Java 8 in action代码总结 1 - filter、Predicate接口

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文章目录

调用filter方法过滤目标集合
自定义的Predicate接口和 filter方法
- JDK8自带的Predicate(谓词)接口

调用filter方法过滤目标集合

//调用filter的代码
//filter方法中ApplePredicate接口当作参数传入，
//在调用filter方法的时候则需要传入这个接口的具体实现，对应接口内的方法也要重写；具体的过滤也在filter重写方法的内部实现
List<Apple> apples = filter(inventory, new ApplePredicate() {@Overridepublic boolean test(Apple a) {return a.getWeight() > 119;}
});
System.out.println(apples);

自定义的Predicate接口和 filter方法

自定义Predicate接口的抽象方法的参数为最后具体要过滤和比较的对象
filter方法的参数是Predicate接口，以及需要过滤的集合或者数组

public static List<Apple> filter(List<Apple> inventory, ApplePredicate p){List<Apple> result = new ArrayList<>();for(Apple apple : inventory){if(p.test(apple)){result.add(apple);}}return result;
}   interface ApplePredicate{public boolean test(Apple a);
}

总结：这边接口作为参数，是行为参数化的体现，是java8 in action中chapter 2的主题 - Passing code with behavior parameterization

JDK8自带的Predicate(谓词)接口

jdk8自带的Predicate接口，用来代替上面的自定义的谓词接口。
该接口中定义了test，negate，or ，and，isEqual等方法。
当调用这个接口的时候，我们需要先定义一个filter()方法，传入Collection操作对象，根据所要做的操作，传入适当个数的Predicate作为参数。

调用代码

List<Apple> greenApples = filterApples(inventory, FilteringApples::isGreenApple);
System.out.println(greenApples);List<Apple> greenApples2 = filterApples(inventory, (Apple a) -> "green".equals(a.getColor()));
System.out.println(greenApples2);

自定义filter方法

public static boolean isGreenApple(Apple apple) {return "green".equals(apple.getColor()); 
}public static List<Apple> filterApples(List<Apple> inventory, Predicate<Apple> p){List<Apple> result = new ArrayList<>();for(Apple apple : inventory){//取反Predicate<Apple> negate = p.negate();//取反静态方法Predicate<Apple> not = Predicate.not(p);if(negate.test(apple)){result.add(apple);}}return result;
}public static List<Apple> filterApples(List<Apple> inventory, Predicate<Apple> p,Predicate<Apple> p1){List<Apple> result = new ArrayList<>();for(Apple apple : inventory){//二合一Predicate<Apple> and = p.and(p1);//二选一Predicate<Apple> or = p.or(p1);if(and.test(apple)){result.add(apple);}}return result;
}

Predicate接口代码

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.* 根据给定参数评估此谓词。** @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.* 返回合并两个谓词逻辑的新的谓词** <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.* 返回一个与给定代码逻辑相反的谓词，取反** @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.* 返回一个符合两个中的一个谓词的新的谓词** <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)}.* 判断两个谓词的效果是否一样** @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* 静态方法，和negate效果一样* {@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();}
}