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Useage of Generic in Java

news 来源：原创 2025/5/13 22:01:03

the usage of generic in class

// 
public class A <T>{     //without specify a parent class or implemented interface as the super bound for generic type
    T 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

public class Main {
    public static <T> void Obj_Print(T a){   //in gerneric method,the type declaration must be placed before the return type
        System.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

    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

    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());
    }