据。
