Android: JSON to objects using GSON fails

Question

I'm a newbee to Android development and have a question about getting a JSON string transformed to class instances using GSON, version 2.6.1.

I have a need for transforming a string like this to objects:

{
"Messages": [{
    "ForwardMsg": true,
    "IsAdmin": true,
    "MsgBody": "Some text",
    "SysInfo": null,
    "Recipients": ["Some test"]
}, {
    "ForwardMsg": true,
    "IsAdmin": false,
    "MsgBody": "Some other text",
    "SysInfo": null,
    "Recipients": ["Some test", "Some more text"]
}]
}

With this ( http://howtodoinjava.com/best-practices/google-gson-tutorial-convert-java-object-to-from-json/ ) as inspiration, I've come up with the following: I have a class DemoMessageList that looks like this:

import java.util.List;

public class DemoMessageList {

private List< DemoMessage> messages;

public DemoMessageList () {
}

public List< DemoMessage > getMessages() {
    return messages;
}

public void setMessages(List< DemoMessage > messages) {
    this.messages = messages;
}

@Override
public String toString()
{
    return "Messages ["+ messages + "]";
}
}

And a class DemoMessage that looks like this:

import java.util.List;

public class DemoMessage {
private Boolean forwardMsg;
private Boolean isAdmin;
private String msgBody;
private String sysInfo;
private List<String> recipients;

public Boolean getForwardMsg() {
    return forwardMsg;
}

public void setForwardMsg(Boolean forwardMsg) {
    this.forwardMsg = forwardMsg;
}

public Boolean doForwardMsg() {
    return forwardMsg;
}

public Boolean getIsAdmin() {
    return isAdmin;
}

public void setIsAdmin(Boolean isAdmin) {
    this.isAdmin = isAdmin;
}

public String getMsgBody() {
    return msgBody;
}

public void setMsgBody(String msgBody) {
    this.msgBody = msgBody;
}

public String getSysInfo() {
    return sysInfo;
}

public void setSysInfo(String sysInfo) {
    this.sysInfo = sysInfo;
}

public List<String> getRecipients() {
    return recipients;
}

public void setRecipients(List<String> recipients) {
    this.recipients = recipients;
}
}

When I do this, to try transform:

public void test() {
String demoData = {"Messages": [{ "ForwardMsg": true, "IsAdmin": false,"MsgBody": "Some other text", "SysInfo": null, "Recipients": ["Some test", "Some more text"]}]}
Log.d("AsData ", "demoData: " + demoData);
Gson gson = new Gson();
DemoMessageList dmList = gson.fromJson(demoData, DemoMessageList.class);
Log.d("AsList ", "dmList: " + dmList.toString());
Log.d("ListSize ", "dmList - Size: " + String.valueOf(dmList.getMessages().size()));
}

I get this logged:

demoData: {"Messages": [{ "ForwardMsg": true, "IsAdmin": false, "MsgBody": "Some other text", "SysInfo": null, "Recipients": ["Some test", "Some more text"]}]}
dmList: Messages [null]
dmList - Size: 0

Why does this fail?? Please help!!!

Answer 1

Your JSON names are different from your class field names. GSON looks at your field names for transformation.

Use your model class like this with custom naming,

@SerializedName("ForwardMsg")
private Boolean forwardMsg;
@SerializedName("IsAdmin")
private Boolean isAdmin;
@SerializedName("MsgBody")
private String msgBody;
@SerializedName("SysInfo")
private String sysInfo;
@SerializedName("Recipients")
private List<String> recipients;

and keep your other class ,

@SerializedName("Messages")
private List< DemoMessage> messages;

Answer 2

Use camel case on your JSON property names:

{
"messages": [{
    "forwardMsg": true,
    "isAdmin": true,
    "msgBody": "Some text",
    "sysInfo": null,
    "recipients": ["Some test"]
}, {
    "forwardMsg": true,
    "isAdmin": false,
    "msgBody": "Some other text",
    "sysInfo": null,
    "recipients": ["Some test", "Some more text"]
}]
}

.. and make the fieldnames match the case of the JSON property names, eg:

private List<DemoMessage> messages;

In short: The JSON property names must match the fields defined in your class(es), both by spelling and letter case.

Answer 3

You rock guys!!

Bharath Mg' answer works perfect in my little test, when adding this:

import com.google.gson.annotations.SerializedName;

I have no control over the string containing the JSON in the real world application as it is provided by a webservice.

This has been bugging me for the 2 days, so I look forward to get on with project.

Thanks again

Answer 4

Bharat's answer is correct. Fields names are case-sensitive.