JVM在原始类型的声明中分配内存，但不为非原始类型分配内存？

Question

I am from a C/C++ background and very new to java, I am having difficulty in understanding variable declaration and memory allocation in java.

when we write,

myclass myobject;

we declare that myobject is a variable of type myclass. We are not allocating memory to it.

int a;

It declares the variable a and also allocates memory equal to size of int in stack.

Is it the case? Does the compiler allocate memory for the primitive data types but not for the non-primitive data types?

I've raised a similar doubt here .

Answer 1

Compiler does't allocate memory. Its JVM who allocate memory

For primitive data type memory is allocated at time of declaration of those variable and memory is taken in that function local stack.
int x;

memory allocated in stack 4 byte

When we use new operator then memory is allocated to heap which is the size of class's data member.
MyClass object;

This is reference variable also takes size of 4 byte

object = new MyClass();

new operator allocate memory in heap and size is sum of all individual data member's size of that class.

Answer 2

Is it the case? Does the compiler allocate memory for the primitive data types but not for the non-primitive data types?

First point to note is Compiler won't allocate any memory. JVM do that.

Coming to actual question, Yes. For primitives weather you initialize it or not later, once you declare, the memory allocates.

Here is the amount of data for primitives

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

For objects, at the time of initialization, the memory allocates. The size of memory really depends on the Class data.

Answer 3

In java the declaration of the variable doesn't mean allocate memory space but means this variable can hold a reference of an object with the same type of its class(or subclass) later.

MyClass myObject; // myObject has null value (declaration only)

The memory space is allocating when you write :

MyClass myObject = new MyClass(); // when we use the new operator (the allocating occurs )

Now myObject has the reference which points to the resulted object from new MyClass() , and the size of this object depends on the data fields in its class declaration.

Conclusion

The statement

int a; // allocate fixed size because int size is known and can't change it

MyClass myObject; // has null value

myObject = new MyClass(); // now has a reference of this object

I wish this help you :) .

Answer 4

In Java, when we declare a variable of a class type, only a reference is created (memory is not allocated for the object). To allocate memory to an object, we must use new() and all objects are dynamically allocated on Heap.

in C++ where objects can be allocated memory either on Stack or on Heap. when we allocate the object using new(), the object is allocated on Heap.

Understanding Memory Management