The class Person {
String STR="aa".
The Person () {
show();
}
Public void show () {
System.out.println(str);
}
}
The class Student extends the Person {
Private String STR="bb".
@ Override
Public void show () {
System.out.println(str);
}
}
The class Test3 {
Public static void main (String [] args) {
Person p=new Student ();
}
}

Why the print null, the structure of the parent class call the show method is what is a subclass of the show

CodePudding user response:

You have new a subclass object, use the parent class to pick up.

CodePudding user response:

Because the creation is a subclass Student object, this point to Student object, so call subclasses Student show
Print null because of class loading sequence, the superclass constructor calls the subclass methods, subclass Student member variable has not been initialized, so is null

CodePudding user response:

Because it is a new Student (), so called the subclass Student. The show (),
Order is instantiated object, instantiate the superclass member variable, the parent class structure, subclasses member variable, subclass structure,
Therefore:
Procedure to initialize the parent class: first instantiation members of STR="aa"; Then perform its structure, for instance is a subclass of call subclasses show method, this time a subclass STR=null, so the output empty
Program and initial anti-fuzzy class: instance, anti-fuzzy class member STR="bb"; Then execute subclass structure, because there is no structure so subclasses will automatically perform a no-parameter constructor, empty

You can add to the same and give the student a construct, anti-fuzzy initial class, because the bb initialization, multi-output bb information

CodePudding user response:

reference 5 floor PNZ. BeijingL response:

because it is a new Student (), so called the subclass Student. The show (),
Order is instantiated object, instantiate the superclass member variable, the parent class structure, subclasses member variable, subclass structure,
Therefore:
Procedure to initialize the parent class: first instantiation members of STR="aa"; Then perform its structure, for instance is a subclass of call subclasses show method, this time a subclass STR=null, so the output empty
Program and initial anti-fuzzy class: instance, anti-fuzzy class member STR="bb"; Then execute subclass structure, because there is no structure so subclasses will automatically perform a no-parameter constructor, empty

You can add to the same and give the student a construct, anti-fuzzy initial class, because the bb initialization, more output bb information

parent class initialization, it aren't show method into the stack, so the method of subclasses