I'm needing to get a unique method identifier to use as a key on a HashMap.
I'm trying to do something using stacktrace and reflection and user the method signature. But the problem is I didn´t find a way to retrive the complete method signature (to avoid methods overload).
Edited
I would like that somethink like this works:
public class Class1 {
HashMap<String, Object> hm;
public Class1() {
hm = new HashMap<String, Object>();
}
public Object method() {
if (!containsKey()) {
Object value;
...
put(value);
}
return get();
}
public Object method(String arg1) {
if (!containsKey()) {
Object value;
...
put(value);
}
return get();
}
public Boolean containsKey() {
if (hm.containsKey(Util.getUniqueID(2)) {
return true;
} else {
return false;
}
}
public void put(Object value) {
hm.put(Util.getUniqueID(2), value);
}
public Object get() {
String key = Util.getUniqueID(2);
if (hm.containsKey(key) {
return hm.get(key);
} else {
return null;
}
}
}
class Util {
public static String getUniqueID(Integer depth) {
StackTraceElement element = Thread.currentThread().getStackTrace()[depth];
return element.getClassName() + ":" + element.getMethodName();
}
}
But the problem is the two methods, with this strategy, will have the same ID.
How can I work around?
You can append + ":" + element.getLineNumber() but you'd still have to worry about the case where two overloaded methods are put on one long line.
Looking at the StackTraceElement methods, it doesn't seem possible to get a unique method identifier this way. Besides, the code is not very readable in my opinion.
I'd suggest you try to be more explicit and do
if (hm.containsKey("getValue(int)") {
...
}
or something similar.
Related
I want to implement a dictionary in android using the following method :
private final Dictionary<String,String> allWords = new Dictionary<String, String>() {
However, whenever I try to add it I get this whole mess below it, and when I tried using it , I keep getting an error.
I know I'm supposed to implement the functions in the abstract class but I'm wondering if there is an easier way to do it than to code it manually.
#Override
public int size() {
return 0;
}
#Override
public boolean isEmpty() {
return false;
}
#Override
public Enumeration<String> keys() {
return null;
}
#Override
public Enumeration<String> elements() {
return null;
}
#Override
public String get(Object key) {
return null;
}
#Override
public String put(String key, String value) {
return null;
}
#Override
public String remove(Object key) {
return null;
}
};
I just used
private final HashMap<String,String> allWords = new HashMap<>();
It is because there is already a built-in class called Dictionary in Java. Therefore the IDE ask you to override the member functions. If you want to implement everything from scratch, use another class name. Don't use Dictionary. E.g. name your calss MyDictionary, CustomDictionary, or something like that. Avoid built-in class names.
I have implemented HashMap for storing hotel booking entry.But I'm getting null values on .get(object) method even it contains all keys and returning keys correctly.I already override equals() & hashCode() methods in two different class (bookingSeason & entry) because bookingSeason class is used in some other class also and it works correctly but in entry class it does not work.
public class Entry {
String code;
List<BookingSeason> booking=new ArrayList<>();
public Entry(String code) {
this.code=code;
}
#Override
public boolean equals(Object o) {
if(o==null)
return false;
if(!(o instanceof Entry))
return false;
Entry room=(Entry) o;
return this.code.equals(room.code)&&
this.booking.equals(room.booking);
}
#Override
public int hashCode() {
return Objects.hash(code,booking);
}
}
public class BookingSeason {
LocalDate startDate;
LocalDate endDate;
public BookingSeason(LocalDate startDate,LocalDate endDate) {
this.startDate=startDate;
this.endDate=endDate;
}
#Override
public boolean equals(Object object)
{
if(object==this)
return true;
if(!(object instanceof BookingSeason))
return false;
BookingSeason bS=(BookingSeason) object;
return Objects.equals(startDate,bS.startDate)&& Objects.equals(
endDate,bS.endDate);
}
#Override
public int hashCode() {
return Objects.hash(startDate,endDate);
}
}
public class Hotel {
List<BookingSeason> bookPeriod=new ArrayList<>();
HashMap<Long,Entry> roomEntry =new HashMap<>();
long num;
Entry newRoom=new Entry();
for(int i=101;i<=199;i++) {
num=i;
newRoom.code="A";
newRoom.code=newRoom.code.concat(String.valueOf(i));
roomEntry.put(num,new Entry(newRoom.code));
System.out.println(roomEntry.get(i));
}
}
roomEntry.put(num,new Entry(newRoom.code));
uses the long value num to enter the new Entry object into the hashmap.
System.out.println(roomEntry.get(i));
uses the int value i to try to get the Entry object.
But, since
Long.valueOf(11).equals(Integer.valueOf(11)) == false
it will not found the entry. You need to pass a long/Long value to the get method. Therefore, either using
System.out.println(roomEntry.get(num));
or using
System.out.println(roomEntry.get((long) i));
will solve your problem.
For reference see also What are the reasons why Map.get(Object key) is not (fully) generic
Solution:
Simply change the for loop from this:
for(int i=101;i<=199;i++) {
to this:
for(long i=101;i<=199;i++) {
Explanation:
What you have done wrong here is that you are trying to pass an int into the get method of HashMap<Long, Entry>.
Since the HashMap uses longs as keys, whenever the get method sees you try to use some other type of incompatible value, like int, it just returns null!
Although the integer i you passed into get has the same hash code as the long num, the get method treats them as "not the same".
Why?
Think about what would happen if get allowed you to use an object that is of unrelated type to the key type of the hash map to access a value:
class A {
#Override
public int hashCode() { return 1; }
}
class B {
#Override
public int hashCode() { return 1; }
}
// in some method
HashMap<A, Object> map = new HashMap<>();
map.put(new A(), "some stuff");
map.get(new B()); // returns "someStuff"! makes no sense, right?
I'm writing a messaging system to queue actions for my program to execute. I need to be able to pass various objects by the messages. I currently have a Msg object that accepts (Action enum, Data<?>...object). The Data object is intended to be a wrapper for any object I might pass.
Currently the Data object uses this code, with generics:
public class Data<T> {
private T data;
public Data(T data){
this.data = data;
}
public T getData(){
return data;
}
}
The Msg object takes Data<?>... type, so Msg has a Data<?>[] field.
If getData() is called on a Data<?> object, it returns the Object type. Obviously not ideal.
I need to be able to pass, say, Image objects as well as String objects. I'm certain there's a better way of passing arbitrary data.
The reason you're having trouble is that you're trying to get the static typing system of Java to do something that it can't. Once you convert from a Data<T> to a Data<?>, whatever T was is effectively lost. There's no clean way to get it back.
The quickest way to get it to work (from what you have right now) is to start throwing casts everywhere, like this:
Data<?> d = new Data("Hello");
String contents = (String)d.getData();
This is kind of a terrible idea, so let's go back to the drawing board.
If (ideally), you have all of the types you could ever need ahead of time (i.e. every Data is either a String or an Image or an Integer), then you can pretty easily (though it's a bit tedious) define a Sum type (aka a union if you're coming from C) of the different types of data you'll have to handle. As a class invariant, we assume that exactly one of the fields is non-null, and the rest are null. For this example I'll assume it can be either a String, an Image, or an Integer, but it's fairly simple to add or remove types from Data as necessary.
public class Data {
private Image imgData;
private String stringData;
private Integer intData;
public Data(Image img) {
this.imgData = img;
}
public Data(String stringData) {
this.stringData = stringData;
}
public Data(Integer intData) {
this.intData = intData;
}
public boolean isImage() {
return imageData != null;
}
public boolean isInteger() {
return intData != null;
}
public boolean isString() {
return stringData != null;
}
public Image asImage() {
if(! isImage()) throw new RuntimeException();
return imgData;
}
public Image asString() {
if(! isString()) throw new RuntimeException();
return stringData;
}
public Image asInt() {
if(! isInt()) throw new RuntimeException();
return intData;
}
}
One necessary side effect is that we cannot wrap null without causing exceptional behavior. Is this is desired, it isn't too difficult to modify the class to allow for it.
With this Data class, it's pretty easy to do if-else logic to parse it.
Data d = ....... //Get a data from somewhere
if(d.isImage()) {
Image img = d.asImage();
//...
} else if (d.isString()) {
String string = d.asString();
//...
} else if (d.isInteger()) {
Integer i = d.asInt();
//...
} else {
throw new RuntimeException("Illegal data " + d + " received");
}
If you call getData().getClass() you will get the class or type that was passed, which doesn't seem to me to be the same as an Object. You might not know what you are getting, but you can either find out or define a common interface for everything you might pass. You could for example, call toString() or getClass() on anything passed. Your question is that you are passing any conceivable object, so my question is what are you going to do with it? If you are going to serialize it into a database you don't need know anything about what type it is, otherwise you can test it or call a common interface.
public class PlayData {
class Msg {
private List<Data<?>> message = new ArrayList<Data<?>>();
public void addData(Data<?> datum) { message.add(datum); }
public void printTypes() { for ( Data<?> datum: message ) { System.out.println(datum.getData().getClass()); } }
}
class Data<T> {
private T value;
public Data(T value) { this.value = value; }
public T getData() { return value; }
}
class Listener {
public void receive(Msg msg) { msg.printTypes(); }
}
class Sender {
private Listener listener;
public Sender(Listener listener) { this.listener = listener; }
public void send(Msg msg) { listener.receive(msg); }
}
class MyPacket {
int i;
public MyPacket(int i) { this.i = i; }
}
public static void main(String[] args) throws Exception { new PlayData().run(); }
public void run() throws Exception {
Sender sender = new Sender(new Listener());
Msg msg = new Msg();
msg.addData(new Data<String>("testing") );
msg.addData(new Data<MyPacket>(new MyPacket(42)) );
sender.send(msg);
}
}
In my program, the user needs to input what type of players the game will have. The players are "human", "good" (for a good AI), "bad" (for a bad AI) and "random" (for a random AI). Each of these players have their own class that extend one abstract class called PlayerType.
My struggle is mapping a String to the object so I can A) create a new object using the String as sort of a key and B) get the related String from an object of its subclass
Ultimately, I just want the implicit String to only appear once in the code so I can change it later if needed without refactoring.
I've tried using just a plain HashMap, but that seems clunky with searching the keys via the values. Also, I'm guessing that I'll have to use the getInstance() method of Class, which is a little less clunky, which is okay if it's the only way.
What I would do is create an enum which essentially functions as a factory for the given type.
public enum PlayerTypes {
GOOD {
#Override
protected PlayerType newPlayer() {
return new GoodPlayer();
}
},
BAD {
#Override
protected PlayerType newPlayer() {
return new BadPlayer();
}
},
RANDOM {
#Override
protected PlayerType newPlayer() {
return new RandomPlayer();
}
};
protected abstract PlayerType newPlayer();
public static PlayerType create(String input) {
for(PlayerTypes player : PlayerTypes.values()) {
if(player.name().equalsIgnoreCase(input)) {
return player.newPlayer();
}
}
throw new IllegalArgumentException("Invalid player type [" + input + "]");
}
)
Because then you can just call it like so:
String input = getInput();
PlayerTypes.create(input);
Of course, you'll get an IllegalArgumentException which you should probably handle by trying to get the input again.
EDIT: Apparently in this particular case, you can replace that loop with just merely
return PlayerTypes.valueOf(input).newPlayer();
And it'll do the same thing. I tend to match for additional constructor parameters in the enum, so I didn't think of using valueOf(), but it's definitely cleaner.
EDIT2: Only way to get that information back is to define an abstract method in your PlayerType class that returns the PlayerTypes enum for that given type.
public class PlayerType {
public abstract PlayerTypes getType();
}
public class GoodPlayer extends PlayerType {
#Override
public PlayerTypes getType() {
return PlayerTypes.GOOD;
}
}
I like the answer provided by Epic but I don't find maps to be clunky. So it's possible to keep a map and get the constructor call directly.
Map<String, Supplier<PlayerType> map = new HashMap<>();
map.put("human", Human::new);
Human h = map.get("human").get();
The two main options I can think of:
Using Class.newInstance(), as you mentioned (not sure if you had this exact way in mind):
// Set up your map
Map<String, Class> classes = new HashMap<String, Class>();
classes.put("int", Integer.class);
classes.put("string", String.class);
// Get your data
Object s = classes.get("string").newInstance();
You could use Class.getDeclaredConstructor.newInstance if you want to use a constructor with arguments (example).
Another option is using switch:
Object getObject(String identifier) {
switch (identifier) {
case "string": return new String();
case "int": return new Integer(4);
}
return null; // or throw an exception or return a default object
}
One potential solution:
public class ForFunFactory {
private ForFunFactory() {
}
public static AThing getTheAppropriateThing(final String thingIdentifier) {
switch (thingIdentifier) {
case ThingImplApple.id:
return new ThingImplApple();
case ThingImplBanana.id:
return new ThingImplBanana();
default:
throw new RuntimeException("AThing with identifier "
+ thingIdentifier + " not found.");
}
}
}
public interface AThing {
void doStuff();
}
class ThingImplApple implements AThing {
static final String id = "Apple";
#Override
public void doStuff() {
System.out.println("I'm an Apple.");
}
}
class ThingImplBanana implements AThing {
static final String id = "Banana";
#Override
public void doStuff() {
System.out.println("I'm a Banana.");
}
}
In a previous post Creating a ToolTip Managed bean
I was able to create a manged bean to collect and display tooltip text with only a single lookup and store them in an Application Scope variable. This has worked great.
I am on the rather steep part of the JAVA learning curve so please forgive me.
I have another managed bean requirement to create a HashMap Application Scope but this time it needs to be of a type String, Object. The application is where I have a single 'master' database that contains most of the code, custom controls, and XPages. This Master Database will point to One or More application databases that will store the Notes Documents specific to the application in question. So I have created in the Master a series of Application Documents that specify the RepIDs of the Application, Help and Rules databases specific to the Application along with a number of other pieces of information about the Application. This should allow me to reuse the same custom control that will open the specific DB by passing it the Application Name. As an example the Master Design DB might point to "Purchasing", "Customer Complaints", "Travel Requests" etc.
The code below works to load and store the HashMap, but I am having trouble retrieving the the data.
The compiler shows two errors one at the public Object get(String key) method and the other at mapValue = this.internalMap.get(key); in the getAppRepID method I think that it is mainly syntax but not sure. I have commented the error in the code where it appears.
/**
*This Class makes the variables that define an application within Workflo!Approval
*available as an ApplicationScope variable.
*/
package ca.wfsystems.wfsAppUtils;
import lotus.domino.Base;
import lotus.domino.Session;
import lotus.domino.Database;
import lotus.domino.View;
import lotus.domino.NotesException;
import lotus.domino.ViewColumn;
import lotus.domino.ViewEntry;
import lotus.domino.ViewEntryCollection;
import lotus.domino.Name;
import java.io.Serializable;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.Vector;
import com.ibm.xsp.extlib.util.ExtLibUtil;
/**
* #author Bill Fox Workflo Systems WFSystems.ca
* July 2014
* This class is provided as part of the Workflo!Approval Product
* and can be reused within this application.
* If copied to a different application please retain this attribution.
*
*/
public abstract class ApplicationUtils implements Serializable, Map<String, Object> {
private static final long serialVersionUID = 1L;
private Session s;
private Name serverName;
private String repID;
private String thisKey;
private ViewEntryCollection formVECol;
private Vector formNames;
private Database thisDB;
private Database appDB;
private View appView;
private View formView;
private ViewEntry formVE;
private ViewEntry tFormVE;
private ViewEntry ve;
private ViewEntry tVE;
private ViewEntryCollection veCol;
private final Map<String, Object> internalMap = new HashMap<String, Object>();
public ApplicationUtils() {
this.populateMap(internalMap);
}
private void populateMap(Map<String, Object> theMap) {
try{
s = ExtLibUtil.getCurrentSession();
//serverName = s.createName(s.getServerName());
thisDB = s.getCurrentDatabase();
appView = thisDB.getView("vwWFSApplications");
veCol = appView.getAllEntries();
ve = veCol.getFirstEntry();
ViewEntry tVE = null;
while (ve != null) {
rtnValue mapValue = new rtnValue();
tVE = veCol.getNextEntry(ve);
Vector colVal = ve.getColumnValues();
thisKey = colVal.get(0).toString();
mapValue.setRepID(colVal.get(2).toString());
// ...... load the rest of the values .......
theMap.put(thisKey, mapValue);
recycleObjects(ve);
ve = tVE;
}
}catch(NotesException e){
System.out.println(e.toString());
}finally{
recycleObjects(ve, veCol, appView, tVE);
}
}
public class rtnValue{
private String RepID;
private String HelpRepID;
private String RuleRepID;
private Vector FormNames;
public String getRepID() {
return RepID;
}
public void setRepID(String repID) {
RepID = repID;
}
public String getHelpRepID() {
return HelpRepID;
}
public void setHelpRepID(String helpRepID) {
HelpRepID = helpRepID;
}
public String getRuleRepID() {
return RuleRepID;
}
public void setRuleRepID(String ruleRepID) {
RuleRepID = ruleRepID;
}
public Vector getFormNames() {
return FormNames;
}
public void setFormNames(Vector formNames) {
FormNames = formNames;
}
}
public void clear() {
this.internalMap.clear();
this.populateMap(this.internalMap);
}
public boolean containsKey(Object key) {
return this.internalMap.containsKey(key);
}
public boolean containsValue(Object value) {
return this.internalMap.containsValue(value);
}
public Set<java.util.Map.Entry<String, Object>> entrySet() {
return this.internalMap.entrySet();
}
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
public boolean isEmpty() {
return this.internalMap.isEmpty();
}
public Set<String> keySet() {
return this.internalMap.keySet();
}
public Object put(String key, Object value) {
return this.internalMap.put(key, value);
}
public Object remove(Object key) {
return this.internalMap.remove(key);
}
public int size() {
return this.internalMap.size();
}
public Collection<Object> values() {
return this.internalMap.values();
}
public void putAll(Map<? extends String, ? extends Object> m) {
this.internalMap.putAll(m);
}
public String getAppRepID(String key){
/*get the Replica Id of the application database
* not sure this is the correct way to call this
*/
rtnValue mapValue = new rtnValue();
mapValue = this.internalMap.get(key);
//error on line above Type Mismatch: can not convert Object to ApplicationUtils.rtnValue
String repID = mapValue.getRepID();
}
public static void recycleObjects(Object... args) {
for (Object o : args) {
if (o != null) {
if (o instanceof Base) {
try {
((Base) o).recycle();
} catch (Throwable t) {
// who cares?
}
}
}
}
}
}
For the get() method, the way I handle that kind of situation is create a variable of the correct data type as null, in my try/catch set the variable, and at the end return the variable. So:
Object retVal = null;
try....
return retVal;
For the other error, if you right-click on the error marker, it might give you the opportunity to cast the variable to rtnValue, so:
mapValue = (rtnValue) this.internalMap.get(key)
If you haven't got it, Head First Java was a useful book for getting my head around some Java concepts. It's also worth downloading the FindBugs plugin for Domino Designer from OpenNTF. It will identify errors as well as bad practices. Just ignore the errors in the "local" package!
The problem is that there is an execution path that do not return nothing
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) { // false
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
rtnValue newMap = new rtnValue();
return newMap;
}
}
if key is not present in the internalMap, nothing is thrown, then that method do not return anything.
To fix the problem, return the newMap at the end.
public Object get(String key) {
//error on Object get Method must return a result of type Object
try {
if (this.internalMap.containsKey(key)) {
return this.internalMap.get(key);
}
} catch (Exception e) {
System.out.println(e.toString());
}
rtnValue newMap = new rtnValue();
return newMap;
}
You can inline the return to save the allocation (which is what the compiler will do anyway). I didn't do it just to make it clear in the example.
But still you have a compiler error in getAppRepID method. You are expecting a rtnValue but you send back an Object. You must cast there.
The appropriate way to handle this is, if you know that all values are of a given type, create the map with the proper type.
Have you tried making your internalMap a map of rtnValue instances (so )?