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Calculating the Memory Usage of Objects and Arrays

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Calculating the Memory Usage of Objects and Arrays

Curious as to how Java uses memory to store objects and arrays? Read on to find out some of the math behind it and more information on just how this is done.

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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 objects. 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 objects types such as String.

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Topics:
java ,memory usage ,array ,object

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