2024-05-23    2024-05-24    3035 字  7 分钟

Supplier接口

java.util.function.Supplier<T> 接口仅包含一个无参的方法T get(),☞为获取一个泛型参数指定类型的对象数据,对应的Labmda表达式需要对外提供一个符合泛型类型的对象数据

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package java.util.function;

/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

示例

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/*
    java.util.function.Supplier<T>接口仅包含一个无参的方法:T get()。用来获取一个泛型参数指定类型的对象数据。

    Supplier<T>接口被称之为生产型接口,指定接口的泛型是什么类型,那么接口中的get方法就会生产什么类型的数据
 */
public class test {
    //定义一个方法,方法的参数传递Supplier<T>接口,泛型执行String,get方法就会返回一个String
    public static String getString(Supplier<String> sup){
        return sup.get();
        // 不要被sup.get()所迷惑,你试试return "hello world",也是可以的,不过我们要想拿到生产型接口的返回值,自然而然就是sup.get()了,
    }

    public static void main(String[] args) {
        //调用getString方法,方法的参数Supplier是一个函数式接口,所以可以传递Lambda表达式
        String s = getString(()->{
            // 生成一个字符串并返回
            return "hello world";
        });
        System.out.println(s);

        //优化Lambda表达式
        String s2 = getString(()->"I love china");
        System.out.println(s2);
    }
}

Consumer 接口

java.util.function.Consumer<T> 接口则正好与Supplier接口相反,它不是生产一个数据,而是消费一个数据, 其数据类型由泛型决定。

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package java.util.function;

import java.util.Objects;

/**
 * Represents an operation that accepts a single input argument and returns no
 * result. Unlike most other functional interfaces, {@code Consumer} is expected
 * to operate via side-effects.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #accept(Object)}.
 *
 * @param <T> the type of the input to the operation
 *
 * @since 1.8
 */
@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); };
    }
}

示例

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import java.util.function.Consumer;
class test {
    /*
        定义一个方法
        方法的参数传递一个字符串的姓名
        方法的参数传递Consumer接口,泛型使用String
        可以使用Consumer接口消费字符串的姓名
     */
    public static void method(String name, Consumer<String> con){
        con.accept(name);
    }

    public static void main(String[] args) {
        //调用method方法,传递字符串,方法的另一个参数是Consumer接口,是一个函数式接口,所以可以传递Lambda表达式
        method("I love china",(String s)->{
            //对传递的字符串进行消费
            //消费方式:直接输出字符串
            //System.out.println(name);
            //消费方式:把字符串转换为大写
            String str = s.toUpperCase();
            System.out.println(str);
        });
    }
}

Predicate接口

有时候我们需要对某种类型的数据进行判断,从而得到一个boolean值结果。这时可以使用 java.util.function.Predicate<T> 接口。

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package java.util.function;

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

示例

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import java.util.function.Predicate;
/*
    java.util.function.Predicate<T>接口
    作用:对某种数据类型的数据进行判断,结果返回一个boolean值

    Predicate接口中包含一个抽象方法:
        boolean test(T t):用来对指定数据类型数据进行判断的方法
            结果:
                符合条件,返回true
                不符合条件,返回false
*/
public class test {
    /*
        定义一个方法
        参数传递一个String类型的字符串
        传递一个Predicate接口,泛型使用String
        使用Predicate中的方法test对字符串进行判断,并把判断的结果返回
     */
    public static boolean checkString(String s, Predicate<String> pre){
        return  pre.test(s);
    }

    public static void main(String[] args) {
        String s = "hello";
        boolean b = checkString(s,str->str.length()>5);
        System.out.println(b);
    }
}

Function接口

java.util.function.Function<T,R> 接口用来根据一个类型的数据得到另一个类型的数据,前者称为前置条件, 后者称为后置条件

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package java.util.function;

import java.util.Objects;

/**
 * Represents a function that accepts one argument and produces a result.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #apply(Object)}.
 *
 * @param <T> the type of the input to the function
 * @param <R> the type of the result of the function
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

示例

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import java.util.function.Function;
/*
    java.util.function.Function<T,R>接口用来根据一个类型的数据得到另一个类型的数据,
        前者称为前置条件,后者称为后置条件。
    Function接口中最主要的抽象方法为:R apply(T t),根据类型T的参数获取类型R的结果。
        使用的场景例如:将String类型转换为Integer类型。
 */
public class test {
    /*
        定义一个方法
        方法的参数传递一个字符串类型的整数
        方法的参数传递一个Function接口,泛型使用<String,Integer>
        使用Function接口中的方法apply,把字符串类型的整数,转换为Integer类型的整数
     */
    public static void change(String s, Function<String,Integer> fun){
        //Integer in = fun.apply(s);
        int in = fun.apply(s);//自动拆箱 Integer->int
        System.out.println(in);
    }

    public static void main(String[] args) {
        //定义一个字符串类型的整数
        String s = "1234";
        //调用change方法,传递字符串类型的整数,和Lambda表达式
        change(s,(String str)->{
            //把字符串类型的整数,转换为Integer类型的整数返回
            return Integer.parseInt(str);
        });
        //优化Lambda
        change(s,str->Integer.parseInt(str));
    }
}