To learn from Mr. Wang. Generics (ix): generics-Supplements

Generic Supplements
Keynote Teacher: Wang Shaohua QQ Group: 483773664 Learning Goals

1. Mastering the difference between a generic method and a type wildcard

2. Generics and Method overloading

3. Generics and Arrays
   

The difference between a generic method and a type wildcard (a) Most of the time, you can use a generic method to replace the type wildcard character

For example, a requirement: Define a method, receive an arbitrary collection, and print out all the elements in the collection. Generic methods and type wildcards can all be implemented

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public class Need {    public <T> void printCollection(Collection<T> c){        for (T t : c) {            System.out.println(t);        }    }        public void printCollection2(Collection<?> c){        for (Object object : c) {            System.out.println(object);        }    }}

(b) Sometimes, we can use both generic methods and wildcard characters

12	public<T> void copy(List<T> dest,List<? extendsT> src){}

Of course, the above method can also be changed into a generic method

123	public<T ,E extends T> void copy2(List<T> dest,List<E> src){            }

(c) types can define variables

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public<T> void printCollection(Collection<T> c){    for(T t : c) {        System.out.println(t);    }}

(iv) Set<e> only allow elements of type E,set<?> equals set<? extends Object> only elements cannot be placed

Add (E)

Ii. generic method and method overloading

Method Overloading: Method overloading refers to defining multiple methods with the same name in a class, but requiring the number of types or parameters for each method to have different parameters.

(i) First case: the upper and lower limits of the generic type

Because generics allow you to set the upper limit of the wildcard character

So the following two methods are method overloads?

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public<T> void copy(List<T> dest,List<? extends T> src){}     public <T> void copy(List<? superT> dest,List<T> src){}

Reason

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publicclass Test {    public static void main(String[] args) {        List<Number> c = new ArrayList<Number>();        List<Integer> c1 = new ArrayList<Integer>();        Need need = newNeed();        need.copy(c, c1);    }}

The compiler was unable to determine which method to call.

Of course, the above two methods at the time of compilation, it will not pass!

(ii) generic type specified differently

1234	publicvoid method(List<String> list) {}public voidmethod(List<Integer> list) {}

This notation is incorrect and the compilation fails.

This is because, from the Java language level, the method overloads depend on the same method name, the number of arguments, the type, the order, and the list<integer> and list<string> types are list<e> after erasure; This does not conform to the requirements of method overloading.

Note:

In Jdk6 and below, the following code compilation will pass, JDK7, compilation will not pass, JDK8 compilation will also pass

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publicString test(List<String> list) {        return "";} public int test(List<Integer> list) {    return1;}

Iii. Generics and Arrays

(a), Java does not supportarray object of generic type1. You cannot create an array object that holds a generic type

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2. Correct definition method

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3. Take value

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To learn from Mr. Wang. Generics (ix): generics-Supplements