How to define data types in Java

1. Explanation

There are two reasons why new T() cannot be used in generic code. One is that the type cannot be determined due to erasure; but it cannot be determined whether T contains No-argument constructor.

We use the factory pattern generic method to create instance objects, create IntegerFactory factories, and create Integer instances. If the code changes later, a new factory type can be added.

2. Example

/**
 * 使用工厂方法来创建实例
 *
 * @param <T>
 */
interface Factory<T>{
    T create();
}
 
class Creater<T>{
    T instance;
    public <F extends Factory<T>> T newInstance(F f) {
     instance=f.create();
     return instance;
    }
}
 
class IntegerFactory implements Factory<Integer>{
    @Override
    public Integer create() {
     Integer integer=new Integer(9);
     return integer;
    }
}

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