Java官方笔记8泛型
泛型
为什么需要泛型?generics enable types (classes and interfaces) to be parameters when defining classes, interfaces and methods.
说白了就像Python动态语言的变量,是动态的,可以指向任意类型。
泛型有个好处是不需要类型转换:
List list = new ArrayList();
list.add("hello");
String s = (String) list.get(0);
ListString>
list = new ArrayListString>
();
list.add("hello");
String s = list.get(0);
// no cast
这个例子的泛型,是指List的实现使用了泛型T>
,从而给使用带来了好处。
定义
class nameT1, T2, ..., Tn>
{
/* ... */ }
比如定义class ListT>
,使用ListString>
将以下代码:
public class Box {
private Object object;
public void set(Object object) {
this.object = object;
}
public Object get() {
return object;
}
}
改为泛型实现:
/**
* Generic version of the Box class.
* @param T>
the type of the value being boxed
*/
public class BoxT>
{
// T stands for "Type"
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
}
T必须是非基本数据类型:any class type, any interface type, any array type, or even another type variable
type parameter命名采用单个大写字母:
- E - Element (used extensively by the Java Collections Framework)
- K - Key
- N - Number
- T - Type
- V - Value
- S, U, V etc. - 2nd, 3rd, 4th types
这样能很好的跟其他命名区分开来。
使用泛型,必须要指定具体的值,比如这里的Integer:
BoxInteger>
integerBox;
这就跟方法调用传参是一个道理。
Diamond,将:
BoxInteger>
integerBox = new BoxInteger>
();
简写为:
BoxInteger>
integerBox = new Box>
();
多个type parameters:
public interface PairK, V>
{
public K getKey();
public V getValue();
}
public class OrderedPairK, V>
implements PairK, V>
{
private K key;
private V value;
public OrderedPair(K key, V value) {
this.key = key;
this.value = value;
}
public K getKey() {
return key;
}
public V getValue() {
return value;
}
}
PairString, Integer>
p1 = new OrderedPairString, Integer>
("Even", 8);
// 这里的int类型8,自动装箱为了I
PairString, String>
p2 = new OrderedPairString, String>
("hello", "world");
简写为diamond:
OrderedPairString, Integer>
p1 = new OrderedPair>
("Even", 8);
OrderedPairString, String>
p2 = new OrderedPair>
("hello", "world");
嵌套:
OrderedPairString, BoxInteger>
>
p = new OrderedPair>
("primes", new BoxInteger>
(...));
Raw Types
A raw type is the name of a generic class or interface without any type arguments.
比如:
public class BoxT>
{
public void set(T t) {
/* ... */ }
// ...
}
BoxInteger>
intBox = new Box>
();
Box rawBox = new Box();
// 这个就是rawType
在IDEA有时候会碰到警告Raw use of parameterized class 'List',就是指的这个玩意。这是老式写法,raw types会绕过泛型的type checks,应该避免使用。
Generic Methods
定义,泛型位置在return type的前面:
public class Util {
public static K, V>
boolean compare(PairK, V>
p1, PairK, V>
p2) {
return p1.getKey().equals(p2.getKey()) &
&
p1.getValue().equals(p2.getValue());
}
}
public class PairK, V>
{
private K key;
private V value;
public Pair(K key, V value) {
this.key = key;
this.value = value;
}
public void setKey(K key) {
this.key = key;
}
public void setValue(V value) {
this.value = value;
}
public K getKey() {
return key;
}
public V getValue() {
return value;
}
}
使用:
PairInteger, String>
p1 = new Pair>
(1, "apple");
PairInteger, String>
p2 = new Pair>
(2, "pear");
boolean same = Util.Integer, String>
compare(p1, p2);
调用泛型方法时也可以省略泛型入参:
PairInteger, String>
p1 = new Pair>
(1, "apple");
PairInteger, String>
p2 = new Pair>
(2, "pear");
boolean same = Util.compare(p1, p2);
// 这里省略了泛型入参
Bounded Type Parameters
有点像Python的typing,限制动态变量的类型,使用extends关键字:
public class BoxT>
{
private T t;
public void set(T t) {
this.t = t;
}
public T get() {
return t;
}
public U extends Number>
void inspect(U u){
System.out.println("T: " + t.getClass().getName());
System.out.println("U: " + u.getClass().getName());
}
public static void main(String[] args) {
BoxInteger>
integerBox = new BoxInteger>
();
integerBox.set(new Integer(10));
integerBox.inspect("some text");
// error: this is still String!
}
}
这样还能进一步调用bounded type parameters的方法:
public class NaturalNumberT extends Integer>
{
private T n;
public NaturalNumber(T n) {
this.n = n;
}
public boolean isEven() {
return n.intValue() % 2 == 0;
// intValue()是Integer的方法
}
// ...
}
Multiple Bounds
Class A {
/* ... */ }
interface B {
/* ... */ }
interface C {
/* ... */ }
class D T extends A &
B &
C>
{
/* ... */ }
class的位置必须在interface前面。
Bounded Type Parameters的用途之一,比如:
public static T>
int countGreaterThan(T[] anArray, T elem) {
int count = 0;
for (T e : anArray)
if (e >
elem) // compiler error
++count;
return count;
}
会编译报错,因为>
符号只适用基本数据类型,如果想支持Object,怎么办呢:
public static T extends ComparableT>
>
int countGreaterThan(T[] anArray, T elem) {
int count = 0;
for (T e : anArray)
if (e.compareTo(elem) >
0)
++count;
return count;
}
extends ComparableT>
以后调用compareTo()
方法,就能既支持基本数据类型又能支持Object了。
泛型在继承时有个注意的点: BoxInteger>
and BoxDouble>
are not subtypes of BoxNumber>
正确的方式:
You can subtype a generic class or interface by extending or implementing it.
interface PayloadListE,P>
extends ListE>
{
void setPayload(int index, P val);
...
}
Type Inference
类型推断:Type inference is a Java compiler's ability to look at each method invocation and corresponding declaration to determine the type argument (or arguments) that make the invocation applicable.(换个理解方式,就是动态变量需要知道绑定哪个类型)
Diamond就是一种Type Inference:
MapString, ListString>
>
myMap = new HashMap>
();
在构造方法中进行推断:
class MyClassX>
{
T>
MyClass(T t) {
// ...
}
}
MyClassInteger>
myObject = new MyClass>
("");
X推断为Integer,T推断为String。
Lambda Expressions也会根据上下文推断target type:
public static void printPersons(ListPerson>
roster, CheckPerson tester)
public void printPersonsWithPredicate(ListPerson>
roster, PredicatePerson>
tester)
printPersons(
people,
p ->
p.getGender() == Person.Sex.MALE
&
&
p.getAge() >
= 18
&
&
p.getAge() = 25);
// 自动推断为CheckPerson
printPersonsWithPredicate(
people,
p ->
p.getGender() == Person.Sex.MALE
&
&
p.getAge() >
= 18
&
&
p.getAge() = 25);
) // 自动推断PredicatePerson>
- Variable declarations
- Assignments
- Return statements
- Array initializers
- Method or constructor arguments
- Lambda expression bodies
- Conditional expressions,
?:
- Cast expressions
再看个例子:
public interface Runnable {
void run();
}
public interface CallableV>
{
V call();
}
void invoke(Runnable r) {
r.run();
}
T>
T invoke(CallableT>
c) {
return c.call();
}
String s = invoke(() ->
"done");
// Lambda
实际推断使用哪个?答案是Callable,因为它有return,而Runnable没有。
Wildcards
使用?
,extends
表示上限:
List? extends Number>
可以是 ListInteger>
, ListDouble>
, and ListNumber>
这里的extends
既是class的extends,也是interface的implements。
ListObject>
和List?>
有什么区别?
①子类型
public static void printList(ListObject>
list) {
for (Object elem : list)
System.out.println(elem + " ");
System.out.println();
}
it prints only a list of Object
instances;
it cannot print ListInteger>
, ListString>
, ListDouble>
, and so on, because they are not subtypes of ListObject>
.
public static void printList(List?>
list) {
for (Object elem: list)
System.out.print(elem + " ");
System.out.println();
}
Because for any concrete type A
, ListA>
is a subtype of List?>
, you can use printList()
to print a list of any type.
②值
You can insert an Object
, or any subtype of Object
, into a ListObject>
. But you can only insert null
into a List?>
.
使用?
,super
表示下限:
public static void addNumbers(List? super Integer>
list) {
for (int i = 1;
i = 10;
i++) {
list.add(i);
}
}
可以是ListInteger>
, ListNumber>
, and ListObject>
— anything that can hold Integer
values
?
能支持集合子类型:
Type Erasure
Type Erasure是Java编译器为了实现泛型做的:
- Replace all type parameters in generic types with their bounds or Object if the type parameters are unbounded. The produced bytecode, therefore, contains only ordinary classes, interfaces, and methods.
- Insert type casts if necessary to preserve type safety.
- Generate bridge methods to preserve polymorphism in extended generic types.
Restriction on Generics
1、不能使用基本数据类型:
class PairK, V>
{
private K key;
private V value;
public Pair(K key, V value) {
this.key = key;
this.value = value;
}
// ...
}
Pairint, char>
p = new Pair>
(8, 'a');
// compile-time error
只能使用包装类:
PairInteger, Character>
p = new Pair>
(8, 'a');
2、不能创建泛型实例:
public static E>
void append(ListE>
list) {
E elem = new E();
// compile-time error
list.add(elem);
}
只能通过类来创建实例:
public static E>
void append(ListE>
list, ClassE>
cls) throws Exception {
E elem = cls.newInstance();
// OK
list.add(elem);
}
ListString>
ls = new ArrayList>
();
append(ls, String.class);
3、static不能使用泛型
public class MobileDeviceT>
{
private static T os;
// 在实例化后,会同时代表3种类型,显然不合理
// ...
}
MobileDeviceSmartphone>
phone = new MobileDevice>
();
MobileDevicePager>
pager = new MobileDevice>
();
MobileDeviceTabletPC>
pc = new MobileDevice>
();
4、不能instanceof
public static E>
void rtti(ListE>
list) {
if (list instanceof ArrayListInteger>
) {
// compile-time error
// ...
}
}
使用?
可以:
public static void rtti(List?>
list) {
if (list instanceof ArrayList?>
) {
// OK;
instanceof requires a reifiable type
// ...
}
}
5、不能创建泛型数组:
ListInteger>
[] arrayOfLists = new ListInteger>
[2];
// compile-time error
6、Cannot Create, Catch, or Throw Objects of Parameterized Types
// Extends Throwable indirectly
class MathExceptionT>
extends Exception {
/* ... */ }
// compile-time error
// Extends Throwable directly
class QueueFullExceptionT>
extends Throwable {
/* ... */ // compile-time error
public static T extends Exception, J>
void execute(ListJ>
jobs) {
try {
for (J job : jobs)
// ...
}
catch (T e) {
// compile-time error
// ...
}
}
throws可以:
class ParserT extends Exception>
{
public void parse(File file) throws T {
// OK
// ...
}
}
7、重载方法不能有擦除后相同的泛型:
public class Example {
public void print(SetString>
strSet) {
}
public void print(SetInteger>
intSet) {
}
// 编译错误
}
参考资料: Generics https://dev.java/learn/generics/
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