I am currently using Java 17 along with ModelMapper to convert objects from one type to another. I came across a confusing scenario where I assign the result of a method that returns a List<Test> to a new ArrayList<Boolean>.
Later, I attempt to add a boolean value to this list, and it surprisingly compiles and gives an output, which I'm struggling to understand.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import .modelmapper.ModelMapper;
import com.google.gson.reflect.TypeToken;

public final class ModelMapperAdapter {

    private static ModelMapper MODEL_MAPPER;

    public static class Test{
        Integer var;
        //getter
        //setter
    }
    
    public static void main(String[] args) {
        MODEL_MAPPER = new ModelMapper();
        Test test = new Test();
        List<Test> list = new ArrayList<>();
        List<Boolean> ids = new ArrayList<>();
        list.add(test);
        ids.addAll(MODEL_MAPPER.map(list, new TypeToken<List<Test>>(){}.getType()));
        System.out.println(ids);//[ModelMapperAdapter.Test(var=null)]
        ids.add(true);
        System.out.println(ids);//[ModelMapperAdapter.Test(var=null), true]     
    }
}

I am currently using Java 17 along with ModelMapper to convert objects from one type to another. I came across a confusing scenario where I assign the result of a method that returns a List<Test> to a new ArrayList<Boolean>.
Later, I attempt to add a boolean value to this list, and it surprisingly compiles and gives an output, which I'm struggling to understand.

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
import .modelmapper.ModelMapper;
import com.google.gson.reflect.TypeToken;

public final class ModelMapperAdapter {

    private static ModelMapper MODEL_MAPPER;

    public static class Test{
        Integer var;
        //getter
        //setter
    }
    
    public static void main(String[] args) {
        MODEL_MAPPER = new ModelMapper();
        Test test = new Test();
        List<Test> list = new ArrayList<>();
        List<Boolean> ids = new ArrayList<>();
        list.add(test);
        ids.addAll(MODEL_MAPPER.map(list, new TypeToken<List<Test>>(){}.getType()));
        System.out.println(ids);//[ModelMapperAdapter.Test(var=null)]
        ids.add(true);
        System.out.println(ids);//[ModelMapperAdapter.Test(var=null), true]     
    }
}

• java
• generics
• collections
• java-17
• modelmapper

1 Answer 1

The problem is that the ModelMapper.map(Object source, Type destinationType) method makes promises that it cannot fulfill.

If you look at the complete method signature:

public <D> D map​(Object source, Type destinationType)

it pretends that for any source object and type descriptor destinationType it will return an instance of D.

The problem is that D and the destinationType don't need to be related.

Due to the way the method signature is written:

• the Java Compiler cannot verify that those two types are related (Type destinationType is not generic). If the Java Compiler could verify it the code would not compile.
• the ModelMapper.map() method has no way of knowing what D is and therefore cannot check that you are using the method correctly. If it could verify what D is it could throw an exception at runtime.

That means that (when using ModelMapper) it is your responsability to make sure that D refers the same type as destinationType.

Note that this problem (together with performance problems) is the reason I stopped using ModelMapper years ago and replaced it with MapStruct