albert's blog: [Java] Lambda Expressions Example

2016/10/03

[Java] Lambda Expressions Example

If I have POJO, which named Person

package albert.practice.lambda;

import java.io.Serializable;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import lombok.ToString;

@Data
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Person implements Serializable{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    
    private String name;
    private Gender gender;
    private int age;
    private String email;
    
}

If I have a List of Person, I would like to do search based on some criteria.

Example 1: I would like to find the person who is male and age is equal or greater than 25
Using Apache Commons Collection Utils:

private List<Person> findByApacheCommonsEx1(List<Person> dummyData) {
     return (List<Person>) CollectionUtils.select(dummyData, new Predicate() {
         @Override
         public boolean evaluate(Object object) {
              Person person = (Person) object;
              Boolean condition = person.getAge() >= 25 && Gender.MALE.equals(person.getGender());
              return condition;
         }
     });
}

Using Lambda Expression:

private List<Person> findByLambdaEx1(List<Person> dummyData) {
     return dummyData.stream()
                      .filter(person -> person.getAge() >= 25 && Gender.MALE.equals(person.getGender()))
                      .collect(Collectors.toList());
}

Example 2: I would like to find the person who is equal or greater than 25, and find the first person from the collection.
Using Apache Commons Collection Utils:

private Person findByApacheCommonsEx2(List<Person> dummyData) {
     List<Person> result = (List<Person>) CollectionUtils.select(dummyData, new Predicate() {
         @Override
         public boolean evaluate(Object object) {
              Person person = (Person) object;
              return person.getAge() >= 25;
         }
     });

     Person person = null;
     if (!result.isEmpty()) {
         person = result.get(0);
     }
     return person;
}

Using Lambda Expression:

 private Person findByLambdaEx2(List<Person> dummyData) {
     Optional<Person> p = dummyData.stream()
                                         .filter(person -> person.getAge() >= 25).findFirst();
     return p.isPresent() ? p.get() : null;
 }

Example 3. Find List of String with criteria, throw RuntimeException if mismatch:

private List<String> getDummyPlayers() {
 return Arrays.asList("王 柏 融", "林 智 勝", "林 泓 育", "蔣 智 賢", "高 國 慶");
}

// using Apache Commons CollectionUtils
private void findPlayer() {
    List<String> players = getDummyPlayers();
    String name = "陳 金 鋒";
    List<String> result = (List<String>) CollectionUtils.select(players, new Predicate() {
            @Override
     public boolean evaluate(Object object) {
      String player = (String) object;
      return name.equals(player);
     }
    });

    if (CollectionUtils.isEmpty(result)) {
        throw new RuntimeException("Cannot find player with name " + name);
    }
}

// Using Lambda expression
private void findPlayer_usingJDK8() {
    List<String> players = getDummyPlayers();
    String name = "陳 金 鋒";
    players.stream().filter(player -> name.equals(player)).findFirst()
                    .orElseThrow(() -> new RuntimeException("Cannot find player with name " + name));
}

Example 4. Remove duplicate item in List of String and do sort:

private List<String> getDummyPlayersWithDuplicateValue() {
    return Arrays.asList("王 柏 融", "林 智 勝", "林 泓 育", "王 柏 融", "林 智 勝", "蔣 智 賢", "高 國 慶");
}

// using Apache Commons CollectionUtils
private void removeDulpicatePlayerAndSort() {
    List<String> players = getDummyPlayersWithDuplicateValue();
    List<String> newList = new ArrayList<>();

    for (String player : players) {
        if (!newList.contains(player)) {
     newList.add(player);
 }
    }

    Collections.sort(newList, new Comparator<String>() {
 @Override
 public int compare(String o1, String o2) {
     return o1.compareTo(o2);
 }
    });

    newList.stream().forEach(player -> log.debug(player.toString()));
}

// Using Lambda expression
private void removeDulpicatePlayerAndSort_usingJDK8() {
    List<String> players = getDummyPlayersWithDuplicateValue();
    List<String> newList = players.stream().map(player -> player).distinct().sorted((o1, o2) -> o1.compareTo(o2))
      .collect(Collectors.toList());
    newList.stream().forEach(player -> log.debug(player.toString()));
}

Reference
[1] https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html
[2] https://dzone.com/articles/why-java-8-1

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2016/10/03

[Java] Lambda Expressions Example

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