Develop Reference

Multiple StringBuilders inside StringBuilder. Does it worth?

I receive a list of models. The number of models could be large. This models has a bunch of properties and any of them could be null potentially.
I need to build a string for every model based of it's properties. If property == null then I add some static part to the result string like "property1 is null".
If else property != null then I add something like this "property1 == 'valueOfThePropertyHere'".
The result string should look something like this:
prop1 == 'value1' and prop2 is null and prop3 == 'value3' and prop4 == 'value4' and prop5 is null and ..... propN == 'valueN'
And I generate such string for every model from the list.
Obviously I do this in for loop and I use StringBuilder for this. The thing is that in append method of StringBuilder I check every field of the model for null using ternary operator and based on this I add the result of this check to the result string. But if a property is not null then I need to add some static part + value of the field itself + some more static stuff. And that means I need to add one more StringBuilder for every property I have. Or I can use '+' which will be transformed into StringBuilder anyway and as far as I know it's a bad practise to use '+' inside StringBuilder (but I have to use it anyway).
Example:
List<Model> models = repository.getModels();
for (Model m: models) {
StringBuilder stringBuilder = new StringBuilder();
stringBuilder
.append(m.getField1() == null ? "field1 is null" : "field1 == '" + new StringBuiler().append(m.getField1()).append("'").append(" and ").toString()))
.append(m.getField2() == null ? "field2 is null" : "field2 == '" + new StringBuiler().append(m.getField2()).append("'").append(" and ").toString()))
...............
.append(m.getFieldN() == null ? "fieldN is null" : "fieldN == '" + new StringBuiler().append(m.getFieldN()).append("'").append(" and ").toString()));
System.out.println(stringBuilder.toString());
}
In my opinion from the performance perspective it doesn't look so well because for every model from a list of models I create another bunch of StringBuilder objects in heap just to get the result string.
Am I missing something? Are there better ways to do so from the performance perspective? Or it's okay because I don't see other options for now.

Go for simple.
Instead of
stringBuilder
.append(m.getField1() == null ? "field1 is null" : "field1 == '" + new StringBuiler().append(m.getField1()).append("'").append(" and ").toString()))
use:
if (m.getField1() == null) {
stringBuilder.append("field1 is null");
} else {
stringBuilder.append("field1 == '").append(m.getField1()).append("'").append(" and ");
}
Aside from the distinct oddness of using a StringBuilder inside a StringBuilder.append call (and why not just use + anyway...), it's really hard to parse where the : is in the conditional expression. Breaking it into lines is much easier.
If you find yourself having to repeat this code pattern a lot, define a method:
void append(StringBuilder stringBuilder, String name, Object value) {
stringBuilder.append(name);
if (value == null) {
stringBuilder.append(" is null");
} else {
stringBuilder.append(" == '").append(value).append("'").append(" and ");
}
}
and then invoke like:
append(stringBuilder, "field1", m.getField1());
append(stringBuilder, "field2", m.getField2());
append(stringBuilder, "field3", m.getField3());

What a mess! Just because you can chain invocations, doesn't mean you should:
List<Model> models = repository.getModels();
for (Model m: models) {
StringBuilder stringBuilder = new StringBuilder();
String field = m.getField1();
if(field==null) {
stringBuilder.append("field1 is null");
} else {
stringBuilder.append("field1 == ").append(m.getField1()).append("'");
}
if(stringBuilder.length()>0) {
stringBuilder.append(" and ");
}
field = m.getField2();
if(field==null) {
stringBuilder.append("field2 is null");
} else {
stringBuilder.append("field2 == ").append(m.getField1()).append("'");
}
if(stringBuilder.length()>0) {
stringBuilder.append(" and ");
}
...
System.out.println(stringBuilder.toString());
}
To avoid all this potential repetition (depending on number of fields):
void appendField(StringBuilder stringBuilder, String fieldName, String value) {
if(stringBuilder.length()>0) {
stringBuilder.append(" and ");
}
stringBuilder.append(fieldName);
if(value==null) {
stringBuilder.append(" is null");
} else {
stringBuilder.append(" == '").append(value).append("'");
}
}
String toString(Model m) {
StringBuilder stringBuilder = new StringBuilder();
appendField(stringBuilder, "field1", m.getField1());
appendField(stringBuilder, "field2", m.getField2());
...
appendField(stringBuilder, "fieldN", m.getFieldN());
return stringBuilder.toString();
}
List<Model> models = repository.getModels();
for (Model m: models) {
System.out.println(toString(m));
}

sign-up form validations in java

i have a signup page connected to sql database.now i want to have validations in signup page like firstname,lastname,username etc can not be empty using java how can i do that
My code is
String fname=Fname.getText();
String lname=Lname.getText();
String uname=Uname.getText();
String emailid=Emailid.getText();
String contact=Contact.getText();
String pass=String.valueOf(Pass.getPassword());
Connection conn=null;
PreparedStatement pstmt=null;
try
{
Class.forName("com.mysql.jdbc.Driver");
conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/zeeshan","root","sHaNi97426");
pstmt=conn.prepareStatement("Insert into signup1 values(?,?,?,?,?,?)");
pstmt.setString(1,fname);
pstmt.setString(2,lname);
pstmt.setString(3,uname);
pstmt.setString(4,emailid);
pstmt.setString(5,contact);
pstmt.setString(6,pass);
int i=pstmt.executeUpdate();
if(i>0)
{
JOptionPane.showMessageDialog(null,"Successfully Registered");
}
else
{
JOptionPane.showMessageDialog(null,"Error");
}
}
catch(Exception e)
{
JOptionPane.showMessageDialog(null,e);
}

First your question is not direct. Validation occurs before database query. You should not proceed to database Connetction or making any query.
What should you do:
public static boolean nullOrEmpty(String value) {
return value == null || value.trim().equals("") ? true : false;
}
public void yourMethod(){
try{
//YourCode Here
String fname=Fname.getText();
if(nullOrEmpty(fname)){
new throw ValidationException("First name should not be null.");
}
//YourCode Here
}catch(ValidationException e){
System.err.println("Exception:"+e.getMessage());
}
}
Check for every string to validate.

that should not be hard, you can do it with simple if and else like below
if(fname != null && fname.isEmpty()){
throw new Exception(fname+" cannot be empty");
}else if(lname != null && lname.isEmpty()){
throw new Exception(fname+" cannot be empty");
}
.....
as a recommendation you should abstract validation and database access objects . see example of MVC here

You may do it just by downloading a jar named org.apache.commons.lang
Stringutils Class Reference
Sample Code
StringUtils.isBlank(null) = true
StringUtils.isBlank("") = true
StringUtils.isBlank(" ") = true
StringUtils.isBlank("bob") = false
StringUtils.isBlank(" bob ") = false
or
StringUtils.isEmpty(obj_String); // Another method to check either null or "";

To check if a String is empty you can use the method .isEmpty(). You'll probably want to use .trim() first, as this removes all the whitespaces at the beginning and ending of the String. For more options check out the full documentation here.

Correct way to initialize wrapper Integer

I have a drop down field in application which displays numbers.
When user doesn't select any value from drop down, I would want to insert as null to database.
How can I initialize an Integer wrapper class to null?
I have tried as
Integer days = new Integer(null);
if (request.getParameter("days").equals("")) {
} else {
days =
Integer.parseInt(request.getParameter("days"));
}
However I am getting the following error, so what is the correct method in declaring Integer variable?
NumberFormatException at test.doPost(Controller.java:23);
How to initialize an Integer variable so that if no values are selected by user then null should get inserted.

You should initialize with null:
Integer days = null;
if (request.getParameter("days") != null && !request.getParameter("days").isEmpty()) {
days = Integer.parseInt(request.getParameter("days"));
}
UPDATE: Better to validate its an integer first:
Integer days = null;
if(request.getParameter("days")!=null && request.getParameter("days").matches("^\\d+$"))
{
days = Integer.parseInt(request.getParameter("days"));
}
UPDATE 2: To be able to insert null in DB:
if (project.getDays() != null)
callablestatement.setInt(2, project.getDays());
else
callablestatement.setNull(2, java.sql.Types.INTEGER);

What about setting the days object to null and then checking to see if the days object is null or is empty:
Integer days = null;
if (request.getParameter("days") != null && !request.getParameter("days").isEmpty()) {
// rest of code.

Apart from the declaration on which others have commented.
You are getting NumberFormatException because request.getParameter("days") is returning a non integer value text or null. parseInt method throws NumberFormatException when it gets a string input which is not a valid integer value.

You cannot trust the value of the days parameter in the request.
What if some client will pass an invalid value? (a non-number string).
For this reason you should catch NumberFormatException when you try to parse the parameter value.
Integer days = null;
String parameterValue = request.getParameter("days");
if (parameterValue != null && !parameterValue.isEmpty()) {
try {
days = Integer.parseInt(parameterValue);
} catch (NumberFormatException e) {
// log or something
}
}

With a try/catch:
Integer days = null;
try {
days = Integer.parseInt(request.getParameter("days"));
} catch (final NumberFormatException ex) {
// ignore
}
or with guava Ints
Integer days = null;
final String param = request.getParameter("days");
if (param != null) {
days = Ints.tryParse(param);
}
This way, you avoid the NumberFormatException if "days" is not parsable.

How about :
Integer days = null;
String param = request.getParameter("days");
if (param != null && !"".equals(param)) {
days = Integer.parseInt(request.getParameter("days"));
}

How to test if a JSONObject is null or doesn't exist

I have a set of JSONObject values which i receive from a server and operate on. Most times I get a JSONObject with a value (let's say statistics) and sometimes, it returns an Error object with a code and a description of the error.
How do I structure my code so that it doesn't break if it returns the error. I thought I could do this, but doesn't work.
public void processResult(JSONObject result) {
try {
if(result.getJSONObject(ERROR) != null ){
JSONObject error = result.getJSONObject(ERROR);
String error_detail = error.getString(DESCRIPTION);
if(!error_detail.equals(null)) {
//show error login here
}
finish();
}
else {
JSONObject info = result.getJSONObject(STATISTICS);
String stats = info.getString("production Stats"));
}
}
}

Use .has(String) and .isNull(String)
A conservative usage could be;
if (record.has("my_object_name") && !record.isNull("my_object_name")) {
// Do something with object.
}

It might be little late(it is for sure) but posting it for future readers
You can use JSONObject optJSONObject (String name) which will not throw any exception and
Returns the value mapped by name if it exists and is a JSONObject, or null otherwise.
so you can do
JSONObject obj = null;
if( (obj = result.optJSONObject("ERROR"))!=null ){
// it's an error , now you can fetch the error object values from obj
}
or if you just want to test nullity without fetching the value then
if( result.optJSONObject("ERROR")!=null ){
// error object found
}
There is whole family of opt functions which either return null or you can also use the overloaded version to make them return any pre-defined values.
e.g
String optString (String name, String fallback)
Returns the value mapped by name if it exists, coercing it if
necessary, or fallback if no such mapping exists.
where coercing mean, it will try to convert the value into String type
A modified version of the #TheMonkeyMan answer to eliminate redundant look-ups
public void processResult(JSONObject result) {
JSONObject obj = null;
if( (obj = result.optJSONObject("ERROR"))!=null ){
//^^^^ either assign null or jsonobject to obj
// if not null then found error object , execute if body
String error_detail = obj.optString("DESCRIPTION","Something went wrong");
//either show error message from server or default string as "Something went wrong"
finish(); // kill the current activity
}
else if( (obj = result.optJSONObject("STATISTICS"))!=null ){
String stats = obj.optString("Production Stats");
//Do something
}
else
{
throw new Exception("Could not parse JSON Object!");
}
}

In JSONObject there is a 'Has' method that you can do to Determaine the key.
I have no idea if this will work but it looks Credible.
public void processResult(JSONObject result) {
if(result.has("ERROR"))
{
JSONObject error = result.getJSONObject("ERROR")
String error_detail = error.getString("DESCRIPTION");
if(error_detail != null)
{
//Show Error Login
finish();
}
}
else if(result.has("STATISTICS"))
{
JSONObject info = result.getJSONObject("STATISTICS");
String stats = info.getString("Production Stats");
//Do something
}
else
{
throw new Exception("Could not parse JSON Object!");
}
}

It is sometimes more convenient and less ambiguous to have a NULL object than to use Java's null value.
JSONObject.NULL.equals(null) returns true.
JSONObject.NULL.toString()returns "null".
Example:
System.out.println(test.get("address").equals(null)); // Preferred way
System.out.println(test.getString("address").equals("null"));
source -- JSONObject oracle docs

Just a note:
With EE8 json specs, I can do an exception-safe get:
result.asJsonObject().getString("ERROR", null);
if, however, I want to do a check I can do it with:
result.asJsonObject().get("ERROR").equals(JsonValue.NULL)

If at any point in your code, org.json.JSONObject json_object becomes null and you wish to avoid NullPointerException (java.lang.NullPointerException), then do check it as below:
if(json_object == null) {
System.out.println("json_object is found as null");
}
else {
System.out.println("json_object is found as not null");
}

If in any case, your jsonobject is null.
Then use this statement for checking jsonobject is null or not.
if (!obj.get("data").isJsonNull()){
//Not Null
}else{
//Null
}
And for checking jsonobject is exist or not, use .has:
if (!obj.has("data")){
//Not Exist
}else{
//Exist
}

XML parsing with attributes that is empty (Android)

i have an xml file with attributes like this:
<folder name = 'somename' description = ''/>
i want to display the description attribute as 'null' but it force closes and throws a FATAL Exception main in the LogCat.
i have this code below at the startElement() method
if (localName.equalsIgnoreCase("folder")) {
/** Get attribute value */
if(attributes.getValue("description")== "null"){
parseList.setFolderdesc(null);
}else{
String desc = attributes.getValue("description");
parseList.setFolderdesc(desc);
}
i tried this code but no luck...
how will i solve this without changing my xml file?

try with the following code
String desc = null;
try{
desc = attributes.getValue("description");
if((desc == null) || (desc.length()<=0)){
desc = null;
}
}catch(Exception ex){
desc = null;
}
if(parseList != null){
parseList.setFolderdesc(desc);
}

This code doesn't do what you expect:
if (attributes.getValue("description") == "null") {
You are comparing the attribute value with the String "null" not with a java null. (And you are testing strings the unsafe way too! Strings should be tested for equality using String.equals() not the == operator.)
That test should be written as follows:
if (attributes.getValue("description") == null) {
or better still:
if (attributes.getValue("description") == null ||
attributes.getValue("description").isEmpty()) {
(I'm not sure whether this will fix you problem, because I don't understand your problem description.)