app-z March 17, 2015 at 11:57

Gson cheat sheet for Map, List and Array

Constantly confronted with parsing, Json always peeps at old projects or at the encountered object implementation on stackoverflow.com .

I decided to collect three main types in a cheat sheet Map, List, Array.

Type itemsMapType = new TypeToken>() {}.getType();
Type itemsListType = new TypeToken>() {}.getType();
Type itemsArrType = new TypeToken() {}.getType();

Serialization / Deserialization class operations are considered:

public class GoodsItem{
    String name;
    float price;
    public GoodsItem(String name, float price) {
    this.name = name;
    this.price = price;
    public String toString(){
	return name + " : " + price;
    }
}

Serialization / Deserialization is done using the Gson library. The GoodsItem class is considered as the “test subject”.

Hashmap

Map mapItems = new HashMap();
mapItems.put(1, new GoodsItem("Samsung", 51200.6f));
mapItems.put(2, new GoodsItem("Lg", 5400.6f));
mapItems.put(3, new GoodsItem("Alcatel", 4500.6f));
String jsonStr = new Gson().toJson(mapItems);
System.out.println(jsonStr);

Serialization Result:

{
"1":{"name":"Samsung","price":51200.6},
"2":{"name":"Lg","price":5400.6},
"3":{"name":"Alcatel","price":4500.6}
}

Map mapItemsDes = new Gson().fromJson(jsonStr, itemsMapType);
System.out.println(mapItemsDes.toString());

Result of Deserialization:

{1=Samsung : 51200.6, 2=Lg : 5400.6, 3=Alcatel : 4500.6}

Arraylist

List listItems = new ArrayList();
listItems.add( new GoodsItem("Samsung" , 51200.6f));
listItems.add( new GoodsItem("Lg" , 5400.6f));
listItems.add( new GoodsItem("Alcatel" , 4500.6f));
String jsonStr = new Gson().toJson(listItems);
System.out.println(jsonStr);

Serialization Result:

[
{"name":"Samsung","price":51200.6},
{"name":"Lg","price":5400.6},
{"name":"Alcatel","price":4500.6}
]

List listItemsDes = new Gson().fromJson(jsonStr,itemsListType);
System.out.println(listItemsDes.toString());

Result of Deserialization:

[Samsung : 51200.6, Lg : 5400.6, Alcatel : 4500.6]

Array

GoodsItem[] arrItems = new GoodsItem[3];
arrItems[0] = new GoodsItem("Samsung", 51200.6f);
arrItems[1] = new GoodsItem("Lg", 5400.6f);
arrItems[2] = new GoodsItem("Alcatel", 4500.6f);
String jsonStr = new Gson().toJson(arrItems);
System.out.println(jsonStr);

Serialization Result:

[
{"name":"Samsung","price":51200.6},
{"name":"Lg","price":5400.6},
{"name":"Alcatel","price":4500.6}
]

GoodsItem[] arrItemsDes = new Gson().fromJson(jsonStr, itemsArrType);
System.out.println(Arrays.toString(arrItemsDes));

Result of Deserialization:

[Samsung : 51200.6, Lg : 5400.6, Alcatel : 4500.6]

As you can see, an ArrayList and a simple array of objects are converted to the same Json string.

Useful tools:
parcelabler.com enables you to generate parcelable methods for Android by class type. For example, for the GoodsItem class:

public class GoodsItem implements Parcelable

public class GoodsItem implements Parcelable {
    String name;
    float price;
    public GoodsItem(String name, float price) {
    this.name = name;
    this.price = price;
    public String toString(){
    return name + " : " + price;
    }
    protected GoodsItem(Parcel in) {
        name = in.readString();
        price = in.readFloat();
    }
    @Override
    public int describeContents() {
        return 0;
    }
    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(name);
        dest.writeFloat(price);
    }
    @SuppressWarnings("unused")
    public static final Parcelable.Creator CREATOR = new Parcelable.Creator() {
        @Override
        public GoodsItem createFromParcel(Parcel in) {
            return new GoodsItem(in);
        }
        @Override
        public GoodsItem[] newArray(int size) {
            return new GoodsItem[size];
        }
    };
}

jsonschema2pojo.org - Generate Plain Old Java Objects from JSON or JSON-Schema.

It would be interesting to know what you use in your projects.

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Gson cheat sheet for Map, List and Array

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