为什么标记为final的对象可以被修改并在Java中调用非final方法？

Question

I am new to Java and I am from C++ background.

I thought final in Java works just like const in C++ but I guess not.

Object initiated as const in C++, only can call const methods and cannot change fields in the object.

But in my code below, I am able to assign value in pet . ie pet.id = new ObjectId(newPetId); .

private void addPet() {
    progressBar.setVisibility(View.VISIBLE);

    final Pet pet;

    try {
        // Locally add and save pet.
        pet = getPetFromUserInput();

    } catch (InvalidInputException e) {
        progressBar.setVisibility(View.GONE);
        return;
    }

    pet.id = new ObjectId(); // Modify field member directly.
    pet.updateName("MyPet"); // Call non-final method.
}

Answer 1

Referencing Erik's answer in comments, I found an easy explanation for C++ programmers.

Pet pet; in Java is like Pet* pet; in C++.

final Pet pet; in Java is like Pet * const pet; in C++ which makes the pointer const but not the value itself.

Note that there is a subtle difference in Java and C++.

In C++, you have to assign a value when declaring a const variable but in Java, it lets you do it later but only once.

Answer 2

In Java, the keyword "final" simply means that once initialized, you cannot change the value of the variable. for example,

final int x = 0;`
//You can't do this!!!
int x=5

It has nothing to do with that variable calling methods.

Answer 3

In java "final" means this

1.If you use "final" before a class, then it means there is no chance to create subclass to that class.

public final class Person {
void getName() {

}

}

Then you can't create like this.

public class Man extends Person{
}
"The type Man cannot subclass the final class Person" will be shown

2. If you write "final" before a method then

    public class Person { final void getName() { } } 

Then you can create subclass to this Person class but you cant override the getName() in subclass.

public class Man extends Person{

@Override
void getName() {
    // TODO Auto-generated method stub
    super.getName();
}

}

"Cannot override the final method from Person" will be shown.

3. If you write "final" before a variable in a class, then you can't alter the value of that variable

exammple:

public  class Person {
public final String name;
void getName() {

}   
}

then in subclass, you can't modify the value.

public class Man extends Person{

public void getName() {
    name = "child";
}

}
"The final field Person.name cannot be assigned" will be shown

all these will shown in compile time itself.

Hope this helps you.