OCAJP 7 Object Lifecycle in Java

What is an Object?

• An object is a collection of data and actions.
• An object is an instance of a class.
• Objects have states and behaviors.

In the real-world, we can find so many objects around us, for example Cars, Birds, Humans etc. All these objects have a state and behavior. If we consider a Car then it have some data speed, lights on, direction, etc. and have some actions turn right, accelerate, turn lights on, etc.

If you compare the java object with a real world object, both of them have similar characteristics. Java objects also have a state and behavior. A Java object's state is stored in fields and behavior is shown via methods.

Technically speaking Car, Bird and Human are considered as Class in Java.  Brian Christopher is an object of human and Vehicle XKMV-669 is the object of car.

Creating Object

Using new keyword is the most common way to create an object in java.

Syntax:-

ClassName Obj.Name = new ClassName();

// Human brianChristopher= new Human();

// Car  vehicleXKMV_669 = new Car();

The first statement creates a new Human object and second statement creates Car object. This single statement performs three actions, Declaration, Instantiation, and Initialization.

Here, Human brianChristopher is a variable declaration which simply declares to the compiler that the name brianChristopher will be used to refer to an object whose type is Human, the new operator instantiates the Human class (thereby creating a new Human object), and Human initializes the object.

Object Lifecycle

In Java, it has seven states in Object lifecycle. They are,

1. Created
2. In use
3. Invisible
4. Unreachable
5. Collected
6. Finalized
7. De-allocated

Created

The following are the some actions performed when an object is created,New memory is allocated for an object.

Once the object has been created, assuming that it is assigned to some variable and then it directly moves to the In Use state.

In use

Objects that are held by at least one strong reference are considered to be “In Use”.

Invisible

An object is in the “Invisible” state when there are no longer any strong references that are accessible to the program, even though there might still be references.

Unreachable

An object enters an “unreachable” state when no more strong references to it exist. When an object is unreachable then it is a state for collection.

It is important to note that not just any strong reference will hold an object in memory. These must be references that chain from a garbage collection root.

Garbage collection roots are a special class of variable that includes,Temporary variables on the stack

Collected

An object is in the “collected” state when the garbage collector has recognized an object as unreachable and readies it for final processing as a precursor to de-allocation. If the object has a finalize method, then it is marked for finalization.

Finalized

An object is in the “finalized” state if it is still unreachable after it’s finalize method, if any, has been run. A finalized object is awaiting de-allocation. If you are considering using a finalizer to ensure that important resources are freed in a timely manner, you might want to reconsider. To lengthening object lifetimes, finalize methods can increase object size.

De-allocated

The de-allocated state is the final step in garbage collection. If an object is still unreachable after all the above work has done, then this is the state for de-allocation.

For more detailed discussion about Object Lifecycle with real-world examples download OCAJP 7 Training Lab from EPractize Labs.