Useage of Generic in Java

ops/2025/3/18 19:10:05/

the usage of generic in class

java">// 
public class A <T>{     //without specify a parent class or implemented interface as the super bound for generic typeT n;public void printstr(){System.out.println(n);}A(T a){this.n=a;System.out.println("A构建成功 "+a.getClass().getSimpleName());}public static void main(String[] args) {A<String> a=new A("hellow world");}
}
//specify Class A or implemented interface as the super bound for gerceric type T
public class B <T extends A>{   //”T extends B & C“ you can use & operator to specify multiple interface when bounding a generic type parameter. If there is a parent,ther parent class must put the first.T a;B(T a){this.a = a;}public void A_method(){a.printstr();}public static void main(String[] args) {B b=new B(new A("testb"));b.A_method();}
}

the usage of generic in method

java">public class Main {public static <T> void Obj_Print(T a){   //in gerneric method,the type declaration must be placed before the return typeSystem.out.println(a.getClass());}public static void main(String[] args) {   Obj_Print("Hello World");}
}

now ,suppose i need create a function,which’s method is printing the elements’type of list,weather the list contains any type;

maybe you think of the fowlling methods,however it's fault

java">    public static void main(String[] args) {List<String> list = new ArrayList<String>();list.add("A");listPrint(list);}public static <T> void listPrint(List<Objects> list){System.out.println(list.getClass());}

the reason of the erro?

Although Object is super class of String,but List<Objects> isn’t the super class of List<String>

the proper way: use wild card

java">    public static void main(String[] args) {List<String> list = new ArrayList<String>();list.add("A");listPrint(list);}public static <T> void listPrint(List<?> list){System.out.println(list.getClass());}