计算一个对象占用的内存

/**
* 计算一个MAP对象占用的内存
* @param args
*/
public static void main(String[] args) {
System.gc();
        long total = Runtime.getRuntime().totalMemory(); //单位：byte
        long m1 = Runtime.getRuntime().freeMemory();
        System.out.println("total:" + total);
        System.out.println("before:" + (total - m1));
         
        Map<Object,Object> map = new HashMap<Object,Object>();
        for(int i=0; i < 400; i++){
            map.put(new Object(),new Object());
        }
        long total1 = Runtime.getRuntime().totalMemory();
        long m2 = Runtime.getRuntime().freeMemory();
        System.out.println("after:" + (total1 - m2));
        System.out.println(map.toString());
}