Java 8 BiFunction示例

ゝ一世哀愁。 2023-02-15 05:08 7阅读 0赞

在Java 8中， [BiFunction][]是功能接口； 它接受两个参数并返回一个对象。

BiFunction.java

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

*  T –函数的第一个参数的类型。
 *  U –函数第二个参数的类型。
 *  R –函数结果的类型。

## 1. BiFunction <T，U，R> ##

1.1本示例使用两个`Integers`并返回`Integer` ， `Double`或`List`

Java8BiFunction1.java

package com.mkyong;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.BiFunction;
    
    public class Java8BiFunction1 {
    
        public static void main(String[] args) {
    
            // takes two Integers and return an Integer
            BiFunction<Integer, Integer, Integer> func = (x1, x2) -> x1 + x2;
    
            Integer result = func.apply(2, 3);
    
            System.out.println(result); // 5
    
            // take two Integers and return an Double
            BiFunction<Integer, Integer, Double> func2 = (x1, x2) -> Math.pow(x1, x2);
    
            Double result2 = func2.apply(2, 4);
    
            System.out.println(result2);    // 16.0
    
            // take two Integers and return a List<Integer>
            BiFunction<Integer, Integer, List<Integer>> func3 = (x1, x2) -> Arrays.asList(x1 + x2);
    
            List<Integer> result3 = func3.apply(2, 3);
    
            System.out.println(result3);
    
        }
    
    }

输出量

5
    16.0
    [5]

## 2. BiFunction <T，U，R> + Function <T，R> ##

2.1此`BiFunction`接受两个`Integer`并返回`Double` ，然后使用`andThen()`将其与`Function`以将`Double`转换为`String` 。

Java8BiFunction2a.java

package com.mkyong;
    
    import java.util.function.BiFunction;
    import java.util.function.Function;
    
    public class Java8BiFunction2a {
    
        public static void main(String[] args) {
    
            // Math.pow(a1, a2) returns Double
            BiFunction<Integer, Integer, Double> func1 = (a1, a2) -> Math.pow(a1, a2);
    
            // takes Double, returns String
            Function<Double, String> func2 = (input) -> "Result : " + String.valueOf(input);
    
            String result = func1.andThen(func2).apply(2, 4);
    
            System.out.println(result);
    
        }
    
    }

输出量

Result : 16.0

2.2本示例将上述程序转换为接受`BiFunction`和`Function`作为参数并将其链接在一起的方法。

Java8BiFunction2b.java

package com.mkyong;
    
    import java.util.function.BiFunction;
    import java.util.function.Function;
    
    public class Java8BiFunction2b {
    
        public static void main(String[] args) {
    
            String result = powToString(2, 4,
                    (a1, a2) -> Math.pow(a1, a2),
                    (r) -> "Result : " + String.valueOf(r));
    
            System.out.println(result); // Result : 16.0
    
        }
    
        public static <R> R powToString(Integer a1, Integer a2,
                                        BiFunction<Integer, Integer, Double> func,
                                        Function<Double, R> func2) {
    
            return func.andThen(func2).apply(a1, a2);
    
        }
    
    }

输出量

Result : 16.0

2.3本示例将上述方法转换为通用方法：

public static <A1, A2, R1, R2> R2 convert(A1 a1, A2 a2,
                                              BiFunction<A1, A2, R1> func,
                                              Function<R1, R2> func2) {
    
        return func.andThen(func2).apply(a1, a2);
    
    }

这种通用方法有很多可能性，让我们看看：

Java8BiFunction2c.java

package com.mkyong;
    
    import java.util.function.BiFunction;
    import java.util.function.Function;
    
    public class Java8BiFunction2c {
    
        public static void main(String[] args) {
    
            // Take two Integers, pow it into a Double, convert Double into a String.
            String result = convert(2, 4,
                    (a1, a2) -> Math.pow(a1, a2),
                    (r) -> "Pow : " + String.valueOf(r));
    
            System.out.println(result);     // Pow : 16.0
    
            // Take two Integers, multiply into an Integer, convert Integer into a String.
            String result2 = convert(2, 4,
                    (a1, a2) -> a1 * a1,
                    (r) -> "Multiply : " + String.valueOf(r));
    
            System.out.println(result2);    // Multiply : 4
    
            // Take two Strings, join both, join "cde"
            String result3 = convert("a", "b",
                    (a1, a2) -> a1 + a2,
                    (r) -> r + "cde");      // abcde
    
            System.out.println(result3);
    
            // Take two Strings, join both, convert it into an Integer
            Integer result4 = convert("100", "200",
                    (a1, a2) -> a1 + a2,
                    (r) -> Integer.valueOf(r));
    
            System.out.println(result4);    // 100200
    
        }
    
        public static <A1, A2, R1, R2> R2 convert(A1 a1, A2 a2,
                                                  BiFunction<A1, A2, R1> func,
                                                  Function<R1, R2> func2) {
    
            return func.andThen(func2).apply(a1, a2);
    
        }
    
    }

输出量

Pow : 16.0
    Multiply : 4
    abcde
    100200

## 3.工厂 ##

3.1此示例使用`BiFunction`创建对象，充当工厂模式。

Java8BiFunction3.java

package com.mkyong;
    
    import java.util.function.BiFunction;
    
    public class Java8BiFunction3 {
    
        public static void main(String[] args) {
    
            GPS obj = factory("40.741895", "-73.989308", GPS::new);
            System.out.println(obj);
    
        }
    
        public static <R extends GPS> R factory(String Latitude, String Longitude,
                                                BiFunction<String, String, R> func) {
            return func.apply(Latitude, Longitude);
        }
    
    }
    
    class GPS {
    
        String Latitude;
        String Longitude;
    
        public GPS(String latitude, String longitude) {
            Latitude = latitude;
            Longitude = longitude;
        }
    
        public String getLatitude() {
            return Latitude;
        }
    
        public void setLatitude(String latitude) {
            Latitude = latitude;
        }
    
        public String getLongitude() {
            return Longitude;
        }
    
        public void setLongitude(String longitude) {
            Longitude = longitude;
        }
    
        @Override
        public String toString() {
            return "GPS{" +
                    "Latitude='" + Latitude + '\'' +
                    ", Longitude='" + Longitude + '\'' +
                    '}';
        }
    }

输出量

GPS{Latitude='40.741895', Longitude='-73.989308'}

`GPS::new`调用以下构造函数，该构造函数接受两个参数并返回一个对象（GPS），因此它与`BiFunction`签名匹配。

public GPS(String latitude, String longitude) {
            Latitude = latitude;
            Longitude = longitude;
        }

## 4.更多 ##

4.1通过某些条件过滤`List` 。

Java8BiFunction4.java

package com.mkyong;
    
    import java.util.ArrayList;
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.BiFunction;
    
    public class Java8BiFunction4 {
    
        public static void main(String[] args) {
    
            Java8BiFunction4 obj = new Java8BiFunction4();
    
            List<String> list = Arrays.asList("node", "c++", "java", "javascript");
    
            List<String> result = obj.filterList(list, 3, obj::filterByLength);
    
            System.out.println(result);   // [node, java, javascript]
    
            List<String> result1 = obj.filterList(list, 3, (l1, size) -> {
                if (l1.length() > size) {
                    return l1;
                } else {
                    return null;
                }
            });
    
            System.out.println(result1);  // [node, java, javascript]
    
            List<String> result2 = obj.filterList(list, "c", (l1, condition) -> {
                if (l1.startsWith(condition)) {
                    return l1;
                } else {
                    return null;
                }
            });
    
            System.out.println(result2);  // [c++]
    
            List<Integer> number = Arrays.asList(1, 2, 3, 4, 5);
    
            List<Integer> result3 = obj.filterList(number, 2, (l1, condition) -> {
                if (l1 % condition == 0) {
                    return l1;
                } else {
                    return null;
                }
            });
    
            System.out.println(result3);  // [2, 4]
    
        }
    
        public String filterByLength(String str, Integer size) {
            if (str.length() > size) {
                return str;
            } else {
                return null;
            }
        }
    
        public <T, U, R> List<R> filterList(List<T> list1, U condition,
                                            BiFunction<T, U, R> func) {
    
            List<R> result = new ArrayList<>();
    
            for (T t : list1) {
                R apply = func.apply(t, condition);
                if (apply != null) {
                    result.add(apply);
                }
            }
    
            return result;
    
        }
    
    }

输出量

[node, java, javascript]
    [node, java, javascript]
    [c++]
    [2, 4]

## 参考文献 ##

*  [函数JavaDoc][JavaDoc]
 *  [BiFunction JavaDoc][BiFunction]
 *  [Java 8函数示例][Java 8]
 *  Java 8教程
 *  Java 8谓词示例
 *  Java 8 BiPredicate示例
 *  Java 8使用者示例
 *  Java 8 BiConsumer示例

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> 翻译自: [https://mkyong.com/java8/java-8-bifunction-examples/][https_mkyong.com_java8_java-8-bifunction-examples]

[BiFunction]: https://docs.oracle.com/javase/8/docs/api/java/util/function/BiFunction.html
[JavaDoc]: https://docs.oracle.com/javase/8/docs/api/java/util/function/Function.html
[Java 8]: https://mkyong.com/java8/java-8-function-examples/
[Link 1]: https://mkyong.com/tag/bifunction/
[Link 2]: https://mkyong.com/tag/chain/
[Link 3]: https://mkyong.com/tag/factory/
[Link 4]: https://mkyong.com/tag/function/
[Link 5]: https://mkyong.com/tag/functional-interface/
[Java 8 1]: https://mkyong.com/tag/java-8/
[https_mkyong.com_java8_java-8-bifunction-examples]: https://mkyong.com/java8/java-8-bifunction-examples/