在Java中将对象作为参数传递

Question

There is a scenario

Say I have a class People & a utility class PeopleUtil with a method computeTotalNumberOfPeople

In a separate class say EvaluatePeople where I have code

People people = new People();
people.setValue(10);
people.setX(45);
PeopleUtil.computeTotalNumberOfPeople(people);
this.persistPeople(people);

In the computeTotalNumberOfPeople method

public void computeTotalNumberOfPeople(People people){
  //logic for computing total no of people & then
  int totalPeople = certainIntValue; 
  // I can return the totalPeople value from this method but I am not doing it just for the sake of this scenario
  people.setTotalNumberOfPeople(totalPeople);
}

When I look at the People object in the db row I see the totalNumberOfPeople value persisted. Which is actually fine.

My question is, I am a little confused about it, shouldn't computeTotalNumberOfPeople method return the people object, which has an extra set value, to the method caller code & then that object should be passed to the peristPeople method as an argument?

I hope you understand what I mean, it doesn't seem right someway

Answer 1

Objects are mutable – they can be changed.

So when you call people.setTotalNumberOfPeople(totalPeople) , you are setting the totalNumberOfPeople variable (or whatever it's called inside the People class) to totalPeople for the people object.

When you exit the computeTotalNumberOfPeople method, the object is still the same one that was modified – the changes from the method persist.

One way to think about it is passing a reference to people . When you call computeTotalNumberOfPeople(people) , you are passing a reference to people . When people is modified, you modifying the same location in memory, and so the changes persist.