Java 8谓词示例

我不是女神ヾ 2023-02-15 05:07 65阅读 0赞

在Java 8中， [谓词][Link 1]是一个功能接口，它接受参数并返回布尔值。 通常，它通常在过滤器中应用于对象集合。

@FunctionalInterface
    public interface Predicate<T> {
      boolean test(T t);
    }

**进一步阅读**  
Java 8 BiPredicate示例

## 1. filter（）中的谓词 ##

`filter()`接受谓词作为参数。

Java8Predicate.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class Java8Predicate {
    
        public static void main(String[] args) {
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    
            List<Integer> collect = list.stream().filter(x -> x > 5).collect(Collectors.toList());
    
            System.out.println(collect); // [6, 7, 8, 9, 10]
    
        }
    
    }

输出量

[6, 7, 8, 9, 10]

Java8Predicate.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class Java8Predicate {
    
        public static void main(String[] args) {
    
            Predicate<Integer> noGreaterThan5 =  x -> x > 5;
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    
            List<Integer> collect = list.stream()
                    .filter(noGreaterThan5)
                    .collect(Collectors.toList());
    
            System.out.println(collect); // [6, 7, 8, 9, 10]
    
        }
    
    }

输出量

[6, 7, 8, 9, 10]

## 2.谓词和（） ##

2.1多个过滤器。

Java8Predicate2.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class Java8Predicate2 {
    
        public static void main(String[] args) {
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    
            // multiple filters
            List<Integer> collect = list.stream()
                    .filter(x -> x > 5 && x < 8).collect(Collectors.toList());
    
            System.out.println(collect);
    
        }
    
    }

输出量

[6, 7]

2.1替换为`Predicate.and()`

Java8Predicate2.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class Java8Predicate2 {
    
        public static void main(String[] args) {
    
            Predicate<Integer> noGreaterThan5 = x -> x > 5;
            Predicate<Integer> noLessThan8 = x -> x < 8;
    
            List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    
            List<Integer> collect = list.stream()
                    .filter(noGreaterThan5.and(noLessThan8))
                    .collect(Collectors.toList());
    
            System.out.println(collect);
    
        }
    
    }

输出量

[6, 7]

## 3. Predicate.or（） ##

Java8Predicate3.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class Java8Predicate3 {
    
        public static void main(String[] args) {
    
            Predicate<String> lengthIs3 = x -> x.length() == 3;
            Predicate<String> startWithA = x -> x.startsWith("A");
    
            List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
    
            List<String> collect = list.stream()
                    .filter(lengthIs3.or(startWithA))
                    .collect(Collectors.toList());
    
            System.out.println(collect);
    
        }
    
    }

输出量

[A, AA, AAA, BBB]

## 4. Predicate.negate（） ##

查找所有不以“ A”开头的元素。

Java8Predicate4.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class Java8Predicate4 {
    
        public static void main(String[] args) {
    
            Predicate<String> startWithA = x -> x.startsWith("A");
    
            List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
    
            List<String> collect = list.stream()
                    .filter(startWithA.negate())
                    .collect(Collectors.toList());
    
            System.out.println(collect);
    
        }
    
    }

输出量

[B, BB, BBB]

## 5.函数中的Predicate.test（） ##

功能谓词。

Java8Predicate5.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class Java8Predicate5 {
    
        public static void main(String[] args) {
    
            List<String> list = Arrays.asList("A", "AA", "AAA", "B", "BB", "BBB");
    
            System.out.println(StringProcessor.filter(
                    list, x -> x.startsWith("A")));                    // [A, AA, AAA]
    
            System.out.println(StringProcessor.filter(
                    list, x -> x.startsWith("A") && x.length() == 3)); // [AAA]
    
        }
    
    }
    
    class StringProcessor {
        static List<String> filter(List<String> list, Predicate<String> predicate) {
            return list.stream().filter(predicate::test).collect(Collectors.toList());
        }
    }

输出量

[A, AA, AAA]
    [AAA]

## 6.谓词链接 ##

我们可以将谓词链接在一起。

Java8Predicate6.java

package com.mkyong.java8;
    
    import java.util.function.Predicate;
    
    public class Java8Predicate6 {
    
        public static void main(String[] args) {
    
            Predicate<String> startWithA = x -> x.startsWith("a");
    
            // start with "a" or "m"
            boolean result = startWithA.or(x -> x.startsWith("m")).test("mkyong");
            System.out.println(result);     // true
    
            // !(start with "a" and length is 3)
            boolean result2 = startWithA.and(x -> x.length() == 3).negate().test("abc");
            System.out.println(result2);    // false
    
        }
    
    }

输出量

true
    false

## 7.宾语谓语 ##

Hosting.java

package com.mkyong.java8;
    
    public class Hosting {
    
        private int Id;
        private String name;
        private String url;
    
        public Hosting(int id, String name, String url) {
            Id = id;
            this.name = name;
            this.url = url;
        }
    
        //... getters and setters, toString()
    }

HostingRespository.java

package com.mkyong.java8;
    
    import java.util.List;
    import java.util.function.Predicate;
    import java.util.stream.Collectors;
    
    public class HostingRespository {
    
        public static List<Hosting> filterHosting(List<Hosting> hosting,
                                                  Predicate<Hosting> predicate) {
            return hosting.stream()
                    .filter(predicate)
                    .collect(Collectors.toList());
        }
    
    }

Java8Predicate7.java

package com.mkyong.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.function.Predicate;
    
    public class Java8Predicate7 {
    
        public static void main(String[] args) {
    
            Hosting h1 = new Hosting(1, "amazon", "aws.amazon.com");
            Hosting h2 = new Hosting(2, "linode", "linode.com");
            Hosting h3 = new Hosting(3, "liquidweb", "liquidweb.com");
            Hosting h4 = new Hosting(4, "google", "google.com");
    
            List<Hosting> list = Arrays.asList(new Hosting[]{h1, h2, h3, h4});
    
            List<Hosting> result = HostingRespository.filterHosting(list, x -> x.getName().startsWith("g"));
            System.out.println("result : " + result);  // google
    
            List<Hosting> result2 = HostingRespository.filterHosting(list, isDeveloperFriendly());
            System.out.println("result2 : " + result2); // linode
    
        }
    
        public static Predicate<Hosting> isDeveloperFriendly() {
            return n -> n.getName().equals("linode");
        }
    }

输出量

result : [Hosting{Id=4, name='google', url='google.com'}]
    result2 : [Hosting{Id=2, name='linode', url='linode.com'}]

做完了

## 参考文献 ##

*  [谓词JavaDoc][Link 1]
 *  Java 8 Streams过滤器示例
 *  Java 8 BiPredicate示例

标签： [过滤器][Link 2] [功能接口][Link 3] [高阶函数][Link 4] [java 8][] [谓词][Link 5] [流][Link 6]

> 翻译自: [https://mkyong.com/java8/java-8-predicate-examples/][https_mkyong.com_java8_java-8-predicate-examples]

[Link 1]: https://docs.oracle.com/javase/8/docs/api/java/util/function/Predicate.html
[Link 2]: https://mkyong.com/tag/filter/
[Link 3]: https://mkyong.com/tag/functional-interface/
[Link 4]: https://mkyong.com/tag/higher-order-function/
[java 8]: https://mkyong.com/tag/java-8/
[Link 5]: https://mkyong.com/tag/predicate/
[Link 6]: https://mkyong.com/tag/stream/
[https_mkyong.com_java8_java-8-predicate-examples]: https://mkyong.com/java8/java-8-predicate-examples/