Reduce Java Object Size with Field Ordering

If you’re seeing higher-than-expected memory usage in your Java application, the problem might be your field ordering.

Yes – the order of your variables in a class can affect object size, heap usage, and even JVM performance.

🧠 Quick Brea…


This content originally appeared on DEV Community and was authored by Prince

If you're seeing higher-than-expected memory usage in your Java application, the problem might be your field ordering.

Yes - the order of your variables in a class can affect object size, heap usage, and even JVM performance.

🧠 Quick Breakdown:

  • JVM aligns fields like long, int, byte for performance
  • Misaligned fields = padding = wasted memory
  • Reordering fields can reduce object size

Example:

// Less efficient
class Product {
    byte status;
    int stock;
    long id;
}

// More efficient
class ProductOptimized {
    long id;
    int stock;
    byte status;
}

💡 JVM Memory Alignment and Padding

The Java Virtual Machine (JVM) aligns fields in memory for performance:

  • long, double → 8-byte alignment
  • int, float → 4-byte alignment
  • byte, boolean → 1-byte (but can mess up the layout)

If a smaller type comes before a larger one, the JVM inserts padding to maintain proper alignment. That’s invisible memory waste.

🧠 Why This Matters

If you're:

  • Working with millions of objects in memory
  • Building memory-sensitive applications
  • Trying to optimize Java heap usage

...then field order becomes a low-effort, high-impact optimization.

📏 Field Order Tip

Follow this order to minimize memory waste:

double → long → int → float → char → short → byte → boolean

🌍 Not Just a Java Thing

This isn’t exclusive to Java. Similar issues exist in:

  • C / C++ → where struct layout is critical
  • Rust → memory safety includes padding awareness
  • Go → field alignment affects struct size
  • Kotlin / Swift → built on VMs or platforms that care about memory layout

So yeah, if you're building for performance anywhere, it helps to know this.

Full Breakdown + Tools + Real Dev Story

👉 Read the full post on Medium


This content originally appeared on DEV Community and was authored by Prince


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