一、引入

1、传统方法：

package generic_;import java.util.ArrayList;
@SuppressWarnings({"all"})
public class Generic01 {public static void main(String[] args) {ArrayList arrayList=new ArrayList();arrayList.add(new Dog("旺财",10));arrayList.add(new Dog("发财",1));arrayList.add(new Dog("招财",5));for(Object o:arrayList){Dog dog=(Dog)o;//向下转型System.out.println(dog.getName()+"-"+dog.getAge());}//旺财-10//发财-1//招财-5//如果不小心加入了一只猫，没有报错arrayList.add(new Cat("加菲猫",8));}
}
class Dog{private String name;private int age;public Dog(String name,int age){this.name=name;this.age=age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}
class Cat{private String name;private int age;public Cat(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

 2、使用泛型：

package generic_;import java.util.ArrayList;
@SuppressWarnings({"all"})
public class Generic01 {public static void main(String[] args) {//解读：//1、ArrayList表示存放到ArrayList集合中的元素是Dog类型//2、如果编译器发现添加的类型，不满足要求，就会报错//3、在遍历时，可以直接取出Dog类型，而不是ObjectArrayList arrayList=new ArrayList();arrayList.add(new Dog("旺财",10));arrayList.add(new Dog("发财",1));arrayList.add(new Dog("招财",5));arrayList.add(new Cat("加菲猫",8));//报错for(Dog dog:arrayList){System.out.println(dog.getName()+"-"+dog.getAge());}//旺财-10//发财-1//招财-5}
}
class Dog{private String name;private int age;public Dog(String name,int age){this.name=name;this.age=age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}
class Cat{private String name;private int age;public Cat(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}
}

3、 使用传统方法的问题分析：
（1）不能对加入到集合ArrayList中的数据类型进行约束（不安全）
（2）遍历的时候，需要进行类型转换，如果集合中的数据量较大，对效率有影响

4、泛型的好处
（1）编译时，检查添加元素的类型，提高了安全性
（2）减少了类型转换的次数，提高效率
 ·不使用泛型
Dog ->加入->object->取出->Dog//放入到Arraylist会先转成Object，在取出时，还需要转换成Dog
·使用泛型
Dog->Dog->Dog//放入时和取出时，不需要类型转换，提高效率
（3）不再提示编译警告

二、基本介绍：（“泛”：a whole group of things, not specific）
1、老韩理解：泛型=>Integer，String，Dog，可以表示数据类型的一种类型
（1）泛型又称参数化类型，是Jdk5.0出现的新特性，解决数据类型的安全性问题
（2）在类声明或实例化时只要指定好需要的具体的类型即可
（3）Java泛型可以保证如果程序在编译时没有发出警告，运行时就不会产生ClassCastException异常。同时，代码更加简洁、健壮
（4）泛型的作用是：可以在类声明时通过一个标识表示类中某个属性的类型，或者是某个方法的返回值的类型，或者是参数类型。

package generic_;public class Generic02 {public static void main(String[] args) {Person person=new Person("韩顺平教育");person.t();//class java.lang.String//此时class Person{}里面的所有的E都用String替换Person person1=new Person(1);person1.t();//class java.lang.Integer//此时class Person{}里面的所有的E都用Integer替换}
}
class Person{E s;//E表示s的数据类型，该数据类型在定义Person对象时指定，即在编译期间，就确定E是什么类型public Person(E s) {//E也可以参数类型this.s = s;}public E f(){//返回类型使用Ereturn s;}public void t(){System.out.println(s.getClass());//显示s的运行类型}
}

2、泛型的语法：

（1）泛型的声明：
interface接口和class类  //比如：List，ArrayList
说明：
1）其中，T，K，V不代表值，而是表示类型
2）任意字母都可以。常用T表示，是Type的缩写
（2）泛型的实例化：
要在类名后面指定类型参数的值（类型）。如：
1）List strList= new ArrayList0;

2） Iterator iterator = customers.iterator0；

（3）

 //我的代码：

package generic_;import java.util.*;@SuppressWarnings({"all"})
public class TestGeneric2 {public static void main(String[] args) {Student student=new Student();Student student1=new Student();Student student2=new Student();HashSet hashSet=new HashSet();hashSet.add(student);hashSet.add(student1);hashSet.add(student2);HashMap hashMap = new HashMap();hashMap.put("jack",student);hashMap.put("tom",student1);hashMap.put("smith",student2);System.out.println("====================");Set keyset=hashMap.keySet();for (Object key :keyset) {System.out.println(key+"-"+hashMap.get(key));}System.out.println("====================");Set entrySet=hashMap.entrySet();for (Object entry:entrySet) {Map.Entry m=(Map.Entry)entry;System.out.println(m.getKey()+"-"+m.getValue());}}
}
class Student{E student;
}
//====================
//tom-generic_.Student@4f3f5b24
//smith-generic_.Student@15aeb7ab
//jack-generic_.Student@27d6c5e0
//====================
//tom-generic_.Student@4f3f5b24
//smith-generic_.Student@15aeb7ab
//jack-generic_.Student@27d6c5e0

//老师的代码：

package generic_;
import java.util.*;@SuppressWarnings({"all"})
public class TestGeneric2 {public static void main(String[] args) {HashSet students=new HashSet();students.add(new Student("jack",18));students.add(new Student("tom",28));students.add(new Student("mary",19));for (Student student :students) {System.out.println(student);}HashMap hm = new HashMap();hm.put("tom",new Student("tom",28));hm.put("smith",new Student("smith",48));hm.put("hsp",new Student("hsp",28));//迭代器/*自动填充的原因(hm.entrySet().var)：public Set> entrySet(){Set> es;return (es=entrySet)==null?(entrySet=new EntrySet()):es;}*/Set> entries = hm.entrySet();/*自动填充的原因(entries.iterator().var)：public final Iterator> iterator(){return new EntryIterator();}*/Iterator> iterator = entries.iterator();System.out.println("===========================");while (iterator.hasNext()) {Map.Entry next=iterator.next();System.out.println(next.getKey()+"-"+next.getValue());}}
}
//Student{name='mary', age=19}
//Student{name='jack', age=18}
//Student{name='tom', age=28}
//===========================
//tom-Student{name='tom', age=28}
//smith-Student{name='smith', age=48}
//hsp-Student{name='hsp', age=28}
class Student{private String name;private int age;public Student(String name, int age) {this.name = name;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic String toString() {return "Student{" +"name='" + name + '\'' +", age=" + age +'}';}
}

3、泛型使用的注意事项和细节：
（1）interface List{}，public class HashSet{}.等等
说明：T，E只能是引用类型，不能是基本数据类型
看看下面语句是否正确？：
List list = new ArrayList();//对
List list2 = new ArrayList();//错
（2）在指定泛型具体类型后，可以传入该类型或者其子类类型
（3）泛型使用形式
List list1 = new Arraylist()；
List list2 = new ArrayList<>()://推荐，因为编译器会自动进行类型推断
（4）如果我们这样写List  list3=new  ArrayList()；默认给它的泛型是[E就是Object]

4、课堂练习题

 //我的代码：

package generic_;import java.util.ArrayList;
import java.util.Comparator;@SuppressWarnings({"all"})
public class GenericExercise01 {public static void main(String[] args) {ArrayList arrayList=new ArrayList();Employee jack = new Employee("jack", 10000, new MyDate(2000, 1, 1));Employee tom = new Employee("tom", 20000, new MyDate(2001, 1, 1));Employee smith = new Employee("smith", 30000, new MyDate(2002, 1, 1));arrayList.add(jack);arrayList.add(tom);arrayList.add(smith);arrayList.sort(new Comparator() {@Overridepublic int compare(Employee o1, Employee o2) {if(o1.getName().equals(o2.getName())){if(o1.getBirthday()==o2.getBirthday()){}else if(o1.getBirthday().getYear()==o2.getBirthday().getYear()){if(o1.getBirthday().getMonth()==o2.getBirthday().getMonth()){if(o1.getBirthday().getDay()==o2.getBirthday().getDay()){return 0;}else{return o1.getBirthday().getDay()-o2.getBirthday().getDay();}}else{return o1.getBirthday().getMonth()-o2.getBirthday().getMonth();}}else{return o1.getBirthday().getYear()-o1.getBirthday().getYear();}}else{return o1.getName().length()-o2.getName().length();}return 0;}});System.out.println(arrayList);//[Employee{name='tom', sal=20000.0, birthday=MyDate{year=2001, month=1, day=1}}, Employee{name='jack', sal=10000.0, birthday=MyDate{year=2000, month=1, day=1}}, Employee{name='smith', sal=30000.0, birthday=MyDate{year=2002, month=1, day=1}}]}
}
class Employee{private String name;private double sal;private MyDate birthday;public Employee(String name, double sal, MyDate birthday) {this.name = name;this.sal = sal;this.birthday = birthday;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getSal() {return sal;}public void setSal(double sal) {this.sal = sal;}public MyDate getBirthday() {return birthday;}public void setBirthday(MyDate birthday) {this.birthday = birthday;}@Overridepublic String toString() {return "Employee{" +"name='" + name + '\'' +", sal=" + sal +", birthday=" + birthday +'}';}
}
class MyDate{private int year;private int month;private int day;public MyDate(int year, int month, int day) {this.year = year;this.month = month;this.day = day;}public int getYear() {return year;}public void setYear(int year) {this.year = year;}public int getMonth() {return month;}public void setMonth(int month) {this.month = month;}public int getDay() {return day;}public void setDay(int day) {this.day = day;}@Overridepublic String toString() {return "MyDate{" +"year=" + year +", month=" + month +", day=" + day +'}';}
}

//老师的代码：

package generic_;import java.util.ArrayList;
import java.util.Comparator;@SuppressWarnings({"all"})
public class GenericExercise01 {public static void main(String[] args) {ArrayList employees=new ArrayList();employees.add(new Employee("jack", 10000, new MyDate(2000, 1, 1)));employees.add(new Employee("tom", 20000, new MyDate(2001, 1, 1)));employees.add(new Employee("smith", 30000, new MyDate(2002, 1, 1)));System.out.println("employees="+employees);employees.sort(new Comparator() {@Overridepublic int compare(Employee o1, Employee o2) {//验证传入的参数if(!(o1 instanceof Employee&&o2 instanceof Employee)){System.out.println("类型不正确");return 0;}//比较nameint i=o1.getName().compareTo(o2.getName());if (i != 0) {return i;}return o1.getBirthday().compareTo(o2.getBirthday());}});System.out.println("=========排序后=========");System.out.println("employees="+employees);}
}
//employees=[
//Employee{name='jack', sal=10000.0, birthday=MyDate{year=2000, month=1, day=1}}, 
//Employee{name='tom', sal=20000.0, birthday=MyDate{year=2001, month=1, day=1}}, 
//Employee{name='smith', sal=30000.0, birthday=MyDate{year=2002, month=1, day=1}}]
//=========排序后=========
//employees=[
//Employee{name='jack', sal=10000.0, birthday=MyDate{year=2000, month=1, day=1}}, 
//Employee{name='smith', sal=30000.0, birthday=MyDate{year=2002, month=1, day=1}}, 
//Employee{name='tom', sal=20000.0, birthday=MyDate{year=2001, month=1, day=1}}]
class Employee{private String name;private double sal;private MyDate birthday;public Employee(String name, double sal, MyDate birthday) {this.name = name;this.sal = sal;this.birthday = birthday;}public String getName() {return name;}public void setName(String name) {this.name = name;}public double getSal() {return sal;}public void setSal(double sal) {this.sal = sal;}public MyDate getBirthday() {return birthday;}public void setBirthday(MyDate birthday) {this.birthday = birthday;}@Overridepublic String toString() {return '\n'+"Employee{" +"name='" + name + '\'' +", sal=" + sal +", birthday=" + birthday +'}';}
}
class MyDate{private int year;private int month;private int day;public MyDate(int year, int month, int day) {this.year = year;this.month = month;this.day = day;}public int getYear() {return year;}public void setYear(int year) {this.year = year;}public int getMonth() {return month;}public void setMonth(int month) {this.month = month;}public int getDay() {return day;}public void setDay(int day) {this.day = day;}@Overridepublic String toString() {return "MyDate{" +"year=" + year +", month=" + month +", day=" + day +'}';}//把对year-month-day的比较封装到MyDate类里面public int compareTo(MyDate o){//比较birthday的yearint yearMinus=year-o.getYear();if (yearMinus != 0) {return yearMinus;}//比较birthday的monthint monthMinus=month-o.getMonth();if (monthMinus != 0) {return monthMinus;}//比较birthday的dayreturn day-o.getDay();}
}

5、自定义泛型（1）基本语法：

class类名{//可以有多个泛型成员}

（2）注意细节：
1）普通成员可以使用泛型（属性、方法）
2）使用泛型的数组，不能初始化
3）静态方法中不能使用类的泛型
4）泛型类的类型，是在创建对象时确定的（因为创建对象时，需要指定确定类型）

5）如果在创建对象时，没有指定类型，默认为Object

package generic_;import java.util.ArrayList;
@SuppressWarnings({"all"})
public class CustomMethodGeneric {public static void main(String[] args) {Car car = new Car();car.fly("宝马",100);//当调用方法时，传入参数，编译器就会确定类型//class java.lang.String//class java.lang.IntegerSystem.out.println("========================");car.fly(300,100.1);//当调用方法时，传入参数，编译器就会确定类型//class java.lang.Integer//class java.lang.DoubleSystem.out.println("========================");Fish fish = new Fish<>();fish.hello(new ArrayList(),11.3f);//class java.util.ArrayList//class java.lang.Float}
}
class Car{//普通类public void run(){}//1、就是泛型//2、是提供给fly使用的，不是返回类型public  void fly(T t,R r){//泛型方法System.out.println(t.getClass());//自动装箱，String / Integer / Double...System.out.println(r.getClass());//自动装箱，String / Integer / Double...}
}
class Fish{//泛型类public void run(){}public  void eat(U u,M m){//泛型方法}//1、hi()方法不是泛型方法//2、是hi()方法使用了类声明的泛型//3、泛型方法 和 方法使用泛型 是不一样的public void hi(T t){}//泛型方法，可以使用类声明的泛型，也可以使用自己声明泛型public  void hello(R r,K k){//(R r,K k)中的R来自类的声明处，K来自方法的定义处System.out.println(r.getClass());System.out.println(k.getClass());}
}

6、泛型的继承和通配符：
（1）泛型不具备继承性
List