在Java中，有没有一种首选的对象转换方法？

Question

I have a superclass person with a subclass student. I have made a student object of type person, and wanted to call a student specific method which does not exist in the person type. I have two methods below that can do this, and I was curious to know is one method superior to the other, or do they both achieve the same outcome?

    Person newStudent = new Student("Scott", 22, "B22334952");

    //method1: make a reference of newStudent and cast to Student
    Student studentRef = (Student) newStudent;
    System.out.println(studentRef.getUserId());

    //method2: cast newStudent, but don't create a reference
    System.out.println(((Student) student2).getUserId());

Answer 1

They'll both have the same outcome.

Create a variable to store the result of the cast if you need to reuse the cast reference.

When you perform a cast, you are setting yourself up for ClassCastException , so always be mindful. You can always check the type before you attempt a cast

if (newStudent instanceof Student) {
    Student studentRef = (Student) newStudent;
}

Answer 2

Both are similar.

Use the second one if you don't want to use that information anymore.

Answer 3

Use whichever you prefer, this is a meaningless micro-optimization and will result in equivalent byte code. You could also use Class.cast(Object) like

System.out.println(Student.class.cast(newStudent).getUserId());

Answer 4

Best of all, don't cast . Casting means your code is making assumptions that cannot be verified at compile time, and that's a recipe for trouble. Declare variable types that capture the full requirements and expectations of their values, and rely only on those declared types.

So,

// Declare a Student because a plain Person won't do
Student newStudent = new Student("Scott", 22, "B22334952");

System.out.println(newStudent.getUserId());