Java 8 Streams filter

末蓝、 2022-06-11 04:12 134阅读 0赞

> 在这篇教程中，我们将用少量的例子向你展示java8中的Streams filter(), collect(), findAny() and orElse().

*  Streams filter() 和 collect()
    
     *  Java 8之前, 过滤一个集合是这样的 :
        
            BeforeJava8.java
            package com.ricky.java8;
            
            import java.util.ArrayList;
            import java.util.Arrays;
            import java.util.List;
            
            public class BeforeJava8 { 
            
                public static void main(String[] args) {
            
                    List<String> lines = Arrays.asList("spring", "node", "ricky");
                    List<String> result = getFilterOutput(lines, "ricky");
                    for (String temp : result) {
                        System.out.println(temp);    //output : spring, node
                    }
            
                }
            
                private static List<String> getFilterOutput(List<String> lines, String filter) {
                    List<String> result = new ArrayList<>();
                    for (String line : lines) {
                        if (!"ricky".equals(line)) { // we dont like ricky
                            result.add(line);
                        }
                    }
                    return result;
                }
            
            }
     *  输出
        
            spring
            node
     *  1.2 java8中等价的例子，使用stream.filter()去过滤集合，使用collect()将stream转化为一个集合。
    
        NowJava8.java
        package com.ricky.java8;
        
        import java.util.Arrays;
        import java.util.List;
        import java.util.stream.Collectors;
        
        public class NowJava8 { 
        
            public static void main(String[] args) {
        
                List<String> lines = Arrays.asList("spring", "node", "ricky");
        
                List<String> result = lines.stream()                // convert list to stream
                        .filter(line -> !"ricky".equals(line))     // we dont like ricky
                        .collect(Collectors.toList());              // collect the output and convert streams to a List
        
                result.forEach(System.out::println);                //output : spring, node
        
            }
        
        }
    
    \-输出
    
        spring
        node
 *  Streams filter(), findAny() and orElse()

Person.java
    package com.ricky.java8;
    
    public class Person { 
    
        private String name;
        private int age;
    
        public Person(String name, int age) {
            this.name = name;
            this.age = age;
        }
    
        //gettersm setters, toString
    }

*  Java 8之前, 你获取Person的name是这样的 :

BeforeJava8.java
            package com.ricky.java8;
    
            import java.util.Arrays;
            import java.util.List;
    
            public class BeforeJava8 { 
    
                public static void main(String[] args) {
    
                    List<Person> persons = Arrays.asList(
                            new Person("ricky", 30),
                            new Person("jack", 20),
                            new Person("lawrence", 40)
                    );
    
                    Person result = getStudentByName(persons, "jack");
                    System.out.println(result);
    
                }
    
                private static Person getStudentByName(List<Person> persons, String name) {
    
                    Person result = null;
                    for (Person temp : persons) {
                        if (name.equals(temp.getName())) {
                            result = temp;
                        }
                    }
                    return result;
                }
            }

输出
    
    ```java
    Person{name='jack', age=20}
    
    ```

*  等价的java8列子，使用stream.filter()过滤集合， .findAny().orElse (null)返回一个符合条件的对象。
    
        NowJava8.java
            package com.ricky.java8;
        
            import java.util.Arrays;
            import java.util.List;
        
            public class NowJava8 { 
        
                public static void main(String[] args) {
        
                    List<Person> persons = Arrays.asList(
                            new Person("ricky", 30),
                            new Person("jack", 20),
                            new Person("lawrence", 40)
                    );
        
                    Person result1 = persons.stream()                        // Convert to steam
                            .filter(x -> "jack".equals(x.getName()))        // we want "jack" only
                            .findAny()                                      // If 'findAny' then return found
                            .orElse(null);                                  // If not found, return null
        
                    System.out.println(result1);
        
                    Person result2 = persons.stream()
                            .filter(x -> "ahmook".equals(x.getName()))
                            .findAny()
                            .orElse(null);
        
                    System.out.println(result2);
        
                }
        
            }
    
    输出
    
        Person{name='jack', age=20}
            null
    
     *  多种条件  
        
        
            NowJava8.java
                package com.ricky.java8;
            
                import java.util.Arrays;
                import java.util.List;
            
                public class NowJava8 { 
            
                    public static void main(String[] args) {
            
                        List<Person> persons = Arrays.asList(
                                new Person("ricky", 30),
                                new Person("jack", 20),
                                new Person("lawrence", 40)
                        );
            
                        Person result1 = persons.stream()
                                .filter((p) -> "jack".equals(p.getName()) && 20 == p.getAge())
                                .findAny()
                                .orElse(null);
            
                        System.out.println("result 1 :" + result1);
            
                        //or like this
                        Person result2 = persons.stream()
                                .filter(p -> {
                                    if ("jack".equals(p.getName()) && 20 == p.getAge()) {
                                        return true;
                                    }
                                    return false;
                                }).findAny()
                                .orElse(null);
            
                        System.out.println("result 2 :" + result2);
            
                    }
            
            
                }
 *  输出
    
        result 1 :Person{name='jack', age=20}
            result 2 :Person{name='jack', age=20}
    
     *  Streams filter() and map()

NowJava8.java
    package com.ricky.java8;
    
    import java.util.Arrays;
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class NowJava8 { 
    
        public static void main(String[] args) {
    
            List<Person> persons = Arrays.asList(
                    new Person("ricky", 30),
                    new Person("jack", 20),
                    new Person("lawrence", 40)
            );
    
            String name = persons.stream()
                    .filter(x -> "jack".equals(x.getName()))
                    .map(Person::getName)                        //convert stream to String
                    .findAny()
                    .orElse("");
    
            System.out.println("name : " + name);
    
            List<String> collect = persons.stream()
                    .map(Person::getName)
                    .collect(Collectors.toList());
    
            collect.forEach(System.out::println);
    
        }
    
    }

*  输出
    
        name : jack
        
        ricky
        jack
        lawrence

# 翻译 #

ricky 20170728

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