Stack Memory and Heap Memory in Java

What is Stack Memory?

The memory space that stores local variables and object references is known as stack memory. The stack memory is created whenever a method is called. The memory allocation takes place in LIFO (Last In, First Out) order. Let’s suppose, with a method call, a thread is created. Then, the stack memory will be allocated to each thread at runtime. Moving forward, let’s understand the workings of stack memory in brief.

Characteristics of Stack Memory

Understanding stack memory in Java is essential to grasp how Java manages method calls and local data during program execution. Stack memory is a key component of the Java memory structure, designed for temporary data storage with high-speed access. Below are the main characteristics that make stack memory unique and important:
 

1. Stores Primitive Variables and Method Calls
Stack memory in Java is used to store primitive local variables and references to objects within methods. When a method is called, its execution context (including parameters and local variables) is pushed onto the stack. This makes stack memory ideal for storing short-lived data needed only during method execution.
 

2. Last-In, First-Out (LIFO) Structure
Stack memory follows the LIFO principle in Java—Last-In, First-Out. The most recently called method is placed on top of the stack and removed first when it completes. This orderly structure helps manage the flow of method calls and returns efficiently, ensuring a clear and predictable execution order.
 

3. Automatically Allocated and Deallocated
One of the key benefits of stack memory is that it is automatically managed by the JVM. Memory is allocated when a method is invoked and automatically deallocated once the method completes. This automatic cleanup helps reduce the risk of memory leaks and simplifies resource management for developers.
 

4. Faster Access Compared to Heap
Due to its fixed size and simple structure, stack memory provides faster access than heap memory. It doesn’t require complex memory management, so reading and writing to the stack is quick. This speed makes it ideal for high-performance tasks involving local variables and function calls.
 

 5. Limited Size and Risk of StackOverflowError
Stack memory is much smaller than heap memory and has a fixed size defined by the JVM. If the program has deep or infinite method recursion, it may exceed this limit, resulting in a StackOverflowError. Developers must write careful recursive logic to avoid such errors.
 

6. Thread-Specific Stack
In Java, each thread has its own private stack memory. This isolation ensures that variables and method calls in one thread do not interfere with those in another. It enhances thread safety and simplifies parallel processing within multi-threaded applications.

Working of Stack Memory

In a program, whenever a method is called during its execution, a memory block is created in the stack memory. Inside this memory block, all object references and local variables are stored. The method is executed, and after its execution is done, the created memory gets cleared from the stack memory automatically. The stack memory becomes again available for use. We can say that stack memory works in such a way that it stores(push) the data temporarily and pop them out after the execution is done. 

Example of Stack Memory

Let’s make a program to understand stack memory more clearly.

Implementation

• Java

Java

class Solution {
    public static void main(String[] args) {
            int n = 10;
            n = ninja(n);
            System.out.println("n = "+n + " is returned and the memory block is cleared.");
    }
    public static int ninja(int n) {
        System.out.println("Inside ninja method.");
        return n;
    }
}

You can also try this code with Online Java Compiler

Run Code

Output

Inside ninja method.
n = 10 is returned and the memory block is cleared.

Explanation

In the above code, we have made a method named ninja. We have declared a variable n inside the main function. A memory block is allocated for the main method, where the value n will be stored. When the ninja method is called, a new block of memory will be allocated in the stack memory. When the execution of the ninja method is completed and the value is returned, then the memory block for the ninja method will be cleared. And now, the main method also completed its execution, so the memory block for the main method will also be cleared.

For better understanding, run this code on Online java compiler.

Advantages of Stack Memory

There are some advantages of using stack memory few of them are mentioned below:

• It has fast access to the memory
   
• Using stack memory, memory is automatically managed and cleaned up
   
• There is no fragmentation in the stack memory

Disadvantages of Stack Memory

Along with some advantages, there are some disadvantages of using stack memory few of them are mentioned below:

• It is of fixed size. Once the stack size is determined, it cannot be changed during runtime
   
• It is of limited memory size. It is limited in size, which may cause a stack overflow error
   
• It is not having any memory sharing

What is Heap Memory?

The memory space a computer program uses to store objects dynamically created during runtime is called heap memory. It is that space of the memory which is shared by all threads of an application and is typically managed by the operating system or runtime environment. The heap is dynamically allocated at runtime. It means that memory can be allocated and deallocated as needed, making it suitable for storing objects of unknown or variable size. The memory allocation takes place dynamically in random order. The size of the heap memory is large. It can be increased and decreased by the JVM options Xmx and Xms(where Xmx and Xms specify the maximum and minimum memory allocation pool for a JVM). Moving forward, let’s understand the working of heap memory in brief.

Characteristics of Heap Memory

Heap memory in Java plays a crucial role in storing objects and managing dynamic memory allocation during program execution. It differs significantly from stack memory in behavior, performance, and purpose. Below are the key characteristics that define heap memory and its role in Java memory management.

1. Stores Objects and Class Instances
Heap memory in Java is used to store all objects, class instances, and arrays created during runtime. Whenever the new keyword is used to create an object, memory for that object is allocated in the heap. Unlike stack variables, heap data persists beyond the execution of a single method.
 

2. Shared Across All Threads
Unlike stack memory, which is thread-specific, heap memory is shared across all threads in a Java application. This shared nature allows multiple threads to access the same objects concurrently. However, this also means developers must handle synchronization carefully to avoid concurrency issues.
 

3. Managed by Garbage Collector
Heap memory is managed automatically by the Java garbage collection system. The JVM continuously monitors heap memory and removes objects that are no longer reachable or referenced, freeing up space. This automated memory cleanup process reduces the burden of manual memory management.
 

 4. Larger but Slower Than Stack Memory
The heap has a larger size compared to the stack, making it suitable for storing complex and long-lived objects. However, because it uses dynamic memory allocation and garbage collection, heap memory in Java is slower in access speed than the stack. This trade-off balances flexibility and performance.
 

5. Prone to Memory Leaks and OutOfMemoryError
If objects in the heap are unintentionally kept referenced, they won't be collected by the garbage collector, leading to memory leaks. Poor memory handling or high object creation rates can fill up the heap space, eventually causing an OutOfMemoryError. Efficient coding and monitoring tools help prevent these issues.
 

 6. Divided into Generations
To optimize Java memory management, the heap is divided into sections: Young Generation, Old Generation, and Permanent Generation (or Metaspace in modern JVMs). This generational structure improves garbage collection efficiency by separating short-lived and long-lived objects.

Working of Heap Memory

The heap memory is further divided into three parts. These parts are

1. New Generation or Young Generation
    
2. Old Generation or Tenured Generation
    
3. Permanent Generation
    

Let’s understand each of them.

New Generation or Young Generation

Inside this memory space, the allocation of objects that are newly created takes place. There are three further parts of a new generation: 

• Eden: This space is responsible for the allocation of newly created objects.
   
• Survivor 1: The objects are moved to Survivor 1 after a minor garbage collection takes place. The minor garbage collection only takes place after Eden is full. 
   
• Survivor 2: Similarly, the objects are moved here from Survivor 1 after the minor garbage collection.
   

Old Generation or Tenured Generation

Inside this memory space, the surviving objects from the new generation are stored. The major garbage collection is run in order to collect dead objects.

Permanent Generation

Inside this memory space, the metadata about the methods and classes are stored. The full garbage collection takes place, and this space is cleaned completely.

Example of Heap Memory

Let’s make a program to understand about heap memory more clearly.

Implementation

• Java

Java

import java.util.ArrayList;
import java.util.List;
 
class Solution {
    public static void main(String[] args) {
            List<String> ninjaList = new ArrayList<>();
 
            ninjaList.add("Ninja 1");
            ninjaList.add("Ninja 2");
            ninjaList.add("Ninja 3");
            ninjaList.add("Ninja 4");
            ninjaList.add("Ninja 5");

            System.out.println(ninjaList);
    }
}

You can also try this code with Online Java Compiler

Run Code

Output

[Ninja 1, Ninja 2, Ninja 3, Ninja 4, Ninja 5]

Explanation

In the above code, the array list of strings named ninjaList is stored in allocated memory in a heap. 

Advantages of Heap Memory

There are some advantages of using heap memory few of them are mentioned below:

• Heap memory has dynamic memory allocation of memory
   
• Using heap memory, memory can be allocated and deallocated whenever needed
   
• It is not having any memory wastage

Disadvantages of Heap Memory

Along with some advantages, there are some disadvantages of using heap memory few of them are mentioned below:

• It is slower than stack memory because of additional overhead in memory allocation and deallocation
   
• Using heap memory, fragmentation can occur, leading to memory being spread across multiple non-contiguous blocks
   
• It can have memory leaks if a program fails to deallocate the memory which is not needed

How JVM Manages Stack and Heap Memory

The Java Virtual Machine (JVM) plays a central role in how memory is allocated and managed during program execution. To ensure efficient performance and stability, the JVM divides memory into different areas and automatically handles memory cleanup using the Java garbage collector. Below, we explore the major JVM memory areas and explain how stack and heap memory are managed.

1. JVM Memory Areas

The JVM memory structure is divided into several key components, each serving a specific role in program execution and Java memory management. Understanding where stack and heap memory in Java fit into this architecture helps developers write more efficient and reliable code.

• Method Area
  This region stores class-level data like method metadata, field information, static variables, and the runtime constant pool. It's shared among all threads and loaded during class loading.
   
• Heap Memory
  The heap is the largest memory area where all objects, class instances, and arrays reside. This memory is shared across all threads and managed by the Java garbage collector. Objects created using the new keyword are stored here.
   
• Stack Memory
  Every thread in Java has its own stack memory, which stores method call frames, local variables, and references to objects in the heap. It operates in a Last-In, First-Out (LIFO) manner and is automatically cleared after method execution.
   
• Program Counter (PC) Register
  This small memory block keeps track of the current instruction being executed in a thread. Each thread has its own PC register for instruction control and sequencing.
   
• Native Method Stack
  This area supports the execution of native (non-Java) code written in languages like C or C++. Like stack memory, it’s also thread-specific and used during JNI (Java Native Interface) calls.

2. Role of the Garbage Collector in Heap Memory

In Java, memory allocation for objects is automatic, and so is memory deallocation—thanks to the Java garbage collector. This internal process of the JVM reclaims heap memory used by objects no longer referenced, helping prevent memory leaks and improving performance.

Automatic Memory Management
The Java garbage collector monitors the heap and automatically clears memory occupied by unreachable objects. This eliminates the need for manual memory cleanup, a common source of bugs in other languages.

Types of Garbage Collectors
The JVM provides multiple GC algorithms to suit different application needs. These include:

• Serial GC – Simple, single-threaded collector for small apps.
   
• Parallel GC – Uses multiple threads for faster cleanup.
   
• Concurrent Mark Sweep (CMS) – Minimizes pause time during garbage collection.
   
• G1 Garbage Collector – Splits heap into regions for more efficient and predictable GC.
   

Generational Collection
The heap is divided into:

• Young Generation – For short-lived objects; collected frequently.
   
• Old Generation (Tenured) – Stores long-lived objects; collected less frequently.
   
• Permanent Generation / Metaspace – Stores metadata for classes and methods (Metaspace is used in Java 8+).

The JVM's intelligent design for stack and heap memory management ensures optimal program execution, thread safety, and minimal memory leaks. Understanding these JVM memory areas and the role of the Java garbage collector equips developers to build more efficient, scalable, and error-free Java applications.

Difference Between Stack Memory And Heap Memory in Java

The differences between stack and heap memory in Java are as follows.

Read more, Difference Between List and Set

Frequently Asked Questions

What is the order memory allocation in heap memory?

In heap memory, there is no order of memory allocation. It takes place dynamically in random order.

Is stack memory RAM or ROM?

Stack memory is typically implemented using RAM (Random Access Memory). RAM is a type of computer memory that allows data to be read from and written to quickly in random order. 

Why stack is faster than heap?

The stack is faster than the heap for memory allocation and deallocation, but the heap is still necessary for more complex data structures and larger amounts of memory that cannot fit on the stack.

What is the size of stack memory?

The size of stack memory is typically limited by the operating system and the hardware architecture of the computer. But the default main stack size is 8MB.

Is stack memory LIFO or FIFO?

Stack memory is managed using a Last-In-First-Out (LIFO) data structure. In other words, the most recently added item to the stack is the first one to be removed.

What are the two types of heap?

Two main types of heap are min heap and max heap. Min heap is where the value of each parent node is less than or equal to the value of its children. On the other hand, the max heap is where the value of each parent node is greater than or equal to the value of its children.

Conclusion

In this article, we have discussed about Stack Memory and Heap Memory in Java and their working. We have also discussed the differences between them. To learn more about Java programming language.

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