'Convert One Entity to Another Entity in Java

I have 2 different Entity Class as below :

@Table(name = "tableA")
public class EntityA{

    @PartitionKey
    @Column(name = "age")
    int age;

    @PartitionKey
    @Column(name = "class")
    int class;

    @Column(name = "rollNo")
    int rollNo;

}

@Table(name = "tableB")
public class EntityA{

    @PartitionKey
    @Column(name = "class")
    int class;

    @Column(name = "rollNo")
    int rollNo;

    // one column less 
}

Now based on some condition I need to persist the data in both the tables.

In my service layer I have a List of EntityA :: List<EntityA> which I am passing to the DAOImpl method where I insert the data as below :

public void insertListItems(List<EntityA> entityAList) {
    // here I need to convert the List<EntityA>  to List<EntityB>
    // before table insert operation.
}

How I can make that conversion where in the EnitityB I have one column less. I don't want to write boilerplate code for the conversion as my entity class is big in actual. Instead use some library that helps with the mapping.

javaobjectmapper


Solution 1:[1]

You can use Jackson's ObjectMapper library to achieve this. In order for this to work you must have getters declared in both EntityA and EntityB class, as well as a default (empty) constructor.

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.util.Collections;
import java.util.List;
import java.util.Objects;
import java.util.Optional;
import java.util.stream.Collectors;

/**
 * Function converts list of objects to another list of objects
 * which' type is specified by the clazz attribute. Clazz attribute
 * must not be null, or it will throw a NullPointerException.
 * 
 * @param list List of target objects
 * @param clazz Class to map list objects to
 * @param <T> Target class
 * @param <F> From class
 * @return F.class List converted to T.class List
 */
public static <T, F> List<T> convertList(List<F> list, Class<T> clazz) {
    Objects.requireNonNull(clazz);
    ObjectMapper mapper = new ObjectMapper();

    // Important: this must be declared so mapper doesn't throw 
    // an exception for all properties which it can't map.
    mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    return Optional.ofNullable(list)
            .orElse(Collections.emptyList())
            .stream()
            .map(obj -> mapper.convertValue(obj, clazz))
            .collect(Collectors.toList());
}

public void insertListItems(List<EntityA> entityAList) {
    List<EntityB> entityBList = convertList(entityAList, EntityB.class);
    // Continue with table insert operation
}

Solution 2:[2]

You could try to get a Map like this Map<String, List<String> columns;

The first value (key) is the column name and the second value (value) is a list of all values of this column.

Then you can just use if statements for selecting a column or not.

So:

  1. Load all columns and their values
  2. Use a loop with if statements.
  3. In this loop, you could also integrate a list of Strings, which are the unallowed or allowed column names.

Solution 3:[3]

?????????????????????,???????sql??.?????????????,??????

If the purpose is purely to store data, I feel that it is better to write native SQL without converting. My English is not good, I use translation software, I hope it can help you

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1
Solution 2 David Weber
Solution 3 yuanfei li

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