How to apply java generics

1. What are generics

Generics allow you to use an identifier to represent the type of a property in the class or the return value and parameter type of a method when defining a class or interface. This type parameter will be determined when using it (for example, inheriting or implementing this interface, declaring a variable with this type, creating an object) (that is, passing in the actual type parameter, also called a type argument).

Generics are a parameterized type.

2. Rules for using generics

• Generics appeared in JDK5.0 and cannot be used in previous versions;

• Generics need to be written in a pair of <>;

• The type of the generic must be a reference type, not a basic data type;

• If it is a basic data type, its wrapper class must be used. If there are multiple generic types, use commas to separate them;

3. Generic application examples

Example:

public class DAO<T> {
    private Map<String ,T> map = new HashMap<>();
    public void save(String id,T entity){
        map.put(id,entity);
    }
    public T get(String id){
        return map.get(id);
    }
    public void update(String id,T entity){
        map.put(id,entity);
    }
    public List<T> list(){
        Collection<T> values = map.values();
        List list = new ArrayList();
        for (T value : values) {
            list.add(values);
        }
        return list;
    }
    public void delete(String id){
        map.remove(id);
    }
}

public class DAOTest {
    public static void main(String[] args) {

        DAO<User> userDAO = new DAO<>();
        User user1 =new User(1,12,"张三");
        User user2 =new User(2,13,"李四");
        User user3 =new User(3,14,"王五");
        userDAO.save("1",user1);
        userDAO.save("2",user2);
        userDAO.save("3",user3);
        System.out.println(userDAO.get("1"));
        userDAO.update("1",new User(1,18,"张三"));
        List<User> list = userDAO.list();
        System.out.println(list);
        userDAO.delete("1");
        System.out.println(userDAO.list());

    }
}

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