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Generic method

The general definition is as follows, that is, the method is preceded by a

Public class FTest {public List f (T t) {...};}

There are three ways to infer generic parameters:

1. Add certain generics directly before f ()

FTest.f (xxx)

2. Determined by input parameters, the following inference is Integer

Int number = 0bot fTest.f (number)

3. It can be determined by the return value.

List list = fTest.f (xxx)

Q: what's wrong with the following code? Is that where toString () is?

Public class A {public static void test (T t) {System.out.println (t.toString ());}}

A:test is a static method, so you can't perceive T in An instance.

Need to be changed to

Public static void test (T t)

There is no problem with toString (). ToString is the method of Object.

Generic parameters and type elimination

Q: what does the generic parameter T become at run time?

A: unified to Object and does not contain any type of information.

Q: can the generic parameter T be compared using instanceof?

Class A {void f (Object arg) if (arg instanceof T) {...}}

A: no, the compiler will report an error.

Q: can the generic parameter T be operated by new T () or new T []?

A: no, the compiler will report an error.

Q: can I call a method in a generic parameter object?

T.f ()

A: only methods of Object can be called.

Q: can I use T for forced conversion?

T t = (T) object

A: it works, but it doesn't really transform, and waring warnings are triggered at compile time.

Problems when creating a new generic object

Suppose there are two classes, the base class Parent and the subclass Child

Class Parent {} class Child extends Parent {}

Answer the following questions:

Q: is there a problem with the following sentence?

List list = new ArrayList ()

A: if there is a problem, the compilation will be wrong. There is no parent-child relationship between List and ArrayList.

Q:

List

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