Java Beginners: What Is an Object Actually?

[u/jadu2115]

I'm a Java learner and I've been learning Java for the past few months, but I still don't truly understand what an object actually is.

I've watched several YouTube videos and used AI tools to understand it, but I keep getting the same explanation: "An object is nothing but data + code."

I understand the definition, but I still can't visualize or feel what an object actually is when I'm writing Java code.

Could someone explain it in a simple, practical way, preferably with a real world analogy and a small Java example?

I feel like I'm missing one fundamental concept here. Any explanation would be really helpful. Thanks!

[u/Leverkaas2516]

A Class is a user-defined data type. It defines the shape of the data (how it is laid out in memory), and what operations can be performed on it.

Start with intrinsic types. An "int" is a collection of bits that can hold a single numeric value. The language provides you ways to add them, subtract them, print them with %d, and so on.

A Class, being a user-defined type, lets YOU define what data is stored and what operations can be used, in the form of methods.

Here's the part that answers the question: an OBJECT is a block of memory at runtime that is known to the JVM as holding the data for an instance of your Class. It's known to the JVM because at runtime it executed a statement like "MyClass myObj = new MyClass();"

At that moment, the JVM allocated a block of memory (maybe a single 32-bit word, maybe more, depending on what properties MyClass is defined to hold) and called the MyClass constructor to initialize the values of those properties.

That's all that happens. Now there's an object, and your program can refer to it because the JVM handed back a reference in your myObj variable. As long as you keep that reference around, the block of memory is kept reserved by the JVM. After your program loses the reference (by returning from the function where myObj was a local variable, for example) the JVM is free to use the memory for something else.

I should add that a class doesn't have to define any data. In that case, though, "new MyClass()" will STILL cause the JVM to allocate a small fragment of memory, in which there is enough information for the JVM to keep track of what class it belongs to and whether it's still potentially in use. You don't have access to this information and can consider it bookkeeping overhead that's used by the JVM...but it means that you can sum it up by saying "an object is a block of memory".