尝试使用GSON反序列化JSON字符串时出现错误

Question

I'm trying to decode a JSON string that's coming from a php script I made that gets results out of my MySQL database. It returns a JSON array.

This is the code for my decoder:

package com.github.viperdream;

import java.lang.reflect.Type;
import java.util.List;

import android.util.Log;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class JSONDecoder {

    public static void decodePin(String data){

        Gson gson = new Gson();

        Type type = new TypeToken<List<Pin>>(){}.getType();
        List<Pin> pin = gson.fromJson(data, type);

        for (Pin newPin : pin){
            Log.d("GSON", newPin.getPinTitle());
        }

    }   
}

This is the Pin object class:

package com.github.viperdream;

public class Pin {

private Integer pinID;
private String pinTitle;
private String pinMessage;
private Integer pinDuration;
private Float pinX;
private Float pinY;
private String pinColor;
private String author;
private Integer pinSQLId;

public Pin(){}

public Pin(int pinID, String pinTitle, String pinMessage, Integer pinDuration, Float pinX, Float pinY, String pinColor, String author, Integer pinSQLId){
    this.pinID = pinID;
    this.pinTitle = pinTitle;
    this.pinMessage = pinMessage;
    this.pinDuration = pinDuration;
    this.pinX = pinX;
    this.pinY = pinY;
    this.pinColor = pinColor;
    this.author = author;
    this.pinSQLId = pinSQLId;
}

public Pin(String pinTitle, String pinMessage, Integer pinDuration, Float pinX, Float pinY, String pinColor, String author, Integer pinSQLId){
    this.pinTitle = pinTitle;
    this.pinMessage = pinMessage;
    this.pinDuration = pinDuration;
    this.pinX = pinX;
    this.pinY = pinY;
    this.pinColor = pinColor;
    this.author = author;
    this.pinSQLId = pinSQLId;
}

//get ------------------------------------------------------
public int getID(){
    return this.pinID;
}

public String getPinTitle(){
    return this.pinTitle;
}

public String getPinMessage(){
    return this.pinMessage;
}

public Integer getPinDuration(){
    return this.pinDuration;
}
public Float getPinX(){
    return this.pinX;
}

public Float getPinY(){
    return this.pinY;
}
public String getPinColor(){
    return this.pinColor;
}
public String getPinAuthor(){
    return this.author;
}
public Integer getPinSQLId(){
    return this.pinSQLId;
}

//set ------------------------------------------------------
public void setPinID(int pinID){
    this.pinID = pinID;
}

public void setPinTitle(String pinTitle){
    this.pinTitle = pinTitle;
}

public void setPinMessage(String pinMessage){
    this.pinMessage = pinMessage;
}

public void setPinDuration(int pinDuration){
    this.pinDuration = pinDuration;
}
public void setPinX(Float pinX){
    this.pinX = pinX;
}

public void setPinY(Float pinY){
    this.pinY = pinY;
}
public void setPinColor(String pinColor){
    this.pinColor = pinColor;
}
public void setPinAuthor(String author){
    this.author = author;
}
public void setPinSQLId(Integer pinSQLId){
    this.pinSQLId = pinSQLId;
}

}

This is the JSON String that I'm trying to decode:

[
  {
    "id":"2",
    "title":"test1",
    "message":"test2",
    "duration":"1",
    "x":"125",
    "y":"754.5",
    "color":"red",
    "author":"viperdream"
  },
  {
    "id":"3",
    "title":"looking for someone",
    "message":"i need to go now",
    "duration":"1",
    "x":"401",
    "y":"472.5",
    "color":"red",
    "author":"viperdream"
  },
  {
    "id":"4",
    "title":"test3",
    "message":"testing:)",
    "duration":"1",
    "x":"195",
    "y":"512.5",
    "color":"red",
    "author":"viperdream"
      }
]

And this is how I make the Json string in PHP

while($pin = mysql_fetch_assoc($query_exec)) {
    $pins[] = $pin;
}

echo json_encode($pins); 

Whenever I run my app, it gives me this error:

java.lang.NullPointerException: println needs a message

Anyone knows what I am doing wrong?

If you need any more information, please do ask!

Thanks in advance!

Answer 1

Gson tries to match the JSON element names to your class' field names. In your case, you have id , title , etc. instead of pinId , pinTitle , etc. If Gson finds an element for which it doesn't find a matching field, it skips it, leaving the field null (or whatever default it has).

The element and field names need to be equal.

Alternatively, you could annotate your field with @SerializedName and give it the value you are expecting from the json.

@SerializedName("id")
private Integer pinID;