对象什么时候初始化？

Question

I want to ask you when is the exact moment when an object is initialized

For example I have this simple Java code:

public class Test {
   public static void main(String args[]) {
      Student student = new Student();
      student.setName("John");
      student.setId(123);
   }
}

So when exactly is the student object initialized? Is it initialized when new Student() is executed? Or when Student student = new Student() is executed? Or after setters are executed? Any feedback will be appreciated!

Answer 1

It's initialized when new + constructor is called.

As states the docs

Each of these statements has three parts (discussed in detail below):

Declaration : The code set in bold are all variable declarations that associate a variable name with an object type.

Instantiation : The new keyword is a Java operator that creates the object.

Initialization : The new operator is followed by a call to a constructor, which initializes the new object.

Answer 2

Is it initialized when new Student() is executed?

Yes. When the resulting object is returned, it has been initialized (by the constructor code).

Or after setters are executed?

The fields of the object are initialized by the time the constructor returns. They may be set to null or "" or 0 or similar, but they're initialized with some value.

If it's not valid for a Student object to have null or whatever for those fields, then the constructor should accept the values for them as parameters, or the class should expose a builder-style interface for building an instance, so that by the time you have a Student instance, you know the fields are filled in with meaningful values. Whether that's necessary is domain-specific.

Answer 3

It depends on your constructor class. Probably line three, but if your mutator methods create new objects then line 4 and 5.

Answer 4

It get initialized when new Student() is executed.

• new Student() - this calls the constructor of the Student Class.

After that, the resulting object is assigned to the ' student ' variable where the data type is Student .

Answer 5

Whenever ' new ' keyword is invoked, the object is instantiated and the constructor is called resulting in object initialization.

First of all, see the difference between a setter and constructor and their pros and cons.

Constructor:

1) A constructor is called when an object is created.

2) They are only called once per object.

3) You may use the constructor to set values at the point of instantiation.

4) It does not allow any return type.

5) A constructor is invoked implicitly.

Setter:

1) A setter is called to change the value of the object after it was initialized.

2) A setter can be called any number of times.

3) You may use the setter to set values after the point of instantiation.

4) It allows return type.

5) A setter is invoked explicitly.

Conclusion:

1) Object is initialized when new Student() is executed

2) Use constructor if you think initialization is mandatory and you have the required values before you can use the object.

3) Use the setter method when the initialization of the variable is non-mandatory and you don't have the values at the time of object initialization.

4) Generally, we should avoid setter as it somehow violates the principle of encapsulation.

Answer 6

Student student = new Student();

When you call new and then the constructor you are creating an object of the same type of your constructor class, the object that is receiving the new + constructor must be the same type.

Another example:

Object that must be the same as your class + name of the object signal = then new plus constructor;

Example exaple = new Example();

Answer 7

When you call

Student student = new Student();

you are creating an object in memory. Once created, you can use the object's methods, like getters and setters.