How to interpret "public <T> T readObjectData(... Class<T> type)" in Java?

Question

I have this Java code.

public <T> T readObjectData(ByteBuffer buffer, Class<T> type) { ... T retVal = (T) summaries; return retVal; 

How to interpret this code? Why do we need public <T> T instead of public T?

How to give the parameter to the 2nd argument (Class<T> type)?

Answer 1

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.

Answer 2

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