A Java object is a member [also called an instance] of a Java class. Each object has an identity, a behavior and a state.
The state of an object is stored in fields [variables], while methods [functions] display the object's behavior. Objects are created at runtime from templates, which are also known as classes.
In Java, an object is created using the keyword "new".
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Techopedia Explains Java Object
Java objects are very similar to the objects we can observe in the real world. A cat, a lighter, a pen, or a car are all objects.
They are characterized by three features:
- Identity
- State
- Behavior
For example, a cat’s state includes its color, size, gender, and age, while its behavior is sleeping, purring, meowing for food, or running around like crazy at 4 AM.
Identity
The identity is a characteristic used to uniquely identify that object – such as a random ID number or an address in memory. Simpler objects like a lighter may have only two states [on and off] and behaviors [turn on, turn off], but they still have an identity [that item’s manufacturing ID, for example].
State
A Java object’s states are stored in fields that represent the individual characteristics of that object. For example, in a first-person shooter video game, a pistol with an eight-bullets clip has nine states in total: one for each bullet [e.g. 8 bullets, 7 bullets, 5 bullets, etc.], plus another one when it’s empty [0 bullets].
Behavior
The object’s behavior is exposed through methods that operate its internal state. For example, the “shooting” behavior will change the state of the pistol from “8 bullets'' to “7 bullets” and so forth every time the player shoots with the gun.
The “reloading” behavior will bring back the pistol into the original “8 bullets'' state.
There are three steps to creating a Java object:
- Declaration of the object.
- Instantiation of the object.
- Initialization of the object.
When a Java object is declared, a name is associated with that object. The object is instantiated so that memory space can be allocated. Initialization is the process of assigning initial values to the object attribute. Object properties are consistent through all objects from the same class, unlike class properties which are applied only to that specific class.
Properties of Java objects
One can usually interact with an object through its methods. Through states and methods, the objects stay in control of how the world can use it.
For example, any value that is greater than “8 bullets” will be rejected by the method since there’s no state for it. This concept is defined as data encapsulation.
Code can be bundled into individual objects and the source code of every one of them can be written and maintained independently of the others. If a program's operation is hindered by a particular object, that object can be easily removed and replaced.
Objects are key to understanding object-oriented technology. Look around right now and you'll find many examples of real-world objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. Dogs have state [name, color, breed, hungry] and behavior [barking, fetching, wagging tail]. Bicycles also have state [current gear, current pedal cadence, current speed] and behavior [changing gear, changing pedal cadence, applying brakes]. Identifying the state and behavior for real-world objects is a great way to begin thinking in terms of object-oriented programming.
Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask yourself two questions: "What possible states can this object be in?" and "What possible behavior can this object perform?". Make sure to write down your observations. As you do, you'll notice that real-world objects vary in complexity; your desktop lamp may have only two possible states [on and off] and two possible behaviors [turn on, turn off], but your desktop radio might have additional states [on, off, current volume, current station] and behavior [turn on, turn off, increase volume, decrease volume, seek, scan, and tune]. You may also notice that some objects, in turn, will also contain other objects. These real-world observations all translate into the world of object-oriented programming.
A software object.
Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores its state in fields [variables in some programming languages] and exposes its behavior through methods [functions in some programming languages]. Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object's methods is known as data encapsulation — a fundamental principle of object-oriented programming.
Consider a bicycle, for example:
A bicycle modeled as a software object.
By attributing state [current speed, current pedal cadence, and current gear] and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.