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Java class method with Class and Field as parameters

How can i create a method that accepts Class and Field as parameters? Like this:
List<SomeClassEntity> list = ...;
// Service to make useful things around a list of objects
UsefulThingsService<SomeClassEntity> usefulThingsService = new UsefulThingsService<>();
// Maybe invoke like this. Did't work
usefulThingsService.makeUsefulThings(list, SomeClassEntity.class, SomeClassEntity::getFieldOne);
// or like this. Will cause delayed runtime erros
usefulThingsService.makeUsefulThings(list, SomeClassEntity.class, "fieldTwo");
public class SomeClassEntity {
Integer fieldOne = 10;
Double fieldThree = 0.123;
public Integer getFieldOne() {
return fieldOne;
}
public void setFieldOne(Integer fieldOne) {
this.fieldOne = fieldOne;
}
public Double getFieldThree() {
return fieldThree;
}
public void setFieldThree(Double fieldThree) {
this.fieldThree = fieldThree;
}
}
public class UsefulThingsService<T> {
public void makeUsefulThings(Class<T> someClassBClass, String fieldName) {
// there is some code
}
}
Want to have correct references on compile stage, not at runtime.
Update:
I need code that would look more convenient than this:
Field fieldOne = null;
try {
fieldOne = SomeClassEntity.class.getDeclaredField("fieldOne");
} catch (NoSuchFieldException e) {
e.printStackTrace();
}
usefulThingsService.makeUsefulThings(SomeClassEntity.class, fieldOne);
I apologize for the next clarification.
Update 2:
- The service compares the list with the previous list, reveals only the changed fields of objects (list items) and updates these fields in the objects in the original list.
- Currently i use annotation on entity's field that is actually ID of the entity and that ID is used to detect identically entities (old and new) when i need to update field of entity in source list.
- Service detect annotated field and use it for next update process.
- I want to refuse to use annotations and provide an Field directly in constructor of service. Or use something other that could establish a relationship between class and field on compilation stage.

Assuming that you want field access because you want to get and set the value, you’d need two functions:
public class UsefulThingsService<T> {
public <V> void makeUsefulThings(List<T> list, Function<T,V> get, BiConsumer<T,V> set) {
for(T object: list) {
V v = get.apply(object);
// there is some code
set.accept(object, v);
}
}
}
and
usefulThingsService.makeUsefulThings(
list, SomeClassEntity::getFieldOne, SomeClassEntity::setFieldOne);
usefulThingsService.makeUsefulThings(
list, SomeClassEntity::getFieldThree, SomeClassEntity::setFieldThree);
There are, however, some things open. E.g., how is this service supposed to do something useful with the field resp. property, without even knowing its actual type. In your example, both are subtypes of Number, so you could declare <V extends Number>, so the method knows how to extract numerical values, however, constructing an appropriate result object would require specifying another function argument.

Iterate Fields of Fields continuously with reflection

Please avoid giving answers in Kotlin only and higher than Android 21.
I'm trying to build an API parser that makes use of class hierarchy logic to represent the API hierarchy itself. With this structure I am able to parse the API in an uncomplicated fashion and I was able to achieve this already, but I'd like to improve it further.
I'll begin explaining what I already have implemented.
This is an example URL that my app will receive via GET, parse and dispatch internally:
http://www.example.com/news/article/1105
In the app the base domain is irrelevant, but what comes after is the API structure.
In this case we have a mixture of commands and variables:
news (command)
article (command)
1105 (variable)
To establish what is a command and what is a variable I built the following class structures:
public class API {
public static final News extends AbstractNews {}
}
public class AbstractNews {
public static final Article extends AbstractArticle {}
}
public class Article {
public static void GET(String articleId) {
// ...
}
}
And I iterate through each class after splitting the URL while matching each command to each class (or subclass) starting from the API class. Until I reach the end of the split URL any matches that fail are stored in a separate list as variables.
The process is as follows for the example provided above:
Split URL each forward slash (ignoring the base domain)
/news/article/1105
List<String> stringList = [
news,
article,
1105
];
Iterate each item in the split list and match agains the API structured classes (the following is just a sample example, it is not 100% of what I currently have implemtend):
List<String> variableList = new ArrayList<>();
Class lastClass = API.class;
for (String stringItem : stringList) {
if ((lastClass = classHasSubClass(lastClass, stringItem)) != null) {
continue;
}
variableList.add(stringItem);
}
Once the end of the list is reached I check if the last class contains the request method (in this case GET) and invoke along with the variable list.
Like I said before this is working perfectly fine, but it leaves every class directly exposed and as a result they can be accessed directly and incorrectly by anyone else working on the project, so I am trying to make the hierarchy more contained.
I want to keep the ability to access the methods via hierarchy as well, so the following can still be possible:
API.News.Article.GET(42334);
While at the same time I don't want it to be possible to do the following as well:
AbstractArticle.GET(42334);
I have tried making each subclass into a class instance field instead
public class API {
// this one is static on purpose to avoid having to instantiate
// the API class before accessing its fields
public static final AbstractNews News = new AbstractNews();
}
public class AbstractNews {
public final AbstractArticle Article = new AbstractArticle();
}
public class Article {
public void GET(String articleId) {
// ...
}
}
This works well for the two points I wanted to achieve before, however I am not able to find a way to iterate the class fields in a way that allows me to invoke the final methods correctly.
For the previous logic all I needed to iterate was the following:
private static Class classHasSubClass(Class<?> currentClass, String fieldName) {
Class[] classes;
classes = currentClass.getClasses();
for (final Class classItem : classes) {
if (classItem.getSimpleName().toLowerCase().equals(fieldName)) {
return classItem;
}
}
return null;
}
But for the second logic attempt with fields I was not able to invoke the final method correctly, probably because the resulting logic was in fact trying to do the following:
AbstractArticle.GET(42334);
Instead of
API.News.Article.GET(42334);
I suspect it is because the first parameter of the invoke method can no longer be null like I was doing before and has to be the correct equivalent of API.News.Article.GET(42334);
Is there a way to make this work or is there a better/different way of doing this?

I discovered that I was on the right path with the instance fields, but was missing part of the necessary information to invoke the method correctly at the end.
When iterating the fields I was only using the Class of each field, which was working perfectly fine before with the static class references since those weren't instances, but now it requires the instance of the field in order to work correctly.
In the end the iterating method used in place of classHasSubClass that got this to work is as follows:
private static Object getFieldClass(Class<?> currentClass, Object currentObject, final String fieldName) {
Field[] fieldList;
fieldList = currentClass.getDeclaredFields();
for (final Field field : fieldList) {
if (field.getName().toLowerCase().equals(fieldName)) {
try {
return field.get(currentObject);
} catch (IllegalAccessException e) {
e.printStackTrace();
break;
}
}
}
return null;
}
With this I always keep an instance object reference to the final field that I want to invoke to pass as the 1st parameter (someMethod.invoke(objectInstance);) instead of null.

I need check if enum element is into enum set

I'm new in java. I need check if enum element is into enum set.
in Delphi:
type
TWeekEnum = (weMonday, weTuesday, weWednesday, weThursday, weFriday, weSaturday, weSunday)
TWeekSetEnum = (weSaturday, weSunday)
if (weSunday in (TWeekSetEnum))
...
Java?

You can define the enum this way, and then also create your subsets as static methods on it.
public enum TWeekEnum {
weMonday, weTuesday, weWednesday, weThursday, weFriday, weSaturday, weSunday;
public static EnumSet<TWeekEnum> getWeekend() {
return EnumSet.of(weSaturday, weSunday);
}
public static EnumSet<TWeekEnum> getWeekDays() {
return EnumSet.complementOf(getWeekend());
}
}
Then you can check if it contains your selected item like this
TWeekEnum.getWeekend().contains(TWeekEnum.weTuesday)

If you prefer the elements to be in the calling code (and not inside the enum) - another solution is to add a normal method named in as follows: -
public enum TWeekEnum {
weMonday, weTuesday, weWednesday, weThursday, weFriday, weSaturday, weSunday;
public boolean in (TWeekEnum ... weekEnum) {
return Arrays.asList(types).contains(this);
}
}
This can be called anywhere in the codebase as follows: -
TWeekEnum weekEnum = TWeekEnum.weSaturday; // <---- If set dynamically, check for null
if (weekEnum.in(TWeekEnum.weSaturday, TWeekEnum.weSunday)) {
// do something
}
... this can look nicer (and read better) if enum values statically imported i.e.
import static com.foo.TWeekEnum.weSaturday;
import static com.foo.TWeekEnum.weSunday;
...
if (weekEnum.in(weSaturday, weSunday)) {
// do something
}

How to use Java Enums being DRY with only a single parameter different between instantiations?

I'm trying to figure out if there is a clean way of doing this. I want to design an ENUM to maintain a list of constant values for different components in my application. Each enum would have the same configuration and same parameters, but would differ at the very least by component name.
In a normal Java class, I could build all the basic logic/code in a base abstract class, and have each component constants extend the abstract class and populate only its own pertinent information. However, Java enums do not allow extending existing classes.
Is there something I can do to avoid having to either push all my constants in a single Enum (ugggg!) or recreate the same enum class each time for each differing component? Definitely not DRY in that case, but I do not know how to avoid the issue.
For a quick use-case example off the top of my head. Say I want to keep a list of all my request mappings in an Enum for use elsewhere in my application. Fairly easy to design an enum that says:
public enum RequestMapping {
INDEX("index"),
GET_ALL_USERS( "getAllUsers");
private String requestMapping = "/users";
private String path;
RatesURI( String path ){
this.path = path;
}
public String getRequestMapping(){
return requestMapping;
}
public String getPath(){
return path;
}
public String getFullRequestPath(){
return requestMapping + "/" + path;
}
}
It becomes easy to use RequestMapping.GET_ALL_USERS.getFullRequestPath().
Now if I want to create this enum on a per-controller basis, I would have to recreate the entire Enum class and change the "requestMapping" value for each one. Granted, this enum has nearly no code in it, so duplicating it would not be difficult, but the concept still remains. The theoretical "clean" way of doing this would be to have an abstract AbstractRequestMapping type that contained all the methods, including an abstract getRequestMapping() method, and only have the extending Enums implement the controller-specific getReqeuestMapping(). Of course, since Enums cannot be extended, I can't think of a non DRY way of doing this.

Have you considered extending a class that takes Enum as a generic parameter? It is an amazingly flexible mechanism.
public class Entity<E extends Enum<E> & Entity.IE> {
// Set of all possible entries.
// Backed by an EnumSet so we have all the efficiency implied along with a defined order.
private final Set<E> all;
public Entity(Class<E> e) {
// Make a set of them.
this.all = Collections.unmodifiableSet(EnumSet.<E>allOf(e));
}
// Demonstration.
public E[] values() {
// Make a new one every time - like Enum.values.
E[] values = makeTArray(all.size());
int i = 0;
for (E it : all) {
values[i++] = it;
}
return values;
}
// Trick to make a T[] of any length.
// Do not pass any parameter for `dummy`.
// public because this is potentially re-useable.
public static <T> T[] makeTArray(int length, T... dummy) {
return Arrays.copyOf(dummy, length);
}
// Example interface to implement.
public interface IE {
#Override
public String toString();
}
}
class Thing extends Entity<Thing.Stuff> {
public Thing() {
super(Stuff.class);
}
enum Stuff implements Entity.IE {
One,
Two;
}
}
You can pass the nature of your implementation up to the parent class in many different ways - I use enum.class for simplicity.
You can even make the enum implement an interface as you can see.
The values method is for demonstration only. Once you have access to the Set<E> in the parent class you can provide all sorts of functionality just by extending Entity.

I will probably split the responsibilities into two parts:
Logic about how a request is structured, and put that into an immutable class.
Actual configurations of each request, stored in enums
The enum will then store an instance of that class, you can add new methods to the class, without modifying the different enums, as long as the constructor remains the same. Note that the class must be immutable, or your enum will not have a constant value.
You can use it like the:
ServiceRequest.INDEX.getRequest().getFullRequestPath()
With these classes:
public interface RequestType {
Request getRequest();
}
public class Request {
private final String requestMapping;
private final String path;
RatesURI(String requestMapping, String path){
this.requestMappint = requestMapping;
this.path = path;
}
public String getRequestMapping(){
return requestMapping;
}
public String getPath(){
return path;
}
public String getFullRequestPath(){
return requestMapping + "/" + path;
}
}
public enum ServiceRequest implements RequestType {
INDEX("index"),
GET_ALL_USERS( "getAllUsers");
private final Request;
ServiceRequest(String path) {
request = new Request("users/", path)
}
public String getRequest{
return request;
}
}

I think what you should be asking yourself is really why you want to use enums for this. First we can review some of the points that make Java enumerated types what they are.
Specifically
A Java enum is a class that extends java.lang.Enum.
Enum constants are static final instances of that class.
There is some special syntax to use them but that is all they boil down to. Because instantiating new Enum instances is disallowed outside of the special syntax (even with reflection, enum types return zero constructors) the following is also ensured to be true:
They can only be instantiated as static final members of the enclosing class.
The instances are therefore explicitly constant.
As a bonus, they are switchable.
What it really boils down to is what it is about the enums that makes them preferable over a simpler OOP design here. One can easily create a simple RequestMapping class:
/* compacted to save space */
public class RequestMapping {
private final String mapping, path;
public RequestMapping(String mapping, String path) {
this.mapping = mapping; this.path = path;
}
public String getMapping() {
return mapping; }
public String getPath() {
return path; }
public String getFullRequestPath() {
return mapping + "/" + path;
}
}
Which can easily be extended to break down the repeated code:
public class UserMapping extends RequestMapping {
public UserMapping(String path) {
super("/users", path);
}
}
/* where ever appropriate for the constants to appear */
public static final RequestMapping INDEX = new UserMapping("index"),
GET_ALL_USERS = new UserMapping("getAllUsers");
But I assume there is something about enums that is attractive to your design, such as the principle that instances of them are highly controlled. Enums cannot be created all willy-nilly like the above class can be. Perhaps it's important that there be no plausible way for spurious instances to be created. Of course anybody can come by and write in an enum with an invalid path but you can be pretty sure nobody will do it "by accident".
Following the Java "static instances of the outer class" enum design, an access modifier structure can be devised that generally abides by the same rule set as Enum. There are, however, two problems which we can't get around easily.
Two Problems
Protected modifier allows package access.
This can easily be surmounted initially by putting the Enum-analog in its own package. The problem becomes what to do when extending. Classes in the same package of the extended class will be able to access constructors again potentially anywhere.
Working with this depends on how stringent you want to be on creating new instances and, conversely, how clear the design ends up. Can't be a whole mess of scopes just so only a few places can do the wrong thing.
Static members are not polymorphic.
Enum surmounts this by not being extendable. Enum types have a static method values that appears "inherited" because the compiler inserts it for you. Being polymorphic, DRY and having some static features means you need instances of the subtype.
Defeating these two issues depends on how stringent you want your design to be and, conversely, how readable and stable you want your implementation to be. Trying to defy OOP principles will get you a design that's hard to break but totally explodes when you call that one method in a way you aren't supposed to (and can't prevent).
First Solution
This is almost identical to the Java enum model but can be extended:
/* 'M' is for 'Mapping' */
public abstract class ReturnMapping<M extends ReturnMapping> {
/* ridiculously long HashMap typing */
private static final HashMap <Class<? extends ReturnMapping>, List<ReturnMapping>>
VALUES = new HashMap<Class<? extends ReturnMapping>, List<ReturnMapping>>();
private final String mapping, path;
protected Mapping(String mapping, String path) {
this.mapping = mapping;
this.path = path;
List vals = VALUES.get(getClass());
if (vals == null) {
vals = new ArrayList<M>(2);
VALUES.put(getClass(), vals);
}
vals.add(this);
}
/* ~~ field getters here, make them final ~~ */
protected static <M extends ReturnMapping> List<M>(Class<M> rm) {
if (rm == ReturnMapping.class) {
throw new IllegalArgumentException(
"ReturnMapping.class is abstract");
}
List<M> vals = (List<M>)VALUES.get(rm);
if (vals == null) {
vals = new ArrayList<M>(2);
VALUES.put(rm, (List)vals);
}
return Collections.unmodifiableList(vals);
}
}
Now extending it:
public final class UserMapping extends ReturnMapping<UserMapping> {
public static final UserMapping INDEX = new UserMapping("index");
public static final UserMapping GET_ALL_USERS = new UserMapping("getAllUsers");
private UserMapping(String path) {
super("/users", path);
}
public static List<UserMapping> values() {
return values(UserMapping.class);
}
}
The huge static HashMap allows almost all of the values work to be done statically in the superclass. Since static members are not properly inherited this is the closest you can get to maintaining a list of values without doing it in the subclass.
Note there are two problems with the Map. The first is that you can call the values with ReturnMapping.class. The map should not contain that key (the class is abstract and the map is only added to in the constructor) so something needs to be done about it. Instead of throwing an exception you could also insert a "dummy" empty list for that key.
The other problem is that you can call values on the superclass before the instances of the subclass are instantiated. The HashMap will return null if this is done before the subclass is accessed. Static problem!
There is one other major problem with this design because the class can be instantiated externally. If it's a nested class, the outer class has private access. You can also extend it and make the constructor public. That leads to design #2.
Second Solution
In this model the constants are an inner class and the outer class is a factory for retrieving new constants.
/* no more generics--the constants are all the same type */
public abstract class ReturnMapping {
/* still need this HashMap if we want to manage our values in the super */
private static final HashMap <Class<? extends ReturnMapping>, List<Value>>
VALUES = new HashMap<Class<? extends ReturnMapping>, List<Value>>();
public ReturnMapping() {
if (!VALUES.containsKey(getClass())) {
VALUES.put(getClass(), new ArrayList<Value>(2));
}
}
public final List<Value> values() {
return Collections.unmodifiableList(VALUES.get(getClass()));
}
protected final Value newValue(String mapping, String path) {
return new Value(getClass(), mapping, path);
}
public final class Value {
private final String mapping, path;
private Value(
Class type,
String mapping,
String path) {
this.mapping = mapping;
this.path = path;
VALUES.get(type).add(this);
}
/* ~~ final class, field getters need not be ~~ */
}
}
Extending it:
public class UserMapping extends ReturnMapping {
public static final Value INDEX, GET_ALL_USERS;
static {
UserMapping factory = new UserMapping();
INDEX = factory.newValue("/users", "index");
GET_ALL_USERS = factory.newValue("/users", "getAllUsers");
}
}
The factory model is nice because it solves two problems:
Instances can only be created from within the extending class.
Anybody can create a new factory but only the class itself can access the newValue method. The constructor for Value is private so new constants can only be created by using this method.
new UserMapping().values() forces the values to be instantiated before returning them.
No more potential errors in this regard. And the ReturnMapping class is empty and instantiating new objects in Java is fast so I wouldn't worry about overhead. You can also easily create a static field for the list or add static methods such as in solution #1 (though this would deflate the design's uniformity).
There are a couple of downsides:
Can't return the subtyped values List.
Now that the constant values are not extended they are all the same class. Can't dip in to generics to return differently-typed Lists.
Can't easily distinguish what subtype a Value is a constant of.
But it's true this could be programmed in. You could add the owning class as a field. Still shaky.
Sum Of It
Bells and whistles can be added to both of these solutions, for example overriding toString so it returns the name of the instance. Java's enum does that for you but one of the first things I personally do is override this behavior so it returns something more meaningful (and formatted).
Both of these designs provide more encapsulation than a regular abstract class and most importantly are far more flexible than Enum. Trying to use Enum for polymorphism is an OOP square peg in a round hole. Less polymorphism is the price to pay for having enumerated types in Java.

Java - Class type from inside static initialization block

Is it possible to get the class type from inside the static initialization block?
This is a simplified version of what I currently have::
class Person extends SuperClass {
String firstName;
static{
// This function is on the "SuperClass":
// I'd for this function to be able to get "Person.class" without me
// having to explicitly type it in but "this.class" does not work in
// a static context.
doSomeReflectionStuff(Person.class); // IN "SuperClass"
}
}
This is closer to what I am doing, which is to initialize a data structure that holds information about the object and its annotations, etc... Perhaps I am using the wrong pattern?
public abstract SuperClass{
static void doSomeReflectionStuff( Class<?> classType, List<FieldData> fieldDataList ){
Field[] fields = classType.getDeclaredFields();
for( Field field : fields ){
// Initialize fieldDataList
}
}
}
public abstract class Person {
#SomeAnnotation
String firstName;
// Holds information on each of the fields, I used a Map<String, FieldData>
// in my actual implementation to map strings to the field information, but that
// seemed a little wordy for this example
static List<FieldData> fieldDataList = new List<FieldData>();
static{
// Again, it seems dangerous to have to type in the "Person.class"
// (or Address.class, PhoneNumber.class, etc...) every time.
// Ideally, I'd liken to eliminate all this code from the Sub class
// since now I have to copy and paste it into each Sub class.
doSomeReflectionStuff(Person.class, fieldDataList);
}
}
Edit
I picked the accepted answer based on what applied best to my problem, however it seems to me that all three of the current answers have their merits.

No, it's not possible without grabbing the stacktrace (which is imo nastier than your initial approach and for which I would in any way prefer Thread#getStackTrace() above new Exception()).
Rather do that job in a non-static initializer (or the default constructor) of the abstract class where you check the initialized status.
public abstract class SuperClass {
{
if (!isInitialized(getClass())) {
initialize(getClass());
}
}
}
The called methods in turn can be safely static.

yes, I use this often to initialize a static Log variable :
e.g. :
public class Project implements Serializable, Cloneable, Comparable<Project> {
private static final Logger LOG = LoggerFactory.getLogger(Project.class);
...

To get a class at runtime, you could do something along the lines of
public class Test {
public static void main(String[] args) {
try{
throw new Exception();
}
catch(Exception e){
StackTraceElement[] sTrace = e.getStackTrace();
// sTrace[0] will be always there
String className = sTrace[0].getClassName();
System.out.println(className);
}
}
}
Not pretty but will do the job (ripped from http://www.artima.com/forums/flat.jsp?forum=1&thread=155230).
This means you still make a call from the subclass (so is in the stack trace), but you don't need to include the XXX.class as an argument.