[reflect] create object

[Alice52]

private static final Map<Class<?>, Class<?>> wrapperPrimitiveMap = new HashMap<>();

static {
    wrapperPrimitiveMap.put(Boolean.class, boolean.class);
    wrapperPrimitiveMap.put(Byte.class, byte.class);
    wrapperPrimitiveMap.put(Character.class, char.class);
    wrapperPrimitiveMap.put(Double.class, double.class);
    wrapperPrimitiveMap.put(Float.class, float.class);
    wrapperPrimitiveMap.put(Integer.class, int.class);
    wrapperPrimitiveMap.put(Long.class, long.class);
    wrapperPrimitiveMap.put(Short.class, short.class);
}

1. basic method[5]

1. new
2. Class.newInstance
3. Constructor.newInstance
4. Clone 方法
5. 反序列化

2. create generic array

1. Array.newInstance(Class, length)

   // T[] array = new T[]; // compile error
   public static <T> T[] createArray(Class<T> componentType, int length) {
   return (T[]) Array.newInstance(componentType, length);
   }
   // Integer[] array = CreateWithGeneric.createArray(Integer.class, 2);

3. create generic list

1. 创建一个普通对象 + add to list

2. Arrays.asList(array)

   @SneakyThrows
   public static <T> List<T> createList(Class<T> componentType) {
       List<T> list = new ArrayList<>();
       T t;
       if (wrapperPrimitiveMap.containsKey(componentType)) {
         t = null;
       } else {
         t = componentType.newInstance();
       }
       list.add(t);
   
       return list;
   }
   
   // List<Person> people = CreateWithGeneric.createList(Person.class); // [Person{name='null', age=0}]
   
   @SneakyThrows
   public static <T> List<T> createList(Class<T> componentType, int length) {
       return Arrays.asList((T[]) Array.newInstance(componentType, length));
   }
   // List<Integer> list1 = CreateWithGeneric.createList(Integer.class, 2); // [null, null]

4. create function

5. create complex object

6. create with Generic

1. reference

[Alice52]

generic erasure

1. instanceof T; // Error
2. new T(); // Error
3. new T[10]; // Error

suggestion

1. pass T
2. and pass Class for type

[Alice52]

notice

1. it's bad thinking to clear T type, please pass Class as args
2. the Subtle of Generic is no type, so donot try clear T type