当间接在数组内时，字节原语是否成为内存中的字节？

Question

I have heard that a byte by itself takes up 4 bytes in memory, and a byte inside a byte array takes 1 byte, but what about a byte member variable inside an object that is within an array?

class SomeObject {
    byte iBite;
}

public static void main(String[] args) {
    SomeObject[] objs = ...
}

Will each SomeObject have its iBite variable only be 1 byte in memory?

Answer 1

A byte as a local variable is implemented as an int , so it takes 4 bytes.

A byte as a field of a class (like in your example) takes 1 byte of memory, but classes in memory are rounded up to multiples of 8 bytes on eg HotSpot JVMs. That said, if you have a class with multiple byte fields (or char or short fields), those will make more efficient use of memory.

Arrays are similar: each byte will take 1 byte, but the array as a whole will be rounded up to a multiple of 8 bytes on eg HotSpot JVMs.

You can experiment with this by hand using http://openjdk.java.net/projects/code-tools/jol/ . If you use it, for example, on

public static class A {
    boolean f;
    byte g;
    int h;
}

I get

Running 64-bit HotSpot VM.
Using compressed oop with 3-bit shift.
Using compressed klass with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

org.openjdk.jol.samples.JOLSample_01_Basic.A object internals:
 OFFSET  SIZE    TYPE DESCRIPTION                    VALUE
      0    12         (object header)                N/A
     12     4     int A.h                            N/A
     16     1 boolean A.f                            N/A
     17     1    byte A.g                            N/A
     18     6         (loss due to the next object alignment)
Instance size: 24 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 6 bytes external = 6 bytes total

which exhibits pretty clearly that boolean and byte take one byte as object fields.

As you'd expect, char and short are 2 bytes, int and float are 4 bytes, long and double are 8 bytes.

https://stackoverflow.com/a/14782255/869736 explains some details on Dalvik, including that currently small fields like byte are in fact implemented with 4 bytes. Remember that these details are going to be VM-dependent.