i need to find duplicates entries using rest service in android

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Mobile

Android

REST

webservice

i have a piece of code that check for duplicate entries while running the code i get an exception "Server Json response might be invalid.

public void onSuccess(String response)
				{
					
					try{
							Toast.makeText(getApplicationContext(), response, Toast.LENGTH_SHORT).show();
							
							JSONObject obj = new JSONObject(response);
								if(obj!=null)
								{
								
								JSONArray data = obj.getJSONArray("CheckForDuplicateSerialResult");
								for(int i=0;i<data.length();i++)
		        				{
		        					JSONObject user = data.getJSONObject(i);
		        					   						
		        					String name=user.getString("Res");
		        					if(name.equals("0")){
		        							
		        					Toast t=Toast.makeText(getApplicationContext(), "Scan Next", Toast.LENGTH_SHORT);
		        						t.setGravity(Gravity.CENTER, 0, 0);
		        						t.show();
		        						}
		        						else{
		        							Toast.makeText(getApplicationContext(), "Already exist", Toast.LENGTH_SHORT).show();
		        							
		        						}
		        					
		        				}
								}
								
								
							
						}catch(JSONException e){
							Toast.makeText(getApplicationContext(), "Error Occured [Server's JSON response might be invalid]!", Toast.LENGTH_LONG).show();
			                e.printStackTrace();
			    		
						}	
				}
					@Override
					public void onFailure(int statusCode, Throwable error,String content){
						//Toast.makeText(getApplicationContext(), content, Toast.LENGTH_LONG).show();
						 if(statusCode == 404){
		                  Toast.makeText(getApplicationContext(), "Requested resource not found", Toast.LENGTH_LONG).show();
		              } 
						 else if(statusCode == 500){
		                  Toast.makeText(getApplicationContext(), "Something went wrong at server end", Toast.LENGTH_LONG).show();
		              }
						 else{
							 //Toast.makeText(getApplicationContext(), content, Toast.LENGTH_LONG).show();
		                  //Toast.makeText(getApplicationContext(), "Unexpected Error occcured! [Most common Error: Device might not be connected to Internet or remote server is not up and running]", Toast.LENGTH_LONG).show();
		              }
					 }

				});
			}

please help me to solve this problem