Java Generics generics---the third part generics, inheritance, and sub-types

generics, inheritance, and subtypes

As you know, you can assign one type of object to another, as long as they are compatible. For example, you can assign an integer object to a value of object .

Object someobject = new Object (), Integer someinteger = new Integer (ten), someobject = Someinteger;   Ok

In object-oriented technology, this is referred to as the "is a" relationship. That is , an integer is an Object in which the assignment is allowed. Again, the following code snippet is valid:

public void SomeMethod (number N) {/* ... */}somemethod (new Integer);   Oksomemethod (New Double (10.1));   Ok

The same principle applies to generics:

box<number> box = new box<number> (); Box.add (new Integer);   Okbox.add (New Double (10.1));  Ok

Limitations consider the following method:

public void Boxtest (box<number> n) {/* ... */}

Now consider what type of parameter this method accepts? Its signature tells us that it only accepts a single parameter box<number>.

What does that mean? Can we box<integer> or box<double>? The answer is in the negative. Because they are not sub-types of box<number>.

This is a common misconception in generic programming, which is a very important concept to learn.

Although Integer is A subtype of number, box<integer> is not a box<number> subtype Note:Any specific type A and B, such asNumber is not relatedto Integer, myclass<a>, and myclass<b>, regardless of whether a and B are related. The common parent class of myclass<a> and Myclass<b> is Object.
When there is a relationship between parameter types, how to establish a relationship between two sub-generic classes like subtypes, refer to wildcard characters and subtypes.
Generic class and sub-type

You can either extend or implement it by using a subclass or interface. The relationship of a type parameter of a class or interface to the type parameter of another class or interface, as determined by the extended or implemented statement.

Use the collection class collections to do An example: arraylist<e> implements list<e>;list<e> Inheritance (Extended) collection<e>.

Therefore,arraylist<string> is the list<string> subclass,list<string> is collection<string> Sub-class. As long as the type parameters are not changed, their subtype relationships are not becoming.

Collections System Example

Now suppose we want to define our own list interface Payloadlist, which associates a generic type P value and with each element. The declaration may appear to be:

Interface Payloadlist<e,p> extends list<e> {  void setpayload (int index, P val);  ...}

below the payloadlist of the list<string> are sub-typed:

• Payloadlist<string,string>
• Payloadlist<string,integer>
• Payloadlist<string,exception>

Payloadlist System Example
Original Address https://docs.oracle.com/javase/tutorial/java/generics/inheritance.html

Java Generics generics---the third part generics, inheritance, and sub-types