2516326-5475e88a458a09e4.png
一，打破砂锅问到底
泛型存在的意义？
泛型类，泛型接口，泛型方法如何定义？
如何限定类型变量？
泛型中使用的约束和局限性有哪些？
泛型类型的继承规则是什么？
泛型中的通配符类型是什么？
如何获取泛型的参数类型？
虚拟机是如何实现泛型的？
在日常开发中是如何运用泛型的？
Java泛型详解.png
二，晓之以理动之以码
1，泛型的定义以及存在意义
泛型，即“参数化类型”。就是将类型由原来的具体的类型参数化，类似于方法中的变量参数，此时类型也定义成参数形式(可以称之为类型形参)，然后在使用/调用时传入具体的类型(类型实参)。
例如：GenericClass{}
一些常用的泛型类型变量：
E：元素(Element)，多用于java集合框架
K：关键字(Key)
N：数字(Number)
T：类型(Type)
V：值(Value)
如果要实现不同类型的加法，每种类型都需要重载一个add方法
package com.jay.java.泛型.needGeneric;
public class NeedGeneric1 {
private static int add(int a, int b) {
System.out.println(a + "+" + b + "=" + (a + b));
return a + b;
}
private static float add(float a, float b) {
System.out.println(a + "+" + b + "=" + (a + b));
return a + b;
}
private static double add(double a, double b) {
System.out.println(a + "+" + b + "=" + (a + b));
return a + b;
}
private static double add(T a, T b) {
System.out.println(a + "+" + b + "=" + (a.doubleValue() + b.doubleValue()));
return a.doubleValue() + b.doubleValue();
}
public static void main(String[] args) {
NeedGeneric1.add(1, 2);
NeedGeneric1.add(1f, 2f);
NeedGeneric1.add(1d, 2d);
NeedGeneric1.add(Integer.valueOf(1), Integer.valueOf(2));
NeedGeneric1.add(Float.valueOf(1), Float.valueOf(2));
NeedGeneric1.add(Double.valueOf(1), Double.valueOf(2));
}
}
取出集合元素时需要人为的强制类型转化到具体的目标类型，且很容易现“java.lang. ClassCast Exception”异常。
package com.jay.java.泛型.needGeneric;
import java.util.ArrayList;
import java.util.List;
public class NeedGeneric2 {
static class C{
}
public static void main(String[] args) {
List list=new ArrayList();
list.add("A");
list.add("B");
list.add(new C());
list.add(100);
//1.当我们将一个对象放入集合中，集合不会记住此对象的类型，当再次从集合中取出此对象时，改对象的编译类型变成了Object类型，但其运行时类型任然为其本身类型。
//2.因此，//1处取出集合元素时需要人为的强制类型转化到具体的目标类型，且很容易出现“java.lang.ClassCastException”异常。
for (int i = 0; i < list.size(); i++) {
// System.out.println(list.get(i));
String value= (String) list.get(i);
System.out.println(value);
}
}
}
所以使用泛型的意义在于
1,适用于多种数据类型执行相同的代码(代码复用)
2, 泛型中的类型在使用时指定，不需要强制类型转换(类型安全，编译器会检查类型)
2，泛型类的使用
定义一个泛型类：public class GenericClass{}
package com.jay.java.泛型.DefineGeneric;
public class GenericClass {
private T data;
public T getData() {
return data;
}
public void setData(T data) {
this.data = data;
}
public static void main(String[] args) {
GenericClass genericClass=new GenericClass<>();
genericClass.setData("Generic Class");
System.out.println(genericClass.getData());
}
}
3，泛型接口的使用
定义一个泛型接口：public interface GenericIntercace{}
public interface GenericIntercace {
T getData();
}
实现泛型接口方式一：public class ImplGenericInterface1 implements GenericIntercace
public class ImplGenericInterface1 implements GenericIntercace {
private T data;
private void setData(T data) {
this.data = data;
}
@Override
public T getData() {
return data;
}
public static void main(String[] args) {
ImplGenericInterface1 implGenericInterface1 = new ImplGenericInterface1<>();
implGenericInterface1.setData("Generic Interface1");
System.out.println(implGenericInterface1.getData());
}
}
实现泛型接口方式二：public class ImplGenericInterface2 implements GenericIntercace {}
public class ImplGenericInterface2 implements GenericIntercace {
@Override
public String getData() {
return "Generic Interface2";
}
public static void main(String[] args) {
ImplGenericInterface2 implGenericInterface2 = new ImplGenericInterface2();
System.out.println(implGenericInterface2.getData());
}
}
4，泛型方法的使用
定义一个泛型方法： private static TgenericAdd(T a, T b) {}
public class GenericMethod1 {
private static int add(int a, int b) {
System.out.println(a + "+" + b + "=" + (a + b));
return a + b;
}
private static T genericAdd(T a, T b) {
System.out.println(a + "+" + b + "="+a+b);
return a;
}
public static void main(String[] args) {
GenericMethod1.add(1, 2);
GenericMethod1.genericAdd("a", "b");
}
}
public class GenericMethod3 {
static class Animal {
@Override
public String toString() {
return "Animal";
}
}
static class Dog extends Animal {
@Override
public String toString() {
return "Dog";
}
}
static class Fruit {
@Override
public String toString() {
return "Fruit";
}
}
static class GenericClass {
public void show01(T t) {
System.out.println(t.toString());
}
public void show02(T t) {
System.out.println(t.toString());
}
public void show03(K k) {
System.out.println(k.toString());
}
}
public static void main(String[] args) {
Animal animal = new Animal();
Dog dog = new Dog();
Fruit fruit = new Fruit();
GenericClass genericClass = new GenericClass<>();
//泛型类在初始化时限制了参数类型
genericClass.show01(dog);
// genericClass.show01(fruit);
//泛型方法的参数类型在使用时指定
genericClass.show02(dog);
genericClass.show02(fruit);
genericClass.show03(animal);
genericClass.show03(dog);
genericClass.show03(fruit);
// genericClass.show03(animal);
}
}
5，限定泛型类型变量
1,对类的限定：public class TypeLimitForClass{}
2,对方法的限定：public static>T getMin(T a, T b) {}
public class TypeLimitForMethod {
// private static T getMin(T a, T b) {
// return (a.compareTo(b) > 0) ? a : b;
// }
public static > T getMin(T a, T b) {
return (a.compareTo(b) < 0) ? a : b;
}
public static void main(String[] args) {
System.out.println(TypeLimitForMethod.getMin(2, 4));
System.out.println(TypeLimitForMethod.getMin("a", "r"));
}
}
public class TypeLimitForClass {
private T data;
public T getData() {
return data;
}
public void setData(T data) {
this.data = data;
}
public static void main(String[] args) {
ArrayList stringArrayList = new ArrayList<>();
stringArrayList.add("A");
stringArrayList.add("B");
ArrayList integerArrayList = new ArrayList<>();
integerArrayList.add(1);
integerArrayList.add(2);
integerArrayList.add(3);
TypeLimitForClass typeLimitForClass01 = new TypeLimitForClass<>();
typeLimitForClass01.setData(stringArrayList);
TypeLimitForClass typeLimitForClass02 = new TypeLimitForClass<>();
typeLimitForClass02.setData(integerArrayList);
System.out.println(getMinListSize(typeLimitForClass01.getData().size(), typeLimitForClass02.getData().size()));
}
public static > T getMinListSize(T a, T b) {
return (a.compareTo(b) < 0) ? a : b;
}
6，泛型中的约束和局限性
1,不能实例化泛型类
2,静态变量或方法不能引用泛型类型变量，但是静态泛型方法是可以的
3,基本类型无法作为泛型类型
4,无法使用instanceof关键字或==判断泛型类的类型
5,泛型类的原生类型与所传递的泛型无关，无论传递什么类型，原生类是一样的
6,泛型数组可以声明但无法实例化
7,泛型类不能继承Exception或者Throwable
8,不能捕获泛型类型限定的异常但可以将泛型限定的异常抛出
public class GenericRestrict1 {
static class NormalClass {
}
private T data;
public void setData() {
//this.data = new T();
}
// private static T result;
// private static T getResult() {
// return result;
// }
private static K getKey(K k) {
return k;
}
public static void main(String[] args) {
NormalClass normalClassA = new NormalClass();
NormalClass normalClassB = new NormalClass();
// GenericRestrict1 genericRestrictInt = new GenericRestrict1<>();
GenericRestrict1 genericRestrictInteger = new GenericRestrict1<>();
GenericRestrict1 genericRestrictString = new GenericRestrict1<>();
// if(genericRestrictInteger instanceof GenericRestrict1){
// return;
// }
// if (genericRestrictInteger == genericRestrictString) {
// return;
// }
System.out.println(normalClassA == normalClassB);//false
System.out.println(genericRestrictInteger == genericRestrictInteger);//
System.out.println(genericRestrictInteger.getClass() == genericRestrictString.getClass()); //true
System.out.println(genericRestrictInteger.getClass());//com.jay.java.泛型.restrict.GenericRestrict1
System.out.println(genericRestrictString.getClass());//com.jay.java.泛型.restrict.GenericRestrict1
GenericRestrict1[] genericRestrict1s;
// genericRestrict1s = new GenericRestrict1[10];
genericRestrict1s = new GenericRestrict1[10];
genericRestrict1s[0]=genericRestrictString;
}
}
public class GenericRestrict2 {
private class MyException extends Exception {
}
// private class MyGenericException extends Exception {
// }
//
// private class MyGenericThrowable extends Throwable {
// }
public void getException(T t) {
// try {
//
// } catch (T e) {
//
// }
}
public void getException(T t) throws T {
try {
} catch (Exception e) {
throw t;
}
}
}
7，泛型类型继承规则
1,对于泛型参数是继承关系的泛型类之间是没有继承关系的
2,泛型类可以继承其它泛型类，例如: public class ArrayList extends AbstractList
3,泛型类的继承关系在使用中同样会受到泛型类型的影响
public class GenericInherit {
private T data1;
private T data2;
public T getData1() {
return data1;
}
public void setData1(T data1) {
this.data1 = data1;
}
public T getData2() {
return data2;
}
public void setData2(T data2) {
this.data2 = data2;
}
public static void setData2(GenericInherit data2) {
}
public static void main(String[] args) {
// Son 继承自 Father
Father father = new Father();
Son son = new Son();
GenericInherit fatherGenericInherit = new GenericInherit<>();
GenericInherit sonGenericInherit = new GenericInherit<>();
SubGenericInherit fatherSubGenericInherit = new SubGenericInherit<>();
SubGenericInherit sonSubGenericInherit = new SubGenericInherit<>();
father = new Son();
// fatherGenericInherit=new GenericInherit();
fatherGenericInherit=new SubGenericInherit();
setData2(fatherGenericInherit);
// setData2(sonGenericInherit);
setData2(fatherSubGenericInherit);
// setData2(sonSubGenericInherit);
}
private static class SubGenericInherit extends GenericInherit {
}
8，通配符类型
1, extends Parent> 指定了泛型类型的上届
2, super Child> 指定了泛型类型的下届
3, > 指定了没有限制的泛型类型
通配符测试类结构.png
public class GenericByWildcard {
private static void print(GenericClass fruitGenericClass) {
System.out.println(fruitGenericClass.getData().getColor());
}
private static void use() {
GenericClass fruitGenericClass = new GenericClass<>();
print(fruitGenericClass);
GenericClass orangeGenericClass = new GenericClass<>();
//类型不匹配,可以使用 extends Parent> 来解决
// print(orangeGenericClass);
}
private static void printExtends(GenericClass extends Fruit> genericClass) {
System.out.println(genericClass.getData().getColor());
}
public static void useExtend() {
GenericClass fruitGenericClass = new GenericClass<>();
printExtends(fruitGenericClass);
GenericClass orangeGenericClass = new GenericClass<>();
printExtends(orangeGenericClass);
GenericClass foodGenericClass = new GenericClass<>();
//Food是Fruit的父类，超过了泛型上届范围，类型不匹配
// printExtends(foodGenericClass);
//表示GenericClass的类型参数的上届是Fruit
GenericClass extends Fruit> extendFruitGenericClass = new GenericClass<>();
Apple apple = new Apple();
Fruit fruit = new Fruit();
// extendFruitGenericClass.setData(apple);
// extendFruitGenericClass.setData(fruit);
fruit = extendFruitGenericClass.getData();
}
public static void printSuper(GenericClass super Apple> genericClass) {
System.out.println(genericClass.getData());
}
public static void useSuper() {
GenericClass foodGenericClass = new GenericClass<>();
printSuper(foodGenericClass);
GenericClass fruitGenericClass = new GenericClass<>();
printSuper(fruitGenericClass);
GenericClass appleGenericClass = new GenericClass<>();
printSuper(appleGenericClass);
GenericClass hongFuShiAppleGenericClass = new GenericClass<>();
// HongFuShiApple 是Apple的子类，达不到泛型下届，类型不匹配
// printSuper(hongFuShiAppleGenericClass);
GenericClass orangeGenericClass = new GenericClass<>();
// Orange和Apple是兄弟关系，没有继承关系，类型不匹配
// printSuper(orangeGenericClass);
//表示GenericClass的类型参数的下界是Apple
GenericClass super Apple> supperAppleGenericClass = new GenericClass<>();
supperAppleGenericClass.setData(new Apple());
supperAppleGenericClass.setData(new HongFuShiApple());
// supperAppleGenericClass.setData(new Fruit());
//get方法只会返回一个Object类型的值。
Object data = supperAppleGenericClass.getData();
}
public static void printNonLimit(GenericClass> genericClass) {
System.out.println(genericClass.getData());
}
public static void useNonLimit() {
GenericClass foodGenericClass = new GenericClass<>();
printNonLimit(foodGenericClass);
GenericClass fruitGenericClass = new GenericClass<>();
printNonLimit(fruitGenericClass);
GenericClass appleGenericClass = new GenericClass<>();
printNonLimit(appleGenericClass);
GenericClass> genericClass = new GenericClass<>();
//setData 方法不能被调用， 甚至不能用 Object 调用；
// genericClass.setData(foodGenericClass);
// genericClass.setData(new Object());
//返回值只能赋给 Object
Object object = genericClass.getData();
}
}
9，获取泛型的参数类型
Type是什么
这里的Type指java.lang.reflect.Type, 是Java中所有类型的公共高级接口, 代表了Java中的所有类型. Type体系中类型的包括：数组类型(GenericArrayType)、参数化类型(ParameterizedType)、类型变量(TypeVariable)、通配符类型(WildcardType)、原始类型(Class)、基本类型(Class), 以上这些类型都实现Type接口.
参数化类型,就是我们平常所用到的泛型List、Map；
数组类型,并不是我们工作中所使用的数组String[] 、byte[]，而是带有泛型的数组，即T[] ；
通配符类型, 指的是>, extends T>等等
原始类型, 不仅仅包含我们平常所指的类，还包括枚举、数组、注解等；
基本类型, 也就是我们所说的java的基本类型，即int,float,double等
public interface ParameterizedType extends Type {
// 返回确切的泛型参数, 如Map返回[String, Integer]
Type[] getActualTypeArguments();
//返回当前class或interface声明的类型, 如List>返回List
Type getRawType();
//返回所属类型. 如,当前类型为O.I, 则返回O. 顶级类型将返回null
Type getOwnerType();
}
public class GenericType {
private T data;
public T getData() {
return data;
}
public void setData(T data) {
this.data = data;
}
public static void main(String[] args) {
GenericType genericType = new GenericType() {};
Type superclass = genericType.getClass().getGenericSuperclass();
//getActualTypeArguments 返回确切的泛型参数, 如Map返回[String, Integer]
Type type = ((ParameterizedType) superclass).getActualTypeArguments()[0];
System.out.println(type);//class java.lang.String
}
}
10，虚拟机是如何实现泛型的
Java泛型是Java1.5之后才引入的，为了向下兼容。Java采用了C++完全不同的实现思想。Java中的泛型更多的看起来像是编译期用的
Java中泛型在运行期是不可见的，会被擦除为它的上级类型。如果是没有限定的泛型参数类型，就会被替换为Object.
GenericClass stringGenericClass=new GenericClass<>();
GenericClass integerGenericClass=new GenericClass<>();
C++中GenericClass和GenericClass是两个不同的类型
Java进行了类型擦除之后统一改为GenericClass
public class GenericTheory {
public static void main(String[] args) {
Map map = new HashMap<>();
map.put("Key", "Value");
System.out.println(map.get("Key"));
GenericClass genericClass = new GenericClass<>();
genericClass.put("Key", "Value");
System.out.println(genericClass.get("Key"));
}
public static class GenericClass {
private K key;
private V value;
public void put(K key, V value) {
this.key = key;
this.value = value;
}
public V get(V key) {
return value;
}
}
private class GenericClass2 {
}
private class GenericClass3 {
}
}
对应的字节码文件
public static void main(String[] args) {
Map map = new HashMap();
map.put("Key", "Value");
System.out.println((String)map.get("Key"));
GenericTheory.GenericClass genericClass = new GenericTheory.GenericClass();
genericClass.put("Key", "Value");
System.out.println((String)genericClass.get("Key"));
}
三，学以致用
1，泛型解析JSON数据封装
api返回的json数据
{
"code":200,
"msg":"成功",
"data":{
"name":"Jay",
"email":"10086"
}
}
BaseResponse .java
public class BaseResponse {
private int code;
private String msg;
public int getCode() {
return code;
}
public void setCode(int code) {
this.code = code;
}
public String getMsg() {
return msg;
}
public void setMsg(String msg) {
this.msg = msg;
}
}
UserResponse.java
public class UserResponse extends BaseResponse {
private T data;
public T getData() {
return data;
}
public void setData(T data) {
this.data = data;
}
}
2，泛型+反射实现巧复用工具类
public class GenericUtils {
public static class Movie {
private String name;
private Date time;
public String getName() {
return name;
}
public Date getTime() {
return time;
}
public Movie(String name, Date time) {
this.name = name;
this.time = time;
}
@Override
public String toString() {
return "Movie{" + "name='" + name + '\'' + ", time=" + time + '}';
}
}
public static void main(String[] args) {
List movieList = new ArrayList<>();
for (int i = 0; i < 5; i++) {
movieList.add(new Movie("movie" + i, new Date()));
}
System.out.println("排序前:" + movieList.toString());
GenericUtils.sortAnyList(movieList, "name", true);
System.out.println("按name正序排：" + movieList.toString());
GenericUtils.sortAnyList(movieList, "name", false);
System.out.println("按name逆序排：" + movieList.toString());
}
public static void sortAnyList(List targetList, final String sortField, final boolean sortMode) {
if (targetList == null || targetList.size() < 2 || sortField == null || sortField.length() == 0) {
return;
}
Collections.sort(targetList, new Comparator() {
@Override
public int compare(Object obj1, Object obj2) {
int retVal = 0;
try {
// 获取getXxx()方法名称
String methodStr = "get" + sortField.substring(0, 1).toUpperCase() + sortField.substring(1);
Method method1 = ((T) obj1).getClass().getMethod(methodStr, null);
Method method2 = ((T) obj2).getClass().getMethod(methodStr, null);
if (sortMode) {
retVal = method1.invoke(((T) obj1), null).toString().compareTo(method2.invoke(((T) obj2), null).toString());
} else {
retVal = method2.invoke(((T) obj2), null).toString().compareTo(method1.invoke(((T) obj1), null).toString());
}
} catch (Exception e) {
System.out.println("List排序异常！");
e.printStackTrace();
}
return retVal;
}
});
}
}
3，Gson库中的泛型的使用-TypeToken
public class GsonGeneric {
public static class Person {
private String name;
private int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
}
public static void main(String[] args) {
Gson gson = new Gson();
List personList = new ArrayList<>();
for (int i = 0; i < 5; i++) {
personList.add(new Person("name" + i, 18 + i));
}
// Serialization
String json = gson.toJson(personList);
System.out.println(json);
// Deserialization
Type personType = new TypeToken>() {}.getType();
List personList2 = gson.fromJson(json, personType);
System.out.println(personList2);
}
}