使用Java 8 Stream和Lambda我想从列表中形成一个特定的列表。详情如下

Question

假设我有一个像列表一样的列表;

{ 
    {1-a12-abc,firstname,john}, 
    {2-a12-abc,firstname,tom}, 
    {1-a12-abc,lastname,doe}, 
    {3-a22-abc,city,Delhi} 
} 

我们可以把每个列表中包含一些PersonId，Attribute和attribute value。我想每个Id，属性列表和相应的值列表一起。

使用Java 8个流API我想输出象下面这样：

{ 
     {{1-a12-abc},{firstname,lastname},{john,doe}}, 
     {{2-a12-abc},{firstname},{tom}}, 
     {{3-a22-abc},{city},{Delhi}} 
    } 

凡分组会随着每个列表的第一个元素上面给出。每个组的第二和第三个元素将形成列表。

请帮忙解决。

Answer 1

这一个版本用java 8流：

// Initial list 
public class AttributeValue { 
    String id; 
    String attribute; 
    String attributeValue; 

    AttributeValue(String id, String attribute, String attributeValue) { 
     this.id = id; 
     this.attribute = attribute; 
     this.attributeValue = attributeValue; 
    } 

    @Override 
    public String toString() { 
     return "{" + id + "," + attribute + "," + attributeValue + "}"; 
    } 

    // ommiting equals() and hashCodes() methods for simplification 
} 

// final list 
public class Record { 
    String id; 
    List<String> attributes; 
    List<String> attributeValues; 

    Record(String id, List<String> attributes, List<String> attributeValues) { 
     this.id = id; 
     this.attributes = attributes; 
     this.attributeValues = attributeValues; 
    } 

    @Override 
    public String toString() { 
     return "{{" + id + "}," 
       + attributes.stream().collect(Collectors.joining(",", "{", "}")) + "," 
       + attributeValues.stream().collect(Collectors.joining(",", "{", "}")) + "}"; 
    } 

    // ommiting equals() and hashCodes() methods for simplification 
} 

private List<Record> transform(List<AttributeValue> values) { 
    return values.stream() 
      .collect(Collectors.groupingBy(value -> value.id, LinkedHashMap::new, Collectors.toList())) 
      .entrySet().stream() 
      .map(pair -> new Record(
       pair.getKey(), 
       pair.getValue().stream().map(value -> value.attribute).collect(Collectors.toList()), 
       pair.getValue().stream().map(value -> value.attributeValue).collect(Collectors.toList()) 
      )) 
      .collect(Collectors.toList()); 
} 

和单元测试使用junit5和assertj用于流利：

@Test 
void parse_a_van_page_with_empty_json_data() throws Exception { 
    List<AttributeValue> initial = Arrays.asList(
      new AttributeValue("1-a12-abc", "firstname", "john"), 
      new AttributeValue("2-a12-abc", "firstname", "tom"), 
      new AttributeValue("1-a12-abc", "lastname", "doe"), 
      new AttributeValue("3-a22-abc", "city", "Delhi") 
    ); 

    List<Record> expected = Arrays.asList(
      new Record("1-a12-abc", Arrays.asList("firstname", "lastname"), Arrays.asList("john", "doe")), 
      new Record("2-a12-abc", Collections.singletonList("firstname"), Collections.singletonList("tom")), 
      new Record("3-a22-abc", Collections.singletonList("city"), Collections.singletonList("Delhi")) 
    ); 

    assertThat(transform(initial)).containsAll(expected); 

    // Some printing 
    System.out.println("initial: "); 
    initial.forEach(System.out::println); 

    System.out.println("expected: "); 
    expected.forEach(System.out::println); 
}