How to get a json list from an Api in JAVA?

I recommend using the library called Jackson, it is very good to do this kind of conversion into java objects.

If you use maven, you can import the following library:

<dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.9.0.pr3</version>
</dependency>

If you do not use, import the following jar into your application:

http://central.maven.org/maven2/com/fasterxml/jackson/core/jackson-databind/2.9.0.pr3/jackson-databind-2.9.0.pr3.jar

Here is a working example of json reading:

Create a class called Champion with the following data:

import java.util.Map;

public class Champion {

    private String type;
    private String version;
    private Map<Integer, DadosChampion> data;

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getVersion() {
        return version;
    }

    public void setVersion(String version) {
        this.version = version;
    }

    public Map<Integer, DadosChampion> getData() {
        return data;
    }

    public void setData(Map<Integer, DadosChampion> data) {
        this.data = data;
    }

    @Override
    public String toString() {
        return "Champion [type=" + type + ", version=" + version + ", data=" + data + "]";
    }
}

Create another class called DataChampion with the following data:

public class DadosChampion {

    private Integer id;
    private String key;
    private String name;
    private String title;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getKey() {
        return key;
    }

    public void setKey(String key) {
        this.key = key;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

Finally, create a last class called Principal with the following data:

import java.io.IOException;
import java.util.Map;

import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Principal {

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {

        String json = "{\"type\":\"champion\",\"version\":\"7.10.1\",\"data\":{\"89\":{\"id\":89,\"key\":\"Leona\",\"name\":\"Leona\",\"title\":\"a Alvorada Radiante\"},\"110\":{\"id\":110,\"key\":\"Varus\",\"name\":\"Varus\",\"title\":\"a Flecha da Vingança\"}}}";

        ObjectMapper mapper = new ObjectMapper();
        Champion champion = mapper.readValue(json, Champion.class);

        System.out.println("Tipo: " + champion.getType());
        System.out.println("Versão: " + champion.getVersion());

        System.out.println();

        System.out.println("Dados do champion:");
        System.out.println("---------------");

        for (DadosChampion dadosChampion : champion.getData().values()) {
            System.out.println("ID: " + dadosChampion.getId());
            System.out.println("KEY: " + dadosChampion.getKey());
            System.out.println("NOME: " + dadosChampion.getName());
            System.out.println("Title: " + dadosChampion.getTitle());
            System.out.println("---------------");
        }

        System.out.println();
        System.out.println("OUTRA FORMA DE PERCORRER: ");
        for (Map.Entry<Integer, DadosChampion> dadosChampion : champion.getData().entrySet()) {
            System.out.println("ID INDICE: " + dadosChampion.getKey());
            System.out.println("ID: " + dadosChampion.getValue().getId());
            System.out.println("KEY: " + dadosChampion.getValue().getKey());
            System.out.println("NOME: " + dadosChampion.getValue().getName());
            System.out.println("Title: " + dadosChampion.getValue().getTitle());
            System.out.println("---------------");
        }
    }

}

The output will be as follows:

Tipo: champion
Versão: 7.10.1

Dados do champion:
---------------
ID: 89
KEY: Leona
NOME: Leona
Title: a Alvorada Radiante
---------------
ID: 110
KEY: Varus
NOME: Varus
Title: a Flecha da Vingança
---------------

Hugs.