Java.Collections.Estimate the amount of memory required to store one byte primitive in ArrayList?

let’s estimate how much memory is used to store one byte primitive in an ArrayList<Byte>.

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🔥 Spoiler: storing a single byte in an ArrayList<Byte> costs way more than 1 byte — closer to 24–32 bytes per element.

Let’s break it down:

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✅ 1. What happens when you do:

ArrayList<Byte> list = new ArrayList<>();
list.add((byte) 42);

You’re actually:

• Boxing the byte into a Byte object
• Storing a reference to that Byte in an internal array

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🔧 Memory components per element:

📦 A. Byte object (the wrapper)

• Object header: ~12 bytes
• Data (byte field): 1 byte
• Padding: ~3 bytes (to align to 8 bytes)

➕ Total for Byte: ~16 bytes

📦 B. Reference in ArrayList array

• 1 reference = 4 bytes (on 32-bit) or 8 bytes (on 64-bit with compressed oops)

➕ Reference: ~4–8 bytes

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📊 Estimated total memory per byte:

Component	Size
Byte object	~16 bytes
Reference in array	~4–8 bytes
Total	~20–24 bytes per byte stored

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🧠 Comparison with byte[]:

byte[] arr = new byte[1_000_000];

• Stores raw primitive bytes
• Total memory: ~1,000,016 bytes (1MB + small overhead)

vs.

ArrayList<Byte> list = new ArrayList<>();
for (int i = 0; i < 1_000_000; i++) {
    list.add((byte)i);
}

• Memory usage: ~20–24 MB 😱

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📌 Summary:

Structure	Memory per byte	Comment
byte[]	✅ ~1 byte	Most efficient
ArrayList<Byte>	❌ ~20–24 bytes	Due to boxing + references
LinkedList<Byte>	❌❌ ~40–50 bytes	Extra overhead from nodes