In this example I have tempSocket1 and tempSocket2 but I really just want one of them. I just included both to show I tried both methods, but I keep getting an error, "the method valueOf(String) in the type Integer is not applicable for the arguments (Optional)." I thought both of these methods were the ones used for converting a string data type to integer, but I'm not sure how the Optional part changes the whole system.
private void showTextInputDialog() {
TextInputDialog changePort = new TextInputDialog("Settings");
changePort.setHeaderText("Change Port");
changePort.setContentText("Please enter port number to be used for establishing connection...");
Optional<String> result = changePort.showAndWait();
result.ifPresent(e -> {
Integer tempSocket1 = Integer.valueOf(result);
Integer tempSocket2 = Integer.parseInt(result);
}
);
}
To convert an Optional to an Integer, it is necessary to invoke the get() method before the conversion.
Optional<String> cadena = Optional.of("333");
Integer num = Integer.valueOf(cadena.get());
You see, Integer.valueOf and Integer.parseInt methods need an argument of type String, but you are passing an Optional<String>. So that's why the error occurred. Optional string and string are not the same.
Just think about this, if Optional<String> were the same as String, would ArrayList<String> be the same as String? Would LinkedList<String> be the same as String? What about HashMap<String, Integer>? Would it be both a String and an Integer?
The chaos that treating generic types the same as their generic type arguments would bring is destructive! Imagine calling charAt on an optional string! Without the implementation, no one knows what will happen...
So yeah, never think that generic types are the same types as the generic type parameters.
Just to extend other answers it may looks better using map method, and even more with lambda and method reference:
Optional<String> result = changePort.showAndWait();
Integer tempSocket = result.map(Integer::valueOf).orElse(8080);
You're trying to pass an Optional<String> instead of a normal String. You need to fetch the string first with .get() before converting your result to an integer. Or use result.ifPresent(e ...) that will automatically unwrap the optional value and convert it to an Integer.
Optional<String> result = changePort.showAndWait();
result.ifPresent(e -> {
Integer tempSocket1 = Integer.valueOf(e);
Integer tempSocket2 = Integer.parseInt(e);
}
);
Previous question
I have the following code:
ArrayList<Object> list = new ArrayList<Object>();
list.add("StringType");
list.add(5);
list.add(new RandomClass());
List<Class<?>> classes = new ArrayList<>();
classes.add(String.class);
classes.add(int.class);
classes.add(RandomClass.class);
for (int i = 0; i < list.size(); i++) {
if (classes.get(i).isInstance(list.get(i))) {
...
}
}
if (isvalid)
mymethod(...);
public void mymethod(String string, int num, RandomClass randomClass){ }
Now I'm trying to cast the object into the right type with a method using a string argument.
Instead of:
mymethod( (String) list.get(0), (int) list.get(1), (RandomClass) list.get(2) );
I would like to reuse the definition created above for the cast.
mymethod( ( define.get(0) ) list.get(0), .... );
I've also tried using the Class.cast(obj) but of course it returns a type '?' which again defeats the purpose of casting it again using (String).
What is type safety?
In computer science, type safety is the extent to which a programming
language discourages or prevents type errors.
If code is type safe, then the compiler can verify, at compile time, that all the types are correct:
String getName() {
return "name";
}
The compiler knows that "name" must be a String so it can verify that this code will never throw a type error.
Once you do something like:
int getNumber() {
(int) number;
}
The need to explicitly cast to int tells you that this code has an error condition, namely when number is not of type int or a type that is assignable to int.
How does it affect you?
Your code:
define.get(0).cast(list.get(0))
You want the return type of this statement to be of the type of get(0). But the compiler has no way of knowing, at compile time, what define.get(0) returns. This is inidcated to you by the return type.
You have a List<Class<?>>, i.e. a List of a class of "I don't care what type". You then use a member of this List to cast a member of your other List - the only result can be an "I don't care what type".
You can hack around this with:
<T> T get(final int i) {
return (T) define.get(i).cast(list.get(i));
}
This will happily compile:
final String thing = get(0);
As will:
final int thing = get(0);
i.e. all that you have done is to endogenise the cast. The error condition still exists.
define.get(0).cast(list.get(0)) would attempt to cast list.get(0) to the required type.
In order to be able to select the appropriate method, the compiler needs to know at compile time what the types of the arguments are. Or at least a general category such as List<?> etc.
This is needed to support overloading of methods. There can be many methods with the same name, but with different parameter types.
Since you are asking the VM to call a method when it can't determine which exact method you want to call, because it doesn't know at compile time what the types of your parameters are, what you ask cannot be done in Java.
Here is the relevant section from the Java Language Specification.
What it says is that the system selects at compile time which method signature to use, and then, at run time, the particular implementation of that method signature that's correct for the given instance.
You don't actually need to store object's class separately
list.get(0).getClass()
will get you the class of the stored object and then you can use what #Eran suggested
and
list.get(0).getClass().getName()
will get you the String name of your class
I am using reflections to map an ResultSet to a Field in bean.
field = clazz.getDeclaredField(str);
field.setAccessible(true);
Object resultSetObject = rs.getObject(str);
Class fieldType = field.getType();
field.set(clazzInst, fieldType.cast(resultSetObject));
The problem is resultSetObject is of type Integer and fieldType is of Long and I cannot cast Intger to Long and getting ClassCastException.
You are essentially asking to duplicate the type knowledge that is available at compile-time with which the compiler generates the correct conversion. The runtime doesn't have this knowledge, so you will have to provide it by building a matrix of types and coding all the conversions you want, explicitly.
You can define this as a matrix indexed by type along both axes (from-type and to-type) or more likely as a Map whose key is a ConversionType object each instance of which defines fromType, toType and a convert() method.
Whether you have
Object ref = new String("object of type string");
or
String ref = new String("still object of type string");
the object referenced will be of type String. Doing
Object obj = (Object) new String("still a string");
does not change that the referenced object is a String. In your case, you'll probably need a conversion strategy to convert between types.
You still have another option, convert long to string ,and then convert to integer.
For example i have a string input "int",can i declare a variable base on that input?
(Not switch check please). I mean something like this (pseudo-code) or similar:
String str="int";
new (variable_name,"int");
// create new variable with int datatype.
You can do this:
String className = "MyClass";
Object obj = Class.forName(className).newInstance();
But it won't work for primitive types.
If instead of using primitive types you will use cannonical name of Object based class you can try to do this
public Object loadClass(String className) {
return Class.forName(className).newInstance(); //this throw some exceptions.
}
Not practically, Java is strongly typed and the type of all variables must be known at compile time if you are to do anything useful with them.
For example, you could do something like this;
String str = "java.lang.Integer";
Class clazz = Class.forName(str);
Object o = clazz.newInstance();
..which will give you an Object o whose type is determined at runtime by the value of the String str. You can't do anything useful with it though without first casting it to the actual type, which must be known at compile time.
I would like to do dynamic casting for a Java variable, the casting type is stored in a different variable.
This is the regular casting:
String a = (String) 5;
This is what I want:
String theType = 'String';
String a = (theType) 5;
Is this possible, and if so how? Thanks!
Update
I'm trying to populate a class with a HashMap that I received.
This is the constructor:
public ConnectParams(HashMap<String,Object> obj) {
for (Map.Entry<String, Object> entry : obj.entrySet()) {
try {
Field f = this.getClass().getField(entry.getKey());
f.set(this, entry.getValue()); /* <= CASTING PROBLEM */
} catch (NoSuchFieldException ex) {
log.error("did not find field '" + entry.getKey() + '"');
} catch (IllegalAccessException ex) {
log.error(ex.getMessage());
}
}
}
The problem here is that some of the class' variables are of type Double, and if the number 3 is received it sees it as Integer and I have type problem.
Yes it is possible using Reflection
Object something = "something";
String theType = "java.lang.String";
Class<?> theClass = Class.forName(theType);
Object obj = theClass.cast(something);
but that doesn't make much sense since the resulting object must be saved in a variable of Object type. If you need the variable be of a given class, you can just cast to that class.
If you want to obtain a given class, Number for example:
Object something = new Integer(123);
String theType = "java.lang.Number";
Class<? extends Number> theClass = Class.forName(theType).asSubclass(Number.class);
Number obj = theClass.cast(something);
but there is still no point doing it so, you could just cast to Number.
Casting of an object does NOT change anything; it is just the way the compiler treats it.
The only reason to do something like that is to check if the object is an instance of the given class or of any subclass of it, but that would be better done using instanceof or Class.isInstance().
Update
according your last update the real problem is that you have an Integer in your HashMap that should be assigned to a Double. What you can do in this case, is check the type of the field and use the xxxValue() methods of Number
...
Field f = this.getClass().getField(entry.getKey());
Object value = entry.getValue();
if (Integer.class.isAssignableFrom(f.getType())) {
value = Integer.valueOf(((Number) entry.getValue()).intValue());
} else if (Double.class.isAssignableFrom(f.getType())) {
value = Double.valueOf(((Number) entry.getValue()).doubleValue());
} // other cases as needed (Long, Float, ...)
f.set(this, value);
...
(not sure if I like the idea of having the wrong type in the Map)
You'll need to write sort of ObjectConverter for this. This is doable if you have both the object which you want to convert and you know the target class to which you'd like to convert to. In this particular case you can get the target class by Field#getDeclaringClass().
You can find here an example of such an ObjectConverter. It should give you the base idea. If you want more conversion possibilities, just add more methods to it with the desired argument and return type.
Regarding your update, the only way to solve this in Java is to write code that covers all cases with lots of if and else and instanceof expressions. What you attempt to do looks as if are used to program with dynamic languages. In static languages, what you attempt to do is almost impossible and one would probably choose a totally different approach for what you attempt to do. Static languages are just not as flexible as dynamic ones :)
Good examples of Java best practice are the answer by BalusC (ie ObjectConverter) and the answer by Andreas_D (ie Adapter) below.
That does not make sense, in
String a = (theType) 5;
the type of a is statically bound to be String so it does not make any sense to have a dynamic cast to this static type.
PS: The first line of your example could be written as Class<String> stringClass = String.class; but still, you cannot use stringClass to cast variables.
You can do this using the Class.cast() method, which dynamically casts the supplied parameter to the type of the class instance you have. To get the class instance of a particular field, you use the getType() method on the field in question. I've given an example below, but note that it omits all error handling and shouldn't be used unmodified.
public class Test {
public String var1;
public Integer var2;
}
public class Main {
public static void main(String[] args) throws Exception {
Map<String, Object> map = new HashMap<String, Object>();
map.put("var1", "test");
map.put("var2", 1);
Test t = new Test();
for (Map.Entry<String, Object> entry : map.entrySet()) {
Field f = Test.class.getField(entry.getKey());
f.set(t, f.getType().cast(entry.getValue()));
}
System.out.println(t.var1);
System.out.println(t.var2);
}
}
You can write a simple castMethod like the one below.
private <T> T castObject(Class<T> clazz, Object object) {
return (T) object;
}
In your method you should be using it like
public ConnectParams(HashMap<String,Object> object) {
for (Map.Entry<String, Object> entry : object.entrySet()) {
try {
Field f = this.getClass().getField(entry.getKey());
f.set(this, castObject(entry.getValue().getClass(), entry.getValue()); /* <= CASTING PROBLEM */
} catch (NoSuchFieldException ex) {
log.error("did not find field '" + entry.getKey() + '"');
} catch (IllegalAccessException ex) {
log.error(ex.getMessage());
}
}
}
It works and there's even a common pattern for your approach: the Adapter pattern. But of course, (1) it does not work for casting java primitives to objects and (2) the class has to be adaptable (usually by implementing a custom interface).
With this pattern you could do something like:
Wolf bigBadWolf = new Wolf();
Sheep sheep = (Sheep) bigBadWolf.getAdapter(Sheep.class);
and the getAdapter method in Wolf class:
public Object getAdapter(Class clazz) {
if (clazz.equals(Sheep.class)) {
// return a Sheep implementation
return getWolfDressedAsSheep(this);
}
if (clazz.equals(String.class)) {
// return a String
return this.getName();
}
return null; // not adaptable
}
For you special idea - that is impossible. You can't use a String value for casting.
Your problem is not the lack of "dynamic casting". Casting Integer to Double isn't possible at all. You seem to want to give Java an object of one type, a field of a possibly incompatible type, and have it somehow automatically figure out how to convert between the types.
This kind of thing is anathema to a strongly typed language like Java, and IMO for very good reasons.
What are you actually trying to do? All that use of reflection looks pretty fishy.
Don't do this. Just have a properly parameterized constructor instead. The set and types of the connection parameters are fixed anyway, so there is no point in doing this all dynamically.
For what it is worth, most scripting languages (like Perl) and non-static compile-time languages (like Pick) support automatic run-time dynamic String to (relatively arbitrary) object conversions. This CAN be accomplished in Java as well without losing type-safety and the good stuff statically-typed languages provide WITHOUT the nasty side-effects of some of the other languages that do evil things with dynamic casting. A Perl example that does some questionable math:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
In Java, this is better accomplished (IMHO) by using a method I call "cross-casting".
With cross-casting, reflection is used in a lazy-loaded cache of constructors and methods that are dynamically discovered via the following static method:
Object fromString (String value, Class targetClass)
Unfortunately, no built-in Java methods such as Class.cast() will do this for String to BigDecimal or String to Integer or any other conversion where there is no supporting class hierarchy. For my part, the point is to provide a fully dynamic way to achieve this - for which I don't think the prior reference is the right approach - having to code every conversion. Simply put, the implementation is just to cast-from-string if it is legal/possible.
So the solution is simple reflection looking for public Members of either:
STRING_CLASS_ARRAY = (new Class[] {String.class});
a) Member member = targetClass.getMethod(method.getName(),STRING_CLASS_ARRAY);
b) Member member = targetClass.getConstructor(STRING_CLASS_ARRAY);
You will find that all of the primitives (Integer, Long, etc) and all of the basics (BigInteger, BigDecimal, etc) and even java.regex.Pattern are all covered via this approach. I have used this with significant success on production projects where there are a huge amount of arbitrary String value inputs where some more strict checking was needed. In this approach, if there is no method or when the method is invoked an exception is thrown (because it is an illegal value such as a non-numeric input to a BigDecimal or illegal RegEx for a Pattern), that provides the checking specific to the target class inherent logic.
There are some downsides to this:
1) You need to understand reflection well (this is a little complicated and not for novices).
2) Some of the Java classes and indeed 3rd-party libraries are (surprise) not coded properly. That is, there are methods that take a single string argument as input and return an instance of the target class but it isn't what you think... Consider the Integer class:
static Integer getInteger(String nm)
Determines the integer value of the system property with the specified name.
The above method really has nothing to do with Integers as objects wrapping primitives ints.
Reflection will find this as a possible candidate for creating an Integer from a String incorrectly versus the decode, valueof and constructor Members - which are all suitable for most arbitrary String conversions where you really don't have control over your input data but just want to know if it is possible an Integer.
To remedy the above, looking for methods that throw Exceptions is a good start because invalid input values that create instances of such objects should throw an Exception. Unfortunately, implementations vary as to whether the Exceptions are declared as checked or not. Integer.valueOf(String) throws a checked NumberFormatException for example, but Pattern.compile() exceptions are not found during reflection lookups. Again, not a failing of this dynamic "cross-casting" approach I think so much as a very non-standard implementation for exception declarations in object creation methods.
If anyone would like more details on how the above was implemented, let me know but I think this solution is much more flexible/extensible and with less code without losing the good parts of type-safety. Of course it is always best to "know thy data" but as many of us find, we are sometimes only recipients of unmanaged content and have to do the best we can to use it properly.
Cheers.
So, this is an old post, however I think I can contribute something to it.
You can always do something like this:
package com.dyna.test;
import java.io.File;
import java.lang.reflect.Constructor;
public class DynamicClass{
#SuppressWarnings("unchecked")
public Object castDynamicClass(String className, String value){
Class<?> dynamicClass;
try
{
//We get the actual .class object associated with the specified name
dynamicClass = Class.forName(className);
/* We get the constructor that received only
a String as a parameter, since the value to be used is a String, but we could
easily change this to be "dynamic" as well, getting the Constructor signature from
the same datasource we get the values from */
Constructor<?> cons =
(Constructor<?>) dynamicClass.getConstructor(new Class<?>[]{String.class});
/*We generate our object, without knowing until runtime
what type it will be, and we place it in an Object as
any Java object extends the Object class) */
Object object = (Object) cons.newInstance(new Object[]{value});
return object;
}
catch (Exception e)
{
e.printStackTrace();
}
return null;
}
public static void main(String[] args)
{
DynamicClass dynaClass = new DynamicClass();
/*
We specify the type of class that should be used to represent
the value "3.0", in this case a Double. Both these parameters
you can get from a file, or a network stream for example. */
System.out.println(dynaClass.castDynamicClass("java.lang.Double", "3.0"));
/*
We specify a different value and type, and it will work as
expected, printing 3.0 in the above case and the test path in the one below, as the Double.toString() and
File.toString() would do. */
System.out.println(dynaClass.castDynamicClass("java.io.File", "C:\\testpath"));
}
Of course, this is not really dynamic casting, as in other languages (Python for example), because java is a statically typed lang. However, this can solve some fringe cases where you actually need to load some data in different ways, depending on some identifier. Also, the part where you get a constructor with a String parameter could be probably made more flexible, by having that parameter passed from the same data source. I.e. from a file, you get the constructor signature you want to use, and the list of values to be used, that way you pair up, say, the first parameter is a String, with the first object, casting it as a String, next object is an Integer, etc, but somehwere along the execution of your program, you get now a File object first, then a Double, etc.
In this way, you can account for those cases, and make a somewhat "dynamic" casting on-the-fly.
Hope this helps anyone as this keeps turning up in Google searches.
Try this for Dynamic Casting. It will work!!!
String something = "1234";
String theType = "java.lang.Integer";
Class<?> theClass = Class.forName(theType);
Constructor<?> cons = theClass.getConstructor(String.class);
Object ob = cons.newInstance(something);
System.out.println(ob.equals(1234));
I recently felt like I had to do this too, but then found another way which possibly makes my code look neater, and uses better OOP.
I have many sibling classes that each implement a certain method doSomething(). In order to access that method, I would have to have an instance of that class first, but I created a superclass for all my sibling classes and now I can access the method from the superclass.
Below I show two ways alternative ways to "dynamic casting".
// Method 1.
mFragment = getFragmentManager().findFragmentByTag(MyHelper.getName(mUnitNum));
switch (mUnitNum) {
case 0:
((MyFragment0) mFragment).sortNames(sortOptionNum);
break;
case 1:
((MyFragment1) mFragment).sortNames(sortOptionNum);
break;
case 2:
((MyFragment2) mFragment).sortNames(sortOptionNum);
break;
}
and my currently used method,
// Method 2.
mSuperFragment = (MySuperFragment) getFragmentManager().findFragmentByTag(MyHelper.getName(mUnitNum));
mSuperFragment.sortNames(sortOptionNum);
Just thought I would post something that I found quite useful and could be possible for someone who experiences similar needs.
The following method was a method I wrote for my JavaFX application to avoid having to cast and also avoid writing if object x instance of object b statements every time the controller was returned.
public <U> Optional<U> getController(Class<U> castKlazz){
try {
return Optional.of(fxmlLoader.<U>getController());
}catch (Exception e){
e.printStackTrace();
}
return Optional.empty();
}
The method declaration for obtaining the controller was
public <T> T getController()
By using type U passed into my method via the class object, it could be forwarded to the method get controller to tell it what type of object to return. An optional object is returned in case the wrong class is supplied and an exception occurs in which case an empty optional will be returned which we can check for.
This is what the final call to the method looked like (if present of the optional object returned takes a Consumer
getController(LoadController.class).ifPresent(controller->controller.onNotifyComplete());