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 */@FunctionalInterfacepublic 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：

@FunctionalInterfacepublic 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);}

@FunctionalInterfacepublic 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);}

@FunctionalInterfacepublic 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);}