What are Generic Classes and Methods? What's the difference? Java Tutorials

Generic classes and generic methods allow you to handle multiple data types without needing redundant copies of classes and methods to handle each data type. Notes: https://therevisionist.org/software-engineering/java/terms/generics/
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Generics are a facility of generic programming that were added to the Java programming language in 2004 within the official version J2SE 5.0. They extend Java's type system to allow “a type or method to operate on objects of various types while providing compile-time type safety.” In particular, the Java collections framework supports generics to specify the type of objects stored in a collection instance.

In 1998, Gilad Bracha, Martin Odersky, David Stoutamire and Philip Wadler created Generic Java, an extension to the Java language to support generic types. Generic Java was incorporated in Java with the addition of wildcards.

Hierarchy and classification

According to Java Language Specification:
A type variable is an unqualified identifier. Type variables are introduced by generic class declarations, generic interface declarations, generic method declarations, and by generic constructor declarations.

A class is generic if it declares one or more type variables. These type variables are known as the type parameters of the class. It defines one or more type variables that act as parameters. A generic class declaration defines a set of parameterized types, one for each possible invocation of the type parameter section. All of these parameterized types share the same class at runtime.

An interface is generic if it declares one or more type variables. These type variables are known as the type parameters of the interface. It defines one or more type variables that act as parameters. A generic interface declaration defines a set of types, one for each possible invocation of the type parameter section. All parameterized types share the same interface at runtime.

A method is generic if it declares one or more type variables. These type variables are known as the formal type parameters of the method. The form of the formal type parameter list is identical to a type parameter list of a class or interface.

A constructor can be declared as generic, independently of whether the class that the constructor is declared in is itself generic. A constructor is generic if it declares one or more type variables. These type variables are known as the formal type parameters of the constructor. The form of the formal type parameter list is identical to a type parameter list of a generic class or interface.

Type wildcards

A type argument for a parameterized type is not limited to a concrete class or interface. Java allows the use of type wildcards to serve as type arguments for parameterized types. Wildcards are type arguments in the form "?", possibly with an upper or lower bound. Given that the exact type represented by a wildcard is unknown, restrictions are placed on the type of methods that may be called on an object which uses parameterized types.
A generic type is a generic class or interface that is parameterized over types.

Java Generics ? , E and T what is the difference?

Well there's no difference between the first two - they're just using different names for the type parameter (E or T).
The third isn't a valid declaration - ? is used as a wildcard which is used when providing a type argument