Understanding Java Heap vs Stack: Memory Allocation, Performance, and Code Examples

Heap and stack are the two areas in Java's RAM where data is stored.

Heap

Java's heap is a runtime data area where class objects allocate space. These objects are created via new and destroyed by the garbage collector.

The heap's advantage is dynamic memory allocation at runtime, eliminating the need to specify size at compile time, but its access speed is slower due to dynamic allocation.

Stack

The stack mainly stores primitive type variables (byte, short, int, long, float, double, boolean, char) and object references.

The stack's advantage is faster access and data sharing, but its size must be determined at compile time, lacking flexibility.

Example: Stack data can be shared

String can be created in two ways:

String str1 = new String("abc");<br/>String str2 = "abc";

The first creates a new object on the heap each time.

The second creates a reference in the stack; if the literal "abc" already exists in the string pool, the reference points to that existing object, otherwise it is added.

Code illustrating the theory:

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

Output: false

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

Output: true

Thus, using the second method creates only one object in memory, saving space and potentially improving performance because the JVM reuses the existing string from the pool.