Java Stack vs Heap Memory Allocation

In Java, memory allocation is primarily divided into two categories, i.e., Stack and Heap memory. Both are used for different purposes, and they have different characteristics.

• Stack Memory: Stores primitive local variables, method call information, and references to objects during program execution.
• Heap Memory: Stores actual objects and dynamic data allocated at runtime. Objects created with new are placed here, and this memory is managed by the Garbage Collector.

Employee ID: 101
Name: Maddy
Salary: 50000.0
Employee ID: 102
Name: Maddy
Salary: 60000.0

Memory representation for the above example:

• emp1 and emp2: references stored on stack.
• new Employee(...): objects stored in heap.
• "Maddy"; stored once in String Pool (heap).
• Both emp1.name and emp2.name: point to the same string object.
• display(emp1): creates a stack frame, parameter e points to emp1.
• (emp1.name == emp2.name): true since both refer to the same pooled string.

Stack Memory Allocation

Stack memory is used for method calls, local variables, and references. Memory is automatically allocated when a method starts and cleared when it ends. Data exists only during the method’s execution. If the stack runs out of space, a StackOverflowError occurs.

The sum is: 30

Heap Memory Allocation

Heap memory is used for objects and instance variables created using the new keyword. The size depends on the class structure. The Garbage Collector manages this area by removing unused objects.

Heap Memory Regions:

• Young Generation: Stores newly created objects.
• Survivor Space: Holds objects that survive garbage collection from Eden space.
• Old Generation: Stores long-lived objects.

Non-Heap Memory Areas:

• Metaspace (earlier Permanent Generation): Stores class metadata and uses native memory (introduced in Java 8).
• Code Cache: Stores JIT-compiled and optimized native code.

Example: Object in heap

Here, the Scanner object is in the heap, while the reference sc is in the stack.

Note: Garbage collection in the heap ensures automatic memory management.

Difference Between Stack and Heap Memory in Java