http://qiita.com/disktnk/items/6d86e162f9ef10da083d

ObjectSizeTest.java

package objectsize;

import java.util.ArrayList;
import java.util.List;

import net.sourceforge.sizeof.SizeOf;

import org.junit.Test;
import org.openjdk.jol.info.GraphLayout;

import com.twitter.common.objectsize.ObjectSizeCalculator;

public class ObjectSizeTest {

    @Test
    public void test() {
        String s = "";
        String s2 = "0123456789";
        List<String> list = new ArrayList<>();
        list.add(s);
        list.add(s2);
        {
            SizeOf.skipFinalField(false);
            SizeOf.skipStaticField(true);
            SizeOf.setMinSizeToLog(40);
            long sSize = SizeOf.deepSizeOf(s);
            long s2Size = SizeOf.deepSizeOf(s2);
            long listSize = SizeOf.deepSizeOf(list);
            System.out.println("[sizeOf]empty string: " + sSize);
            System.out.println("[sizeOf]string      : " + s2Size);
            System.out.println("[sizeOf]string list : " + listSize);
        }
        {
            long sSize = ObjectSizeCalculator.getObjectSize(s);
            long s2Size = ObjectSizeCalculator.getObjectSize(s2);
            long listSize = ObjectSizeCalculator.getObjectSize(list);
            System.out.println("[tc]empty string: " + sSize);
            System.out.println("[tc]string      : " + s2Size);
            System.out.println("[tc]string list : " + listSize);
        }
        {
            long sSize = GraphLayout.parseInstance(s).totalSize();
            long s2Size = GraphLayout.parseInstance(s2).totalSize();
            long listSize = GraphLayout.parseInstance(list).totalSize();
            System.out.println("[jol]empty string: " + sSize);
            System.out.println("[jol]string      : " + s2Size);
            System.out.println("[jol]string list : " + listSize);
        }
    }
}