A rare one that is explained more clearly:（

Generic wildcard<? Extends T>is used to receive the returned data. Generic collections with this notation cannot use the add method, while<? Super T>cannot use the get method, as it is prone to errors when assigning values as an interface call.

Explanation: To elaborate on the PECS (Producer Extend Consumer Super) principle:

  1. For those who frequently read content outside, it is suitable to use upper bound extends.
  2. Often inserted, suitable for using the lower bound Super.

A few specific examples (note: java.util.java is the parent class of java.sql.Timestamp)

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# 上界用extends关键字声明，表示参数化的类型可能是所指定的类型，或者是此类型的子类
public void upperBound(List<? extends Date> list, Date date) {    
    Date now = list.get(0);    
    System.out.println("now==>" + now);    
    //list.add(date); //这句话无法编译    
    list.add(null);//这句可以编译，因为null没有类型信息    
}

public void testUpperBound() {    
    List<Timestamp> list = new ArrayList<Timestamp>();    
    Date date = new Date();    
    upperBound(list,date);    
}

# 泛型方法解决方法上面add问题
public <T extends Date> void upperBound2(List<T> list, T date) {    
    list.add(date);    
}  

public void testUpperBound2() {    
    List<Timestamp> list = new ArrayList<Timestamp>();    
    Date date = new Date();    
    Timestamp time = new Timestamp(date.getTime());    
    upperBound2(list,time);    
    //upperBound2(list,date);//这句同样无法编译    
}  

# 下界用super进行声明，表示参数化的类型可能是所指定的类型，或者是此类型的父类型，直至Object
public void lowerBound(List<? super Timestamp> list) {    
    Timestamp now = new Timestamp(System.currentTimeMillis());    
    list.add(now); 
    //下面语句不能编译，类型察除后变为：Timestamp time = (Timestamp)list.get(0);   
    //Timestamp time = list.get(0);  
}  

public void testLowerBound() {    
    List<Date> list = new ArrayList<Date>();    
    list.add(new Date());    
    lowerBound(list);    
}

###Reference
阿里java规范

< http://fyting.iteye.com/blog/122732 >