Insert Custom list and get that list in Room database using TypeConverter

Aman Gupta

29 June 2019

We have now that Google announced a set of new libraries for designing Android application’s architecture — Android Architecture Components. And the Room persistent library is one of them.

Before reading this blog you have to sound knowledge of room persistent library, Please read the below blog first,

Android Room Persistence Library

Now we can start.

What is TypeConverters in Room?

When you want to store some custom types objects like the e.g lists, models, list of models etc, in the database, you can use Type Converters. It converts your custom object type into a known type in terms of database types. This is most useful features of Room persistent library.

How to create your TypeConverter

Below is the simple Entity class,

@Entity public class Options extends BaseObservable implements Serializable { @ColumnInfo(name = "option_name") private String optionName; @ColumnInfo(name = "option_values") private List<OptionValues> optionValues; // custom type object @Bindable public String getOptionName() { if (optionName == null) return ""; return optionName; } public void setOptionName(String optionName) { this.optionName = optionName; notifyPropertyChanged(BR.optionName); } public List<OptionValues> getOptionValues() { if (optionValues == null) { optionValues = new ArrayList<>(); return optionValues; } return optionValues; } public void setOptionValues(List<OptionValues> optionValues) { this.optionValues = optionValues; } }

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@Entity
public class Options extends BaseObservable implements Serializable {

    @ColumnInfo(name = "option_name")
    private String optionName;

    @ColumnInfo(name = "option_values")
    private List<OptionValues> optionValues;  // custom type object

    @Bindable
    public String getOptionName() {
        if (optionName == null)
            return "";
        return optionName;
    }

    public void setOptionName(String optionName) {
        this.optionName = optionName;
        notifyPropertyChanged(BR.optionName);
    }

    public List<OptionValues> getOptionValues() {
        if (optionValues == null) {
            optionValues = new ArrayList<>();
            return optionValues;
        }
        return optionValues;
    }

    public void setOptionValues(List<OptionValues> optionValues) {
        this.optionValues = optionValues;
    }
}

We can easily see that the option_values column is the list of OptionValues model. So if you run the project it will show some error,

So the question is how to insert the custom types in DB, and your answer is type converters, but how to add?

adding the Type converters

First of all simply create a type converter class, Named DataConverter

public class DataConverter implements Serializable { @TypeConverter // note this annotation public String fromOptionValuesList(List<OptionValues> optionValues) { if (optionValues == null) { return (null); } Gson gson = new Gson(); Type type = new TypeToken<List<OptionValues>>() { }.getType(); String json = gson.toJson(optionValues, type); return json; } @TypeConverter // note this annotation public List<OptionValues> toOptionValuesList(String optionValuesString) { if (optionValuesString == null) { return (null); } Gson gson = new Gson(); Type type = new TypeToken<List<OptionValues>>() { }.getType(); List<OptionValues> productCategoriesList = gson.fromJson(optionValuesString, type); return productCategoriesList; } }

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public class DataConverter implements Serializable {

@TypeConverter // note this annotation
    public String fromOptionValuesList(List<OptionValues> optionValues) {
        if (optionValues == null) {
            return (null);
        }
        Gson gson = new Gson();
        Type type = new TypeToken<List<OptionValues>>() {
        }.getType();
        String json = gson.toJson(optionValues, type);
        return json;
    }

    @TypeConverter // note this annotation
    public List<OptionValues> toOptionValuesList(String optionValuesString) {
        if (optionValuesString == null) {
            return (null);
        }
        Gson gson = new Gson();
        Type type = new TypeToken<List<OptionValues>>() {
        }.getType();
        List<OptionValues> productCategoriesList = gson.fromJson(optionValuesString, type);
        return productCategoriesList;
    }

}

Note we’ve added a new annotation TypeConverter, which allows you to persists a specific custom type into a database. In above class functions(or type converters) simply convert your list of models into Strings and vise-versa.

if you want to use this type converter, you can simply @TypeConverters(DataConverter.class) put in above your objects or add it to your app database class.

E.g,

@Entity public class Options extends BaseObservable implements Serializable { // ... @TypeConverters(DataConverter.class) // add here @ColumnInfo(name = "option_values") private List<OptionValues> optionValues; // .... }

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@Entity
public class Options extends BaseObservable implements Serializable {

//   ...

    @TypeConverters(DataConverter.class) // add here
    @ColumnInfo(name = "option_values")
    private List<OptionValues> optionValues;

//   ....

}

or,

@Database(entities = {Options.class, OptionValues.class}, version = 1) @TypeConverters(DataConverter.class) // add here public abstract class AppDatabase extends RoomDatabase { // ... }

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@Database(entities = {Options.class, OptionValues.class}, version = 1)
@TypeConverters(DataConverter.class)   // add here
public abstract class AppDatabase extends RoomDatabase {

//           ...

}

Wow now you have successfully added type converters in your app, now you can store the custom type of objects in Database.

Thanks for reading this blog, I hope this blog is helpful to you. If you have any query reading this blog or room database, You can ask in comments.

Stay coollll and updated :).