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Java declare a class object from the string of that class?

I have a script provided from the client like this
segment-id Integer
segment-description String
Now I want to build a class with the following methods
Sample sample = new Sample();
// casting to the type specified in the script
(map.get("segment-id")) segmentId = sample.get("segment-id");
// Now it can be used as an Integer
Integer result = segmentId + 2;
Is it possible to do something like
Class<map.get("segment-id")> segmentId = new Class<map.get("segment-id")>();
Or any better solution...? I need a way create objects of specific type, which I don't know in advance.
My current solution is
public Integer getInteger(String key) {
return map.get(key);
}
but in this way I have to know in advanced segment-id is of type Integer.

You can use Class.forName() to get the class and .newInstance() to get a new instance:
Object createdObject = Class.forName("java.lang.String").newInstance();
or ...
Object createdObject = Class.forName("java.lang.Integer").newInstance();
If you need to know if it's a String:
if(createdObject instanceof String) {
String castValue = (String) createdObject;
...
}
if(createdObject instanceof Integer) {
Integer castValue = (Integer) createdObject;
...
}
But you could just test the incoming string:
if("java.lang.String".equals(nameOfClassToCreate)) {
....
}

Get the data type of JSON element in Java

I get a JSON value from Kafka queue and I want to get the right data type to save it in the DB.
Value can be: String, int, double or array.
How can i detect automatically the right datatype and create a Java Object from it?
My first steps:
check if json is an array or not:
if (jsonValue.isJsonPrimitive()) {
// create new Object
//ToDo need to parse int, double not only to string
new ValueObject(time,jsonValue.getAsString);
} else if (jsonValue.isJsonArray()) {
//create new Object
//ToDo need to parse int, double string
new ValueObject(time,jsonValue.getAsJsonArray());
}
How can I design the ValueObject class to convert the value to the corresponding data type and return the right object?
Thanks for any ideas

have you tried:
//this instanciates an object of the getClass() method output
Object output = jsonValue.getClass().cast(jsonValue);
if that didn't work, you can try instanceof:
if(jsonValue instanceof int){
int output = (int) jsonValue;
}...
I hope that will do.

if you are using jackson lib, you can do like this:
JsonNode rootNode = objectMapper.readTree(json);
Iterator<String> fields = rootNode.fieldNames();
while(fields.hasNext()){
String field = fields.next();
JsonNode obj = rootNode.get(field);
System.out.println("value " + obj);
if (obj.isInt()) {
System.out.println("Integer");
}
if (obj.isDouble()) {
System.out.println("Double");
}
if (obj.isTextual()) {
System.out.println("String");
}
}

Java Reflection - get method without specific parameters type

I am trying to get the method by java reflection, but i don't want to be specific about the parameters classes in getMethod().
public Object prepareFilter(String filter, String sort) {
Class filterClass = this.filterClass;
try {
Method createCriteriaMethod = filterClass.getMethod(CREATE_CRITERIA_METHOD);
Method orderByClauseMethod = filterClass.getMethod(ORDER_BY_CLAUSE_METHOD, String.class);
Class criteriaClass = createCriteriaMethod.getReturnType();
Object filterObject = filterClass.newInstance();
Object criteriaObject = createCriteriaMethod.invoke(filterObject);
for (ExtFilterRequest extFilter : ExtFilterRequest.decodeJson(filter)) {
StringBuilder sb = new StringBuilder()
.append(AND)
.append(WordUtils.capitalize(extFilter.getProperty()))
.append(extFilter.getCondition());
Method criteriaConditionMethod = criteriaClass.getMethod(sb.toString(), ????); // earlier extFilter.getTransformedValue().getClass()
criteriaConditionMethod.invoke(criteriaObject, extFilter.getTransformedValue());
}
String orderByClause = ExtSortRequest.getOrderByString(sort);
if (orderByClause != null)
orderByClauseMethod.invoke(filterObject, orderByClause);
return filterObject;
} catch (Exception e) {
// later
}
return null;
}
I have the methods generated by MyBatis and I want to call them by reflection with the decoded json that comes from extjs client. It looks like: operator, value and property. Following code is working with string values but i dont know what to put in the place of question marks when I call for example method which get a Date value (decoded value is always a string).
Is it possible to call getMethod with some hmm.. generic type and get the specific method? Or should I do it in another way?
Summarizing - if I have method like this:
public Criteria andSomeReferenceIsEqualTo(String value) {
addCriterion("some_ref =", value, "someRef");
return (Criteria) this;
}
and this
public Criteria andPrimDateEqualTo(Date value) {
addCriterionForJDBCDate("prim_date =", value, "primDate");
return (Criteria) this;
}
I want to call them in the same way by method I specify earlier - even if its boolean, list of values, string or integer.

Java reflection nested methods not modifying underlying object

I am taking in an array of methods and I want to chain them together to modify an object that I am working in.
For example I start with
"getStuff().get(1).get(3).setMoreStuff().put(stuff,6)"
I split it into an array called methods, and clean up the parameters inside each method and I try to modify this.
Object res = this;
String[] methods = targetString.split("\\.(?=\\D)");
for (String m : methods){
List<Object> params = new ArrayList<Object>();
List<Object> params = new ArrayList<Object>();
for (String p : m.split("\\(|,|\\)")) {
try {
if (p.indexOf(".") != -1){
double tempD = Double.parseDouble(p);
params.add(tempD);
} else {
int tempP = Integer.parseInt(p);
params.add(tempP);
}
} catch (Exception ex) { //not a number
params.add(p);
}
}
switch (params.size()) {
case 1:
res = res.getClass().getMethod(
params.get(0)
).invoke(res);
break;
case 2:
res = res.getClass().getMethod(
params.get(0),
params.get(1).getClass()
).invoke(res, params.get(1));
break;
case 3:
res = res.getClass().getMethod(
params.get(0),
params.get(1).getClass(),
params.get(2).getClass()
).invoke(res, params.get(1), params.get(2));
break;
}
in the end I notice that res has been modified the way that I expect. All the getters and setters are called correctly. But of course the underlying object "this" refers to has not been changed!
I guess I'm just calling the getters and setters of the copy I made in the first line!
now I can't just use
this.getClass().getMethod(...).invoke(...)
because I need to call the same getMethod on the object returned by this call.
To clarify:
Object res = this;
creates a "pointer" to this. So that when I call
res.getStuff().setStuff(foo)
this will also be modified.
but it seem that when I call
res = res.getStuff();
res = res.setStuff();
like I do in my loop,
this does not modify the underlying object this refers to?
Edit: Included more code as per request.
Edit2: added anther example, to clarify my problem.
Edit3: tried to add more code, its a bit hard to add a working program without including every class

Your general approach should be fine (although your approach to parameter conversion is somewhat ugly) - it's the specifics that are presumably causing you problems. Here's a short but complete program demonstrating calling methods and then seeing the difference afterwards:
import java.lang.reflect.*;
class Person {
private String name = "default";
public String getName() {
return name;
}
// Obviously this would normally take a parameter
public void setName() {
name = "name has been set";
}
}
class Test {
private Person person = new Person();
public Person getPerson() {
return person;
}
// Note that we're only declaring throws Exception for convenience
// here - diagnostic code only, *not* production code!
public void callMethods(String... methodNames) throws Exception {
Object res = this;
for (String methodName : methodNames) {
Method method = res.getClass().getMethod(methodName);
res = method.invoke(res);
}
}
public static void main(String[] args) throws Exception {
Test test = new Test();
test.callMethods("getPerson", "setName");
System.out.println(test.getPerson().getName());
}
}
The output is "name has been set" just as I'd expect. So see if you can simplify your code bit by bit, removing extra dependencies etc until you've got something similarly short but complete, but which doesn't work. I suspect you'll actually find the problem as you go.

Object does not change reference, its VALUE changes. So if you will call this.get("some key"), you will get value that the same value that you put using reflection.
Right?

Iterating through hashmap and creating unique objects - trying to prevent duplicates

I explain what I am trying to do in comments above the parts in the method:
public int addPatron(String name) throws PatronException {
int i = 0;
//1. Iterate through a hashmap, and confirm the new name I am trying to add to the record doesn't already exist in the hashmap
for (Map.Entry<Integer, Patron> entry : patrons.entrySet()) {
Patron nameTest = entry.getValue();
//2. If the name I am trying to add already exists, we want to throw an exception saying as much.
if (nameTest.getName() == name) {
throw new PatronException ("This patron already exists");
//3. If the name is unique, we want to get the largest key value (customer number) already in the hash, an increment by one.
} else if (nameTest.getName() != name) {
Map.Entry<Integer,Patron> maxEntry = null;
for(Map.Entry<Integer, Patron> entryCheck : patrons.entrySet()) {
if (maxEntry == null || entryCheck.getKey() > maxEntry.getKey()) {
maxEntry = entryCheck;
i = maxEntry.getKey();
i++;
}
}
} else {
throw new PatronException("Something's not working!");
}
//4. If everything is ok up to this point, we want to us the name and the new customer id number, and use those to create a new Patron object, which then gets added to a hashmap for this class which contains all the patrons.
Patron newPatron = new Patron(name, i);
patrons.put(i, newPatron);
}
return i;
}
When I try and run a simple unit test that will fail if I successfully add the same name for addPatron twice in a row, the test fails.
try {
testLibrary.addPatron("Dude");
testLibrary.addPatron("Dude");
fail("This shouldn't have worked");
The test fails, telling me the addPatron method is able to use the same name twice.
#Jon Skeet:
My Patron class looks like this:
public class Patron {
//attributes
private String name = null;
private int cardNumber = 0;
//operations
public Patron (String name, int cardNumber){
this.name = name;
this.cardNumber = cardNumber;
}
public String getName(){
return name;
}
public int getCardNumber(){
return cardNumber;
}
}

As others have said, the use of == for comparing strings is almost certainly inappropriate. However, it shouldn't actually have caused a problem in your test case, as you're using the same constant string twice, so == should have worked. Of course, you should still fix the code to use equals.
It's also not clear what the Patron constructor or getName methods do - either of those could cause a problem (e.g. if they create a new copy of the string - that would cause your test to fail, but would also be unnecessary usually).
What's slightly more worrying to me is this comment:
// 3. If the name is unique, we want to get the largest key value (customer number)
// already in the hash, an increment by one.
This comment is within the main loop. So by that point we don't know that the name is unique - we only know that it doesn't match the name of the patron in this iteration.
Even more worrying - and I've only just noticed this - you perform the add within the iteration block too. It seems to me that you should have something more like this:
public int addPatron(String name) throws PatronException {
int maxKey = -1;
for (Map.Entry<Integer, Patron> entry : patrons.entrySet()) {
if (entry.getValue().getName().equals(name)) {
// TODO: Consider using IllegalArgumentException
throw new PatronException("This patron already exists");
}
maxKey = Math.max(maxKey, entry.getKey());
}
int newKey = maxKey + 1;
Patron newPatron = new Patron(name, newKey);
patrons.put(newKey, newPatron);
return newKey;
}
Additionally, it sounds like really you want a map from name to patron, possibly as well as the id to patron map.

You need to use equals to compare String objects in java, not ==. So replace:
if (nameTest.getName() == name) {
with:
if (nameTest.getName().equals(name)) {

Try to use
nameTest.getName().equals(name)
instead of
nameTest.getName() == name
because now you're comparing references and not the value of the String.
it's explained here
Took another look on your code
Well i took another look on your code and the problem is, that your HashMap is empty at the start of the Test. So the loop will never be runned ==> there will never bee a Patron added or an Exception thrown.

The cause of the problem is how you have used the compare operator ==.
When you use this operator against two objects, what you test is that variable point to the same reference.
To test two objects for value equality, you should use equals() method or compareTo if available.
For String class, invoke of equals is sufficient the check that the store same characters more.
What is equals method ?
To compare the values of Object
The problem is how you compare names.