Java – How to Calculate Size of Objects & Arrays

This article represents a list of web pages which can help one understand the memory usage of Java objects and arrays along with examples. Please feel free to comment/suggest any other cool pages. Also, sorry for the typos.

The in-memory size of the object depends on the architecture, mainly on whether the VM is 32 or 64-bit. The actual VM implementation also matters.

  • How to calculate memory usage of a Java object?: Very simplified explanation of how one could calculate a memory of any Java object. For example, lets say, you want to calculate the memory of a Java object which holds two int variables, one boolean variable, one Long object, and a reference to other object. The memory would turn out to be following:
    • 8 bytes for the object header
    • 2 x 4 = 8 bytes for two int variables
    • 1 byte for a boolean variable
    • 8 bytes (object reference) + 8 bytes for long data type = 16 bytes for long object
    • 4 bytes for reference to some other object

    The total size of the above mentioned object will be 8 + 8 + 1 + 16 + 4 = 37 bytes + 3 bytes (for padding) = 40 bytes.

  • How to calculate memory usage of a Java array?: The page presents examples on how to calculate size of a Java array object. For example, lets say a Java array consisting of 20 Integer objects. Following is the detail on the size:
    • 12 bytes for array header object (8 bytes for header and 4 bytes for storing length of the array)
    • 20 x 16 bytes = 320 bytes for integer objects.

    The total size of the said Java array object = 12 + 320 bytes = 332 bytes + 4 bytes (padding) = 336 bytes.

  • Memory usage of both Java Objects and Array: This article presents more examples on memory usage of obejcts types such as String.
Ajitesh Kumar

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