Java Basic Knowledge Generics

Generic type

The parameter type of a generic type must be a reference type
such as arraylist<int> list = new arraylist<int> (); Error
arraylist<integer> list = new arraylist<integer> (); That's right
List.add (5); Proper automatic packing (packing) mechanism
int i = list.get (0); correctly, if the element is a wrapper type, such as Integer,double,character, you can assign the element directly to a variable of the basic type. This process is called the automatic unboxing mechanism.

• Defining generic Classes

Importjava.util.ArrayList; Public classGenericstack<e> {         Privatearraylist<e> list =NewArraylist<e>();  Public BooleanIsEmpty () {returnList.isEmpty (); }          Public intGetSize () {returnlist.size (); }          PublicE Peek () {returnList.get (GetSize ()-1); }          Publice pop () {e o= List.get (GetSize ()-1) ; List.remove (GetSize ()-1); returno; }
public void push (E o) {
List.add (o);
}
Public String toString () {
Return "stack:" + list.tostring ();
}
}

Genericstack the constructor form after erasing the parameter type is Genericstack (), not genericstack<> ();
Generic class if you have more than one parameter type, place multiple arguments in one angle bracket, such as <E1,E2,E3>.
You can define a class or interface as a subtype of a generic class or generic interface, such as the Java API, where the Java.lang.String class implements the comparable interface: public class String implements comparable <String>;

Generics can be specified as a subtype of another type (or generic), such that a generic type is called a restricted
public class boundedtypedemo{
public static <e extends Geometricobject> boolean Equalarea (e Object1, E object2)
{
return Object1.getarea () = = Object2.getarea ();
}
}
Note: The location of the generic type placement should precede the return type of the method (defining a generic method) or after the class name (when defining a generic class)

• Original type

You can use the primitive types of generics (Generic) without specifying a specific type. Such as:
Genericstack stack = new Genericstack (); Equivalent to
genericstatck<object> stack = new genericstack<object> ();
This genericstack generic class with no type parameters is called the original type. The original type can be backwards compatible with earlier versions of Java. such as the comparable type

• Pass-to-match generics

Three different forms:
？ , non-restricted wildcard, equivalent to? Extends Object
？ Extends T, restricted pass, denotes subclasses of T or T
? Super T, lower bound, indicates the parent type of T or T

Java Basic Knowledge Generics