I want to delete an object I created, (a oval which follows you), but how would I do this?
delete follower1;
didn't work.
EDIT:
Okay, I'll give some more context. I'm making a small game with a oval you can control, and a oval which follows you. Now I've got files named: DrawPanel.class, this class draws everything on the screen, and handles collisions, sounds, etc. I got an enemy.class, which is the oval following the player. I got an entity.class, which is the player you can control. And if the player intersects with the follower, I want my player object to get deleted. The way I'm doing it:
public void checkCollisions(){
if(player.getBounds().intersects(follower1.getBounds())){
Follower1Alive = false;
player.health = player.health - 10;
}
}
You should remove the references to it by assigning null or leaving the block where it was declared. After that, it will be automatically deleted by the garbage collector (not immediately, but eventually).
Example 1:
Object a = new Object();
a = null; // after this, if there is no reference to the object,
// it will be deleted by the garbage collector
Example 2:
if (something) {
Object o = new Object();
} // as you leave the block, the reference is deleted.
// Later on, the garbage collector will delete the object itself.
Not something that you are currently looking for, but FYI: you can invoke the garbage collector with the call System.gc()
Your C++ is showing.
There is no delete in java, and all objects are created on the heap. The JVM has a garbage collector that relies on reference counts.
Once there are no more references to an object, it becomes available for collection by the garbage collector.
myObject = null may not do it; for example:
Foo myObject = new Foo(); // 1 reference
Foo myOtherObject = myObject; // 2 references
myObject = null; // 1 reference
All this does is set the reference myObject to null, it does not affect the object myObject once pointed to except to simply decrement the reference count by 1. Since myOtherObject still refers to that object, it is not yet available to be collected.
If you want help an object go away, set its reference to null.
String x = "sadfasdfasd";
// do stuff
x = null;
Setting reference to null will make it more likely that the object will be garbage collected, as long as there are no other references to the object.
You don't need to delete objects in java. When there is no reference to an object, it will be collected by the garbage collector automatically.
You can remove the reference using null.
Let's say You have class A:
A a = new A();
a=null;
last statement will remove the reference of the object a and that object will be "garbage collected" by JVM.
It is one of the easiest ways to do this.
Java has a Garbage Collector, it will delete the object for you if no reference is held to it anymore.
//Just use a List
//create the list
public final List<Object> myObjects;
//instantiate the list
myObjects = new ArrayList<Object>();
//add objects to the list
Object object = myObject;
myObjects.add(object);
//remove the object calling this method if you have more than 1 objects still works with 1
//object too.
private void removeObject(){
int len = myObjects.size();
for(int i = 0;i<len; i++){
Objects object = myObjects.get(i);
myObjects.remove(object);
}
}
Related
I want to delete an object I created, (a oval which follows you), but how would I do this?
delete follower1;
didn't work.
EDIT:
Okay, I'll give some more context. I'm making a small game with a oval you can control, and a oval which follows you. Now I've got files named: DrawPanel.class, this class draws everything on the screen, and handles collisions, sounds, etc. I got an enemy.class, which is the oval following the player. I got an entity.class, which is the player you can control. And if the player intersects with the follower, I want my player object to get deleted. The way I'm doing it:
public void checkCollisions(){
if(player.getBounds().intersects(follower1.getBounds())){
Follower1Alive = false;
player.health = player.health - 10;
}
}
You should remove the references to it by assigning null or leaving the block where it was declared. After that, it will be automatically deleted by the garbage collector (not immediately, but eventually).
Example 1:
Object a = new Object();
a = null; // after this, if there is no reference to the object,
// it will be deleted by the garbage collector
Example 2:
if (something) {
Object o = new Object();
} // as you leave the block, the reference is deleted.
// Later on, the garbage collector will delete the object itself.
Not something that you are currently looking for, but FYI: you can invoke the garbage collector with the call System.gc()
Your C++ is showing.
There is no delete in java, and all objects are created on the heap. The JVM has a garbage collector that relies on reference counts.
Once there are no more references to an object, it becomes available for collection by the garbage collector.
myObject = null may not do it; for example:
Foo myObject = new Foo(); // 1 reference
Foo myOtherObject = myObject; // 2 references
myObject = null; // 1 reference
All this does is set the reference myObject to null, it does not affect the object myObject once pointed to except to simply decrement the reference count by 1. Since myOtherObject still refers to that object, it is not yet available to be collected.
If you want help an object go away, set its reference to null.
String x = "sadfasdfasd";
// do stuff
x = null;
Setting reference to null will make it more likely that the object will be garbage collected, as long as there are no other references to the object.
You don't need to delete objects in java. When there is no reference to an object, it will be collected by the garbage collector automatically.
You can remove the reference using null.
Let's say You have class A:
A a = new A();
a=null;
last statement will remove the reference of the object a and that object will be "garbage collected" by JVM.
It is one of the easiest ways to do this.
Java has a Garbage Collector, it will delete the object for you if no reference is held to it anymore.
//Just use a List
//create the list
public final List<Object> myObjects;
//instantiate the list
myObjects = new ArrayList<Object>();
//add objects to the list
Object object = myObject;
myObjects.add(object);
//remove the object calling this method if you have more than 1 objects still works with 1
//object too.
private void removeObject(){
int len = myObjects.size();
for(int i = 0;i<len; i++){
Objects object = myObjects.get(i);
myObjects.remove(object);
}
}
I have an array of game objects in the GameWorld and they can get removed from that world. The problem is that some game objects have references to other game objects. E.g. Player class has a reference to a Bird. Bird gets randomly removed from GameWorld, but Player still has a reference to it. I currently do a null check to check whether the GameObject is still valid and in the world. However, removing an object from the array does not make that reference null. So how can I make it null?
Here is an example:
// GameWorld creates bird
ArrayList<Object> gameObjects = new ArrayList<>();
Object bird = new Object();
gameObjects.add(bird);
// Player references it
Object referencedBird = gameObjects.get(0);
// later in GameWorld, another scope, there is no access to the 'bird' object, trying to remove the bird from the world
Object objectToRemove = gameObjects.get(0);
gameObjects.remove(0);
objectToRemove = null;
// back in the Player class
Debug.log("is null " + (referencedBird == null)); // false! I need it to be true
You can't make an object null, you can only make a reference null. Updating one reference to an object doesn't change other references to the same object.
Think about it like us both having the same person's number in our phones: if I delete the number, it doesn't get deleted from your phone. And if we both delete it, that person's phone number doesn't cease to exist: just neither of us can call them.
The only things you can do are:
Set referencedBird = null explicitly;
Or refer to it via gameObjects.get(0) instead of via the referencedBird variable.
Okay since Andy pointed out that you can not destroy a object itself you can still have a work-around.
interface RemoveAble<T>{
boolean isRemoved();
T prepareRemoveAndGet();
}
class Bird implements RemoveAble<Bird>{
boolean shouldRemove = false;
#Override
public boolean isRemoved() {
return shouldRemove;
}
#Override
public Bird prepareRemoveAndGet() {
shouldRemove = true;
return this;
}
}
And use it like this:
// GameWorld creates bird
ArrayList<RemoveAble> gameObjects = new ArrayList<>();
Bird bird = new Bird();
gameObjects.add(bird);
// Player references it
Bird referencedBird = gameObjects.get(0);
// later in GameWorld, another scope, there is no access to the 'bird' object, trying to remove the bird from the world
Object objectToRemove = gameObjects.get(0).prepareRemoveAndGet;
// back in the Player class
Debug.log("is null " + (referencedBird.isRemoved == true)); // false! I need it to be true
And then null it by yourself
e.g.
if(referencedBird.isRemoved) referencedBird = null;
First you need to better understand how Java variables work:
I you set a variable to null that does not affect the object behind it at all. You would need to somehow invalidate the referenced object (using a boolean variable) or the better solution would be to always get the object directly from the ArrayList.
Unlike C and C++ in java you cant manipulate memory directly, the only option you have is to remove all the Strong (regular) reference to obj and leave to JVM to remove it from memory whenever it needs memory.
// GameWorld creates bird
ArrayList<Object> gameObjects = new ArrayList<>();
Object bird = new Object();
gameObjects.add(bird);
// a weak reference to array Item
WeakReference<Object> referencedBird = new WeakReference(gameObjects.get(0));
// manipulating and accessing array item via weak reference
referencedBird.get().toString();
// after this remove there is no strong reference to array element (0) and it is available for JVM to revoke
//memory allocated to it in up-coming garbage collection cycle
gameObjects.remove(0);
//garbage collection may remove the obj from memory so this method may return null
referencedBird.get();
Also you have the option to make your gameObjects list, memory efficient by making array of type WeakReference
ArrayList<WeakReference> weakReferenceToGameObjects = new ArrayList<>();
Object bird1 = new Object();
weakReferenceToGameObjects.add((WeakReference) bird1);
bird1 = null;
// now if you try to get the element you may get null!
weakReferenceToGameObjects.get(0).get();
I have some custom object like:
public class House
{
private Roof r;
private Wall w;
private Floor f;
public House()
{
this.r = new Roof();
this.w = new Wall();
this.f = new Floor();
}
}
Then I create a new instance of House object:
public static void main()
{
House h = new House();
}
So, now I have four objects:
House h
Roof r
Wall w
Floor f
Roof r, Wall w and Floor f absolutely depends on House.
So if I will delete House h, I want them to be removed as well.
What is the correct way to do it?
What if I will add this method to House class:
public void remove()
{
this.r = null;
this.w = null;
this.f = null;
}
And when I will need to remove a House h with all its components write:
public static void main()
{
h.remove();
h = null;
}
I think this should work, but not sure.
Also, maybe there is a better way?
And yes, there is nothing about public houses, inns etc. :)
You can never depend on garbage collection occurring at all. Even if you call System.gc(), you have no guarantee that it will do anything. When the system decides to run a garbage collection, unreachable references may be collected, but if you run a small application, the collector may never run.
If you need to be notified when an object is disposed, you should use a java.lang.ref.PhantomReference and a java.lang.ref.ReferenceQueue. Do not depend on Object.finalize(). But that does not seem to be your issue; you simply want to clean up after yourself. You should care only if your application's performance requires it.
You dont need to null out all instance variables in House since they are automatically garbage collected when there is no reference to the instance of House.
What if they are referenced from somewhere else? But I want them to be removed with the removal of "parent" object.
For this you should use weak references each time you reference the objects from somewhere outside of the House class. This way the objects will get garbage collected when there no longer is a reference to House, since the only strong references pointing to them are inside of House.
The solution you have given is almost correct. Once you have marked an object reference as null, then that object will be destroyed and removed from memory by the JVM garbage collector whenever that garbage collector will start running .
You can also manually instruct garbage collector to collect garbage after setting reference variables as NULL .
I have the following code:
class Test {
public static void main(String[] args) {
String a,b,c;
a = new String(args[0]);
b = a;
a = null;
b = null;
}
}
Can someone tell me when a will be eligible for garbage collection. I think it's after b is made null because don't a and b reference the same object ?
Pitching my answer too. As the other answers say, the String/ object is available for Garbage Collection once it is no longer accessible (you no longer have a handle to it).
So if you had a one-direction linked list... [1] -> [2] -> [3] and you had a handle to [1] (which has a handle to [2] and onwards). If you set your handle to [1] to null, you would put the entire list available to the Garbage collector. As this answer says, you are able to call System.gc() to request the Garbage Collector run, but it is not guaranteed that it will.
I believe the main focus to this answer is that objects are available to the garbage collector when they are inaccessible and that does not necessarily mean that there are no references to it. In my above example, even though [1] had a handle to [2], [2] was available for the garbage collector because there was no handle to [1].
The object you've created, new String(args[0]);, will be eligible for collection once there are no longer any references to it. So let's step through the code:
a = new String(args[0]);
a points to your String, it is not eligible for collection.
b = a;
a and b point to your String, not eligible.
a = null;
b points to your String, not eligible.
b = null;
No references to your String, Garbage Colelctor is happy!
What do you mean by a? a is a reference, and only objects are garbage collected.
The string referenced by a, and then b, is eligible for garbage collection when nothing references it anymore. In this case, this is after both references to the string (a and b) have been changed to reference something else; in this case, null.
You must reason in terms of allocated objects: the String you allocated is not referenced by anyone as soon as you nullify both a and b. From thereon the garbage collector is entitled to do its job.
class Test {
public static void main(String[] args) {
String a,b,c; // you dont have objects yet
a = new String(args[0]); // here u make a new object
b = a; // b references to the same object as a
a = null; // a points to null, reference is removed.
b = null; // b point to null, reference is removed
}
}
in your program the object that was created and assigned to a and b will be garbage collected after references are removed.
Garbage collector will clean up the object that has no roots and not being referenced by anything.
I wish to confirm which scenario will cause a Garbage Collection on the object myObj:
Scenario 1
ArrayList arList = new ArrayList();
while(someCondition)
{
myObj = new MyObect(); // a custom object
arList.add(myObj);
}
Scenario 2
ArrayList arList = new ArrayList();
while(someCondition)
{
myObj = new MyObect(); // a custom object
arList.add(myObj);
myObj=null;
}
Does explicitly setting an object as null improves garbage collection behavior or will be it same when I reset the object using the new constructor ?
You don't specify the scope of myObj and its important. If its a local variable it almost certainly doesn't matter. If its an instance variable then that could be a long-lived and unnecessary reference in which case setting to null will be useful.
Update: given the updated information that myObj is local to the method, it will be of zero value to set it to null at the end of each iteration of the loop. Consider this example:
public void process(String text) {
String[] lines = text.split("\n");
List<MyObject> list = new ArrayList<MyObject>();
Object myObj;
for (String line : lines) {
myObj = new MyObject(line);
list.add(myObj);
// 1. set myObj = null here
}
list = null; // 2
// 3. do some other stuff
}
public class MyObject {
private final String line;
public MyObject(String line) {
this.line = line;
}
}
Now in this example, let's say that at step 3, it took a long time. Say 10 minutes. During that 10 minutes myObj is pointing to the last line processed. Doesn't sound like a problem? Well it could be. The way substrings work in Java is that they reference the original string. So if you do:
String s = ... // 100 megabytes
String s2 = s.substring(100, 101);
you're actually keeping the entire 100MB in memory because s2 references s.
So in the function I have above, myObj references a line which references the entire file. Changing step 1 to myObj = null; would actually help that because this reference is preventing the object being garbage collected.
Note: step 2 is important here because if you didn't nullify the list all the references would exist anyway.
You just need to think about how references work. An object won't be garbage collected while a reference to it exists. This means clearing long-lived references and keeping variables scoped as tightly as possible. The correct solution for the above is:
for (String line : lines) {
Object myObj = new MyObject(line);
...
}
and then myObj is scoped inside the loop so as soon as the loop ends or another iteration begins it has gone out of scope, which is much better.
Setting it to null will have no effect, since the object is still reachable via arList.
That is, your MyObect instances will live at least as long as arList.
EDIT: Based on your comment, it does sound like myObj is longer-lived. In that case, set it to null after the end of your loop.
I think that this is the root of your misunderstanding.
hmm.. but I don't wish to keep 2 copies of myObj , one in arList and one in the original variable. How can I flush myObj once I add it to arLsit ?
You do NOT "keep two copies of myObj". In your examples, there is only ever one "copy" of each MyObject instance created by the loop. The sequence is:
You create a MyObject instance, assigning its reference to myObj.
You add the reference to the instance to the ArrayList that arList refers to.
You assign null to the reference in myObj.
Note that adding the reference to the list does NOT create a copy of the MyObject instance. It simply means that that you have the reference in two places instead of
one. And when you assign the null you once again have the reference in just one place.
The other thing to note is that assigning null to something will never CAUSE the garbage collector to run. All it does is to (explicitly) remove a potential copy of a reference from consideration the next time the garbage collector is run.
Finally, if we assume that the scoping is as follows, then the line C will have no discernible effect ... unless either line A or line B triggers a garbage collection.
{
MyObject myObj;
ArrayList arList = new ArrayList();
while (someCondition) { // A
myObj = new MyObect(); // B
arList.add(myObj);
myObj = null; // C
}
}
Because it is in a while, myObj is always overwritten (the reference). So in Scenario 1 only one object (the last added in arList) will not be null.
It would be better if you declare it in the while statement:
while(someCondition)
{
MyObect myObj = new MyObect(); // a custom object
arList.add(myObj);
}