java generic type parameters (Type Parameters) Type Parameters Bound (Bound)

After declaring the type parameter <T>, the compiler does not know Twhat type it is. If we need to call some class methods, the compiler will report an error.

class Test<T>{
    
    
    void fn(T t){
    
    
        System.out.println(t.charAt(0 )); //假设我们指定T是String或String的子类,我们想调用其charAt方法,这时编译器会报错
    }
}

At this time, we need to give Ta subclass of the previous <T extends String>, which means Tyes Stringor String, then the compilation can pass

package com;

class Test<T extends String>{
    
    
    void fn(T t){
    
    
        System.out.println(t.charAt(0 ));
    }
}

public class App2 {
    
    
    public static void main(String[] args) {
    
    
        Test<String> test = new Test();
        test.fn("123");

    }
}

Boundary declarations have two purposes:
1. A method to obtain a boundary type
2. Only the boundary type or its subclasses can be instantiated, which is equivalent to a restriction

 Test<Integer> test = new Test(); //error

There is no lower bound,<T super String>

Reference:
http://www.angelikalanger.com/GenericsFAQ/FAQSections/TypeParameters.html#FAQ107