在java中更新对象的问题

Question

If I have an instance of an object, and within that object is a variable that holds the data of another object. If I ever update the second object will the copy of that object be updated as well or do I need to simultaneously update all copies of said object.

For example:

public class Object()
{
    int x = xValue;
    Object linked = saidObject;
}

public class doStuff()
{
   saidObject.x++;
   if(linked.equals(saidObject))
       return true;
}

will this code (not compilable obviously just fill in blanks) return true?

Answer 1

By doing:

Object linked = saidObject;

you are not copying the object, just creating another pointer to it, it means you have two different pointers that point to the same object.

copying or cloning an object can be useful in some cases but its not the usual case.

Answer 2

if(linked.equals(saidObject)) will return true as the two variables do point to the same object.

In Java all variables and fields are references to an actual Object that lives somewhere in memory.

When you assign one variable to another, it's like copying the address of the object so that they both point to the same object in memory.

eg

Object a = new Object();  // this actually creates the Object in memory
Object b = a;             // this copies the reference to Object from a to b
// At this point, a and b point to exactly the same object in memory. Therefore ...
a.equals(b);              // returns true.

In fact a == b returns true too, which is a better way of comparing for this case as == compares if two variables point to the same object (they do), whereas equals() often compares by value, which is unnecessary here.

It doesn't matter if b is actually a field within a (eg class Obj { Obj b; }; Obj a = new Obj(); ab = a; ) and it points to the same type of object, the principle is the same: a = b means they point to same object, nothing new is created.

Answer 3

An object instance is itself and is distinct from every other instance.

That is, mutating an object (by reassigning a field) someplace modifies it everywhere .. as, well, it is what it is. Likewise, mutating a different object .. is, well, changing a different object.