Java8 - 新功能之函数式接口BinaryOperator

Posted 2021-04-09 不会coding

BinaryOperator<T>函数式接口是BiFunction<T,T,T>的子接口，它接受2个都是T类型的参数，并且返回T类型的结果。

下面是这个接口的定义：

package java.util.function;

import java.util.Objects;
import java.util.Comparator;

/**
 * Represents an operation upon two operands of the same type, producing a result
 * of the same type as the operands. This is a specialization of
 * {@link BiFunction} for the case where the operands and the result are all of
 * the same type.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object, Object)}.
 *
 * @param <T> the type of the operands and result of the operator
 *
 * @see BiFunction
 * @see UnaryOperator
 * @since 1.8
 */
@FunctionalInterface
public interface BinaryOperator<T> extends BiFunction<T,T,T> {
    /**
 * Returns a {@link BinaryOperator} which returns the lesser of two elements
 * according to the specified {@code Comparator}.
 *
 * @param <T> the type of the input arguments of the comparator
 * @param comparator a {@code Comparator} for comparing the two values
 * @return a {@code BinaryOperator} which returns the lesser of its operands,
 * according to the supplied {@code Comparator}
 * @throws NullPointerException if the argument is null
 */
    public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
    }

    /**
 * Returns a {@link BinaryOperator} which returns the greater of two elements
 * according to the specified {@code Comparator}.
 *
 * @param <T> the type of the input arguments of the comparator
 * @param comparator a {@code Comparator} for comparing the two values
 * @return a {@code BinaryOperator} which returns the greater of its operands,
 * according to the supplied {@code Comparator}
 * @throws NullPointerException if the argument is null
 */
    public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {
        Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
    }
}

下面对这个接口的方法如何使用进行举例说明：

例子1：apply抽象方法

private static void testBaseFunction() {
        BinaryOperator<String> joinOperator = (t1,t2)-> t1 + "," + t2;
        String str = joinOperator.apply("Hello", "World");
        System.out.println("str:" + str);
  }

输出：str:Hello,World

例子2：minBy静态方法

private static void testMinBy() {
        //取2个整数的最小值
 BinaryOperator<Integer> minOperator = BinaryOperator.minBy(Comparator.naturalOrder());
 Integer minInt = minOperator.apply(203,500);
 System.out.println("minInt:" + minInt);
 }

输出：minInt:203

例子3：maxBy静态方法

private static void testMaxBy() {
        //取2个字符串的最大值，小写字母排序要比大写字母大
 BinaryOperator<String> maxOperator = BinaryOperator.maxBy(Comparator.naturalOrder());
 String maxStr = maxOperator.apply("ABC","anc");
 System.out.println("maxStr:" + maxStr);
 }

输出：maxStr:anc

java的泛型不支持基础数据类型，因为jdk特别增加了DoubleBinaryOperator，IntBinaryOperator和LongBinaryOperator三个支持基础数据类型的函数式接口。它们的定义如下：

@FunctionalInterface
public interface DoubleBinaryOperator {
    /**
 * Applies this operator to the given operands.
 *
 * @param left the first operand
 * @param right the second operand
 * @return the operator result
 */
    double applyAsDouble(double left, double right);
}

@FunctionalInterface
public interface IntBinaryOperator {

    /**
 * Applies this operator to the given operands.
 *
 * @param left the first operand
 * @param right the second operand
 * @return the operator result
 */
    int applyAsInt(int left, int right);
}

@FunctionalInterface
public interface LongBinaryOperator {

    /**
 * Applies this operator to the given operands.
 *
 * @param left the first operand
 * @param right the second operand
 * @return the operator result
 */
    long applyAsLong(long left, long right);
}

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