Example usage of Guava's `TypeToInstanceMap` for maps of non-trivial types

Converter.java

public interface Converter<T>
{
    String toString(T thing);
    
    T fromString(String string);
}

ConverterRegistry.java

import com.google.common.reflect.MutableTypeToInstanceMap;
import com.google.common.reflect.TypeParameter;
import com.google.common.reflect.TypeToInstanceMap;
import com.google.common.reflect.TypeToken;

// Suppress nags about TypeToken - we don't intend to serialise this class anyway.
@SuppressWarnings("serial")
public class ConverterRegistry
{
    private final TypeToInstanceMap<Converter<?>> converters = new MutableTypeToInstanceMap<>();

    public ConverterRegistry()
    {
        converters.putInstance(new TypeToken<Converter<String>>(){}, new StringConverter());
        converters.putInstance(new TypeToken<Converter<Integer>>(){}, new IntegerConverter());
    }

    public <T> Converter<T> getConverter(Class<T> clazz)
    {
        return converters.getInstance(new TypeToken<Converter<T>>(){}.where(new TypeParameter<T>(){}, clazz));
    }
}

IntegerConverter.java

class IntegerConverter implements Converter<Integer>
{
    @Override
    public String toString(Integer thing)
    {
        return Integer.toString(thing);
    }

    @Override
    public Integer fromString(String string)
    {
        return Integer.parseInt(string);
    }
}

StringConverter.java

class StringConverter implements Converter<String>
{
    @Override
    public String toString(String thing)
    {
        return thing;
    }

    @Override
    public String fromString(String string)
    {
        return string;
    }
}

UsageExample.java

public class UsageExample
{
    public static void main(String[] args)
    {
        ConverterRegistry registry = new ConverterRegistry();

        Converter<String> stringConverter = registry.getConverter(String.class);
        System.out.println(stringConverter);

        Converter<Integer> integerConverter = registry.getConverter(Integer.class);
        System.out.println(integerConverter);
    }
}