Understanding Java Heap and Stack Memory with Examples

In Java, both heap and stack are areas in RAM used to store data during program execution.

Heap : It is a runtime data area where class objects are allocated via new and reclaimed by the garbage collector. The heap allows dynamic memory allocation, which provides flexibility but incurs slower access speed.

Stack : It primarily stores primitive type variables (byte, short, int, long, float, double, boolean, char) and object references. Stack access is fast and data can be shared, but the size of each stack frame must be determined at compile time, limiting flexibility.

To illustrate stack data sharing, consider two ways to create a String in Java.

First way (using new):

String str1 = new String("abc");
String str2 = "abc";

The object created with new resides in the heap, and each call creates a new object.

Second way (string literal): the JVM checks the string pool (often associated with the stack) for an existing "abc". If it exists, the reference points to that single object; otherwise, the literal is added to the pool.

Code example demonstrating the first approach:

public static void main(String[] args) {
    String str1 = new String("abc");
    String str2 = new String("abc");
    System.out.println(str1 == str2);
}

Output: false because two distinct objects are created on the heap.

Code example demonstrating the second approach:

public static void main(String[] args) {
    String str1 = "abc";
    String str2 = "abc";
    System.out.println(str1 == str2);
}

Output: true because both references point to the same interned string in the pool.

Thus, using string literals can save memory and improve performance, as the JVM reuses existing objects instead of allocating new ones each time.