Demonstrate overhead of primitives stored in an object

PrimitiveOverhead.java

/**
 * Demonstrate overhead of primitives stored in an object.  Note that each
 * object has 12 bytes of overhead for the class pointer, flags, and monitor and
 * the JVM pads each object to the next 8 byte alignment.  Thus an object with
 * two byte fields will consume 16 bytes and an object with two int fields will
 * consume 24 bytes:
 * 
 * $ javac PrimitiveOverhead.java ; java PrimitiveOverhead
 * 
 *  num     #instances         #bytes  class name
 * ----------------------------------------------
 *    1:      16777216      402653184  PrimitiveOverhead$IntClass
 *    2:      16777216      268435456  PrimitiveOverhead$ByteClass
 *    3:           315      134230920  [Ljava.lang.Object;
 */

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class PrimitiveOverhead {
    static final class ByteClass { byte x, y; }
    static final class IntClass { int x, y; } 
    
    private static int LENGTH = 16 * 1024 * 1024;
    private static final Object[] byteArray = new Object[LENGTH];
    private static final Object[] intArray = new Object[LENGTH];
    
    public static void main(String[] args) throws Exception {
        for (int i = 0; i < LENGTH; ++i) { 
            byteArray[i] = new ByteClass();
            intArray[i] = new IntClass();
        }   
        System.out.print(runCommand(new String[] {
                "sh", "-c", "jmap -histo:live $PPID | head -6"}));
    }           
    
    private static String runCommand(final String[] args) throws IOException {
        StringBuilder builder = new StringBuilder();
        Process child = Runtime.getRuntime().exec(args);
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(
                child.getInputStream()))) {
            String line;
            while ((line = reader.readLine()) != null) {
                builder.append(line).append('\n');
            }
        }
        return builder.toString();
    }