How to interpret "public <T> T readObjectData(... Class<T> type)" in Java?
This declares the readObjectData
method generic, with one type parameter, T
.
public <T> ...
Then the return type is T
.
... T readObjectData(...
Without the initial <T>
, which is the generic type declaration, the symbol T
will be undefined.
In the parameter list, Class<T> type
is one of the parameters. Because the return type and this parameter both reference T
, this ensures that if you pass in a Class<String>
, then it will return a String
. If you pass in a Class<Double>
, then it will return a Double
.
To pass in the parameter, pass in any Class
object, e.g. String.class
.
The <T>
part is declaring a generic type argument T
. If you were to omit this part, the compiler would likely complain that the type T
doesn't exist.
In this case, T
serves as a placeholder for an actual type, which will only be determined when the method is actually called with non-generic type arguments.
public <T> T readObjectData(...
^ ^
| + Return type
+ Generic type argument
<T>
is a parameter class. There is no class named T
. You can use this method with any class specified via second method argument named type
.
since method is defined as following:
public <T> T readObjectData(ByteBuffer buffer, Class<T> type)
You can call it as written below:
MyClass obj = o.readObjectData(buffer, MyClass.class);
Please pay attention that you do not have to cast return value of readOjectData()
to MyClass
. Once upon a time, before java 5 this method would be defined as:
public Object readObjectData(ByteBuffer)
and its usage looked like:
MyClass obj = (MyClass)o.readObjectData(buffer);
Since casting may cause ClassCastException
this is a bad practice. This was a reason for invention of generics.