for example:
Student[] stu = new Student[10];
Student stu = new Student();
what's the differences between them?
and what happened when initializing an array of objects? why doesn't it call the constructor?
7 Answers
Initializing an array creates empty spaces for each element. It does not create any individual element. This
Student[] stus = new Student[10];
therefore creates ten empty spaces, into each of which can be placed a Student object. No other kind--unless it sub-classes Student. Each of these ten spaces are null until you explicitely place a new object into them. Such as with
stus[1] = new Student(); //1 is the *second* element
Initializing an object creates a new object of that type. This
Student stu = new Student();
creates a new Student object. Alternatively, this
Student stu;
declares the object, but does not create it. It's just like when you initialize the array. This allots a space for the (one) Student object, but does not create it.
To repeat, this creates (initializes) it--meaning places it in that declared space:
stu = new Student();
As a final note, as mentioned by @FlorentBayle, you should not name these objects the same. Consider naming the array as I do above: stus, which is more indicative of its actual value.
More information:
Comments
Given Student.java:
public class Student
{
private String name;
public Student () { name = ""; }
public String getName () { return name; }
public void setName (String name) { this.name = name; }
}
Then we have:
Student[] A = null;
Ais a reference of typeStudent[]which means Array of Student references- A reference is a memory address
Ahasnullvalue; meaning it refers to nothingStudent[] B = new Student[10];Bis a reference of typeStudent[]new Student[10];will reserve a memory of 10 memory references of typeStudentBtakes the reference returned bynewwhich reference the 10 blocks ofStudentreferences- All of the reference blocks reserved by
neware initialized tonull Attempting
B[0].getName();will compile, but will throw aNullPointerExceptionon run-timeB[0] = new Student();B[0]is the first element in the arrayBinitially refers tonullnew Student();will create an Object of typeStudentby invoking its constructorStudent()known as the default constructorAfter this, you can call Student functions like
B[0].getName();Student C = null;Cis a reference of typeStudentChas the value ofnullwhich means it refers to nothingAttempting
C.getName();will compile, but will throw aNullPointerExceptionon run-timeC = new Student();Cwill take the reference returned bynew Student();new Student();will create an object of typeStudentby calling its constructorStudent()and return its memory reference
Comments
When you initialize an array, all the elements are null, initially. When you make a Student object, it actually calls the Student constructor.
Comments
In the array instantiation/initialization you are allocating enough memory to hold 10 Student objects. In your instantiation/initialization of stu, Your stu variable refers to one Student object in memory
Comments
I agree with PakkuDon, don't know what you mean by "Structure Function."
Initializing an array of objects will make a new array of Student objects, which is basically a list of size Student[x] where x is the number of objects in the array.
When you actually add a Student object to the array, it will call the Student constructor, and create the Student object.
Simply creating a Student object will actually create the object, not just a list that can hold them.
Comments
in Student[] stu = new Student[10]; you initialize an array of 10 elements that take up 10xsize of the Student object. But with stu = new Student(); it takes up only 1xsize of the Student object.
Comments
Student[] stu = new Student[10]; creates an an array that can hold 10 Student objects. Each element is initialised to null.
Student stu = new Student(); creates a single Student.
As for your question on why the constructor isn't called when constructing an array of objects, I'd ask "Why would it be?" The compiler only knows that the array can store objects that are an instance of Student. How is the compiler supposed to know which constructor it is supposed to call to construct these objects? What should the compiler do in the event that your class, Student, lacks a no-args constructor?
There's also the fact that the programmer may wish to populate the array with data retrieved from an external source during runtime. It would make more sense to default to null rather than to populate the array with a bunch of arbitrary instances that are going to be discarded later since that'll add an unnecessary overhead to the program.
Structure function? Are you referring to the constructor?stu = new Student();will not work ifstuis the array ofStudentyou just declared the line before. If it's not, you should really choose another name.