The Java documentation for Class says:
Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader.
What are these Class objects? Are they the same as objects instantiated from a class by calling new?
Also, for example object.getClass().getName() how can everything be typecasted to superclass Class, even if I don't inherit from java.lang.Class?
Nothing gets typecasted to Class. Every Object in Java belongs to a certain class. That's why the Object class, which is inherited by all other classes, defines the getClass() method.
getClass(), or the class-literal - Foo.class return a Class object, which contains some metadata about the class:
name
package
methods
fields
constructors
annotations
and some useful methods like casting and various checks (isAbstract(), isPrimitive(), etc). the javadoc shows exactly what information you can obtain about a class.
So, for example, if a method of yours is given an object, and you want to process it in case it is annotated with the #Processable annotation, then:
public void process(Object obj) {
if (obj.getClass().isAnnotationPresent(Processable.class)) {
// process somehow;
}
}
In this example, you obtain the metadata about the class of the given object (whatever it is), and check if it has a given annotation. Many of the methods on a Class instance are called "reflective operations", or simply "reflection. Read here about reflection, why and when it is used.
Note also that Class object represents enums and intefaces along with classes in a running Java application, and have the respective metadata.
To summarize - each object in java has (belongs to) a class, and has a respective Class object, which contains metadata about it, that is accessible at runtime.
A Class object is sort of a meta object describing the class of an object. It is used mostly with the reflection capabilities of Java. You can think of it like a "blueprint" of the actual class. E.g. you have a class Car like this:
public class Car {
public String brand;
}
You can then construct a Class object which describes your "Car" class.
Class myCarClass = Class.forName("Car");
Now you can do all sorts of querying on your Car class on that Class object:
myCarClass.getName() - returns "Car"
myCarClass.getDeclaredField("brand") - returns a Field object describing the "brand" field
and so on. Every java object has a method getClass() which returns the Class object describing the Class of the Java object. So you could do something like:
Car myCar = new Car();
Class myCarClass = myCar.getClass();
This also works for objects you don't know, e.g objects you get from the outside:
public void tellMeWhatThisObjectsClassIs(Object obj) {
System.out.println(obj.getClass().getName());
}
You could feed this method any java object and it will print the actual class of the object you have given to it.
When working with Java, most of the time you don't need to worry about Class objects. They have some handy use cases though. E.g. they allow you to programmatically instanciate objects of a certain class, which is used often for object serialization and deserialization (e.g. converting Java Objects back and forth to/from XML or JSON).
Class myCarClass = Class.forName("Car");
Car myCar = myCarClass.newInstance(); // is roughly equivalent to = new Car();
You could also use it to find out all declared fields or methods of your class etc, which is very useful in certain cases. So e.g. if your method gets handed an unknown object and you need to know more about it, like if it imlements some interface etc, the Class class is your friend here.
So long story short, the Class, Field, Method, etc. classes which are in the java.lang.reflect package allow you to analyze your defined classes, methods, fields, create new instances of them, call methods all kinds of other stuff and they allow you to do this dynamically at runtime.
getClass() is a method that returns an object that is an instance of java.lang.Class... there is no casting involved. Casting would look like this:
Class<?> type = (Class<?>) object;
I would also like to add to ColinD 's answer that getClass will return the same object for instances of same type. This will print true:
MyOtherClass foo = new MyOtherClass();
MyOtherClass bar = new MyOtherClass();
System.out.println(foo.getClass()==bar.getClass());
Note that it is not equals, I am using ==.
In order to fully understand the class object, let go back in and understand we get the class object in the first place. You see, every .java file you create, when you compile that .java file, the jvm will creates a .class file, this file contains all the information about the class, namely:
Fully qualified name of the class
Parent of class
Method information
Variable fields
Constructor
Modifier information
Constant pool
The list you see above is what you typically see in a typical class. Now, up to this point, your .java file and .class file exists on your hard-disk, when you actually need to use the class i.e. executing code in main() method, the jvm will use that .class file in your hard drive and load it into one of 5 memory areas in jvm, which is the method area, immediately after loading the .class file into the method area, the jvm will use that information and a Class object that represents that class that exists in the heap memory area.
Here is the top level view,
.java --compile--> .class -->when you execute your script--> .class loads into method area --jvm creates class object from method area--> a class object is born
With a class object, you are obtain information such as class name, and method names, everything about the class.
Also to keep in mind, there shall only be one class object for every class you use in the script.
Hope this makes sense
A Class object is an instance of Class (java.lang.Class). Below quote taken from javadoc of class should answer your question
Class has no public constructor. Instead Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader.
The Object class is the parent class of all the classes in java by default. In other words, it is the topmost class of java.
The Object class is beneficial if you want to refer any object whose type you don't know. Notice that parent class reference variable can refer the child class object, know as upcasting.
Let's take an example, there is getObject() method that returns an object but it can be of any type like Employee,Student etc, we can use Object class reference to refer that object. For example:
Object obj=getObject();//we don't know what object will be returned from this method
Related
Have this requirement where i am getting the classes names at run time from a particular package . I dont know the classes before hand
List<Class<?>> classes = PlayGround.find("MyOwn.newpackage");
Now i need to instantiate a new Object for each class in the above list and add it as members in another class (say Class Model)
for (int i=0;i<classes.size();i++){
Class tmp = classes.get(i);
}
is possible to create a object from the above tmp class and add it as member variable for Class Model in real time in Java?
The not so clean way would be to edit the .java filestream but was wondering can i do it from program itself.
You want to use Constructor#newInstance(Object...). This is preferable to Class#newInstance because the latter will propagate any exception thrown by the no-args constructor, even if it is a checked one. This means that you don't get any of the compile-time type-checks for checked exception. Constructor#newInstance by contrast will throw an InvocationTargetException that wraps the checked exception thrown by the constructor.
So what you want to do is the following (exception-handling omitted for brevity):
Constructor<?> constructor = tmp.getConstructor();
Now you can create a new instance by calling constructor.newInstance(). As far as "adding it as a member variable to another class", why do you need to do that? It would be much easier for the target class to have a list of some sort and then you can just insert the new instance into that list. Or, if you know exactly what type of object the target class needs, create member variables beforehand, and then you can just provide it directly (through a constructor or through a setter).
we can use class.forName() which is used for loading the class dynamically at run time.
eg:
class.forName(name of the class to be loaded)
if you want to create object for the loaded class you can use new instance().
eg:
Class.forName("Employee").newInstance() you will get object of employee.
in this way we can load class dynamically and create object of that loaded class at runtime.
In your case you can traverse through the list and load the class and instantiate object for those loaded class and use it as per your requirement.
I'm reading Java Inheritance Docs and got this paragraph, but I don't what is mean of this. Can any body explain?
Excepting Object, which has no superclass, every class has one and
only one direct superclass (single inheritance). In the absence of any
other explicit superclass, every class is implicitly a subclass of
Object.
Edited
Another point related to this question:
And due to Object is superclass, NetBeans show me extra method list when I try to call any Member of class with object reference? All those methods are declared in Object class?
It means that,in java the Object class is the parent class of all the classes by default. In other words, it is the topmost class or the base class of java. All other classes inherit the properties of Object class.
Meaning of implicitly - suggested though not directly expressed.
If samething applied in Java classes. Consider there is a class called Test.
If you write
public class Test {....}
That is equivalent to
public class Test extends Object {....}
Though you didn't write it, it's equivalent to it. You need not to write extends Object manually. Internally JVM treats that your class is extends Object. Since the part after extends is your class, here Object is your super class.
But,
When you write
public class Test extends BigTest{....}
Things changed now. You are telling BigTest is my parent class. That means you are implicitly writing yourself that BigTest is my parent. Interesting part here is though BigTest is your direct Parent class, internally Object also your Parent.
So now you have 2 parent classes. One is BigTest which you mentioned and other is Object. If you didn't mention anything, only Object is your Parent.
Edit :
This is the reason that you will see extra list of method when you will try to call any member of class with object-reference. Those methods are declared in Object class.
why java do this? any feature of this?
Yes. There are benefits of it. Main reason is to reduce the code duplication.
Continue reading here .... Why Object as super class in Java ?
It means that the following two classes are the same:
class MyClass {
}
class MyClass extends Object {
}
If a class definition doesn't specify extends, the class implicitly extends Object.
The Object class comes with the JRE, you use any class without it. Java does this so you can have a reference to any object e.g.
Object o = x;
This works because no matter what x is, it is also an Object.
Note: even int[] class is a sub-class of Object.
I implemented the code from this tutorial http://www.javablogging.com/dynamic-in-memory-compilation/comment-page-1/#comment-65105 which in a nut shell is dynamically creating a class and only saving it to memory (versus writing to a file with extension .java and compiling that file with extension .class). At the end of his main he instantiates the class with
Object instance = fileManager.getClassLoader(null)
.loadClass(fullName).newInstance();
System.out.println(instance);
And in the file we create we defined the toString method so when we print out instance the class name is printed. But when I add methods to the class and try to call them I get the error method is undefined for type object.
I know that in normal circumstances I would need to use Object like Object obj = new Foo(); and then I could call Foo's members but with the code I showed above I cannot stick new in front of fileManager...newInstance();
Conclusion:
How can I create an instance of the DynaClass (the dynamically created class) from the tutorial and call its members.
UPDATE:
I'm not sure if it's possible to cast the object because we are creating the class dynamically and everything is in strings. As far as I know you cannot cast something with a string name.
cast it?
Foo instance= (Foo) fileManager.getClassLoader(null)
.loadClass(fullName).newInstance();
EDIT
Oh apparently you create the class code as a String so there is nothing to link to at compile time...
So there's nothing you can do aside from calling methods via reflection I guess?
Method method = instance.getClass().getMethod("myMethod", noparams);
method.invoke(instance, null);
It might be better to make an interface with the methods you want to use and your dynamic class implements that interface. Then you can cast your object to that interface. That's usually how plug-ins work.
What does .class mean in, for example, MyClass.class? I understand that when you put the name of a class and the point it is used to access its static fields or methods, but that doesn't relate to '.class'
SomeClass.class gets the Class<SomeClass> type which you can use for programming using the reflection API.
You can get the same type if you have an instance of the class using instance.getClass().
You can check out the documentation here. For example, you can:
get the names of fields and methods, annotations, etc.
invoke methods, get constructor to create new instances of the class
get the name of the class, the superclass, package, etc
When you write .class after a class name, it references the Class object that represents the given class (formally, it is a named class literal). The the type of NombreClase.class is Class<NombreClase>.
E.g., NombreClase.class is an object that represents the class NombreClase on runtime. It is the same object that is returned by the getClass() method of any (direct) instance of NombreClase.
NombreClase obj = new NombreClase();
System.out.println(NombreClase.class.getName());
System.out.println(obj.getClass().getName())
You can add .class to the name of any class to retrieve an instance of its Class object.
When you use Integer.class you reference an instance of Class<Integer>, which is a typed class object.
I always thought this was a field member that got added by the compiler, but it looks like it's really just syntactic sugar.
What is the java class type used for? I am confused about what it means and how it is different than declaring an object type:
Class className;
Thanks
There are several uses for a Class object. For example, say I want to create an instance of a class based on some class name stored in a config file.
String className = config.myClass;
Class clazz = Class.forName(className);
Object myClassInstance = clazz.newInstance();
It represents the runtime type of the object. The actual programmatic use of the Class type is often found in reflection and generics.
For example, loading a JDBC driver abstractly with help of Class#forName():
String jdbcDriverClassName = getItFromSomeExternalConfigurationFile();
Class.forName(jdbcDriverClassName);
Or typecasting an generic Object to a beforeknown type:
public static <T> T findAttribute(String key, Class<T> type) {
return type.cast(attributeMap.get(key)); // It's a Map<String, Object>.
}
...which can be used as
SomeType instance = findAttribute("someKey", SomeType.class);
A more extended example can be found here in flavor of a "generic object converter".
Actually, reading the java.lang.Class javadoc, including all of the available methods, should give you an idea what it can be used for.
Class is a special type of Object, i.e Class is a sub class of Object. Every class you define has its own Class object. You can access this as MyObject.class or myInstance.getClass(). In another word, any class you define has a Class attribute where as any class is an Object. I agree it is slightly confusing to a newbie.
javadoc says:
Instances of the class Class represent classes and interfaces in a running Java application. An enum is a kind of class and an annotation is a kind of interface. Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions. The primitive Java types (boolean, byte, char, short, int, long, float, and double), and the keyword void are also represented as Class objects.
Class has no public constructor. Instead Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader.
You can use it when checking the type of some variable or check for inheritance runtime.
It's also used in reflection, to load dynamically types and execute methods on them.
From the book Thinking in Java:
The Class object
To understand how Run Time Type Information (RTTI) works in Java, you must first know how type information is represented at run time. This is accomplished through a
special kind of object called the Class object, which contains information
about the class. In fact, the Class object is used to create all of the
'regular' objects of your class.