【函数式接口】java.util.function.Predicate

java.util.function.Predicate 是 Java 8 引入的一个函数式接口，位于 java.util.function 包中。它表示一个断言（布尔值函数），接受一个输入参数并返回一个布尔值。

核心特性

1. 函数式接口：只有一个抽象方法
2. 泛型支持：Predicate<T> 可以处理任何类型的对象
3. 常用场景：用于条件检查、过滤操作等

接口定义

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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);
    }

    /**
     * Returns a predicate that is the negation of the supplied predicate.
     * This is accomplished by returning result of the calling
     * {@code target.negate()}.
     *
     * @param <T>     the type of arguments to the specified predicate
     * @param target  predicate to negate
     *
     * @return a predicate that negates the results of the supplied
     *         predicate
     *
     * @throws NullPointerException if target is null
     *
     * @since 11
     */
    @SuppressWarnings("unchecked")
    static <T> Predicate<T> not(Predicate<? super T> target) {
        Objects.requireNonNull(target);
        return (Predicate<T>)target.negate();
    }
}

主要方法

1. 抽象方法：
   • boolean test(T t)：对给定参数进行断言测试
2. 默认方法：
   • and(Predicate other)：逻辑与组合
   • or(Predicate other)：逻辑或组合
   • negate()：逻辑非操作
3. 静态方法：
   • isEqual(Object target)：创建一个测试输入是否等于目标值的谓词

使用示例

基本用法

Predicate<String> isEmpty = s -> s.isEmpty();
System.out.println(isEmpty.test("")); // true
System.out.println(isEmpty.test("Hello")); // false

方法组合

Predicate<String> isLongerThan5 = s -> s.length() > 5;
Predicate<String> startsWithA = s -> s.startsWith("A");

// 组合两个条件
Predicate<String> combined = isLongerThan5.and(startsWithA);
System.out.println(combined.test("Apple")); // false (长度不大于5)
System.out.println(combined.test("Avocado")); // true

常用场景 - 过滤集合

List<String> names = Arrays.asList("Alice", "Bob", "Charlie", "David", "Eve");

// 使用Predicate过滤
List<String> longNames = names.stream()
    .filter(name -> name.length() > 4)
    .collect(Collectors.toList());
// 结果: ["Alice", "Charlie", "David"]

静态方法使用

List<String> words = Arrays.asList("Java", "Python", "C++", "");
Predicate<String> isJava = Predicate.isEqual("Java");
words.stream().filter(isJava).forEach(System.out::println); // 输出: Java

实际应用场景

1. 集合过滤（Stream API 中的 filter() 方法）
2. 条件验证
3. 动态条件组合
4. 作为方法参数传递条件逻辑

Predicate 接口是函数式编程在 Java 中的重要体现，它使得条件逻辑可以像对象一样被传递和组合，大大增强了代码的灵活性和可重用性。