オブジェクトのサイズを調べる簡単なコード

Test.java

import java.util.ArrayList;

public class Test {
	private Class<?>[] CLASSES = new Class<?>[] {
			Object.class,
			HasOneField.class,
			HasTwoField.class,
			HasThreeField.class,
			HasLongField.class,
			HasTwoLongField.class,
			HasIntAndLongField.class,
			HasObjectField.class,
			HasTwoObjectField.class,
			HasEightByteField.class,
			HasNineByteField.class,
			ArrayList.class
	};

	public static void main(String[] args) {
		try {
			for (int i = 0; i < 2; ++i) {
				new Test().test(i);
			}
		} catch (InstantiationException e) {
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			e.printStackTrace();
		}
	}

	private void test(int i) throws InstantiationException, IllegalAccessException {
		for (Class<?> c : CLASSES) {
			System.gc();
			long beforeFree = Runtime.getRuntime().freeMemory();

			Object instance = c.newInstance();

			System.gc();
			long afterFree = Runtime.getRuntime().freeMemory();

			if (i != 0) {
				System.out.printf(
						"%30s,%2d fields,%3d bytes%n",
						instance.getClass().getName(),
						c.getDeclaredFields().length,
						beforeFree - afterFree);
			}
		}
	}

	static class HasOneField {
		private int field;
	}

	static class HasTwoField {
		private int field1;
		private int field2;
	}

	static class HasThreeField {
		private int field1;
		private int field2;
		private int field3;
	}

	static class HasLongField {
		private long field1;
	}

	static class HasTwoLongField {
		private long field1;
		private long field2;
	}

	static class HasIntAndLongField {
		private int field1;
		private long field2;
	}

	static class HasObjectField {
		private Object field1;
	}

	static class HasTwoObjectField {
		private Object field1;
		private Object field2;
	}

	static class HasNineByteField {
		private byte field1;
		private byte field2;
		private byte field3;
		private byte field4;
		private byte field5;
		private byte field6;
		private byte field7;
		private byte field8;
		private byte field9;
	}

	static class HasEightByteField {
		private byte field1;
		private byte field2;
		private byte field3;
		private byte field4;
		private byte field5;
		private byte field6;
		private byte field7;
		private byte field8;
	}
}