Java: memory usage issue

I have set maxmimum memory to this value

java -Xmx280m -jar ...

      

        
        
        
      

    

Code for using memory usage information (on button click):

    int mb = 1024*1024;
    //Getting the runtime reference from system
    Runtime runtime = Runtime.getRuntime();
    System.out.println("##### Heap utilization statistics [MB] #####");
    //Print used memory
    System.out.println("Used Memory:"
        + (runtime.totalMemory() - runtime.freeMemory()) / mb);
    //Print free memory
    System.out.println("Free Memory:"
        + runtime.freeMemory() / mb);
    //Print total available memory
    System.out.println("Total Memory:" + runtime.totalMemory() / mb);
    //Print Maximum available memory
    System.out.println("Max Memory:" + runtime.maxMemory() / mb);

      

        
        
        
      

    

The program shows the following output (2 clicks + new tabs, etc.):

Used Memory:41
Free Memory:123
Total Memory:165
Max Memory:249
Used Memory:67
Free Memory:97
Total Memory:165
Max Memory:249

      

        
        
        
      

    

However, when I look at the Linux performance monitor (Centos 7) it shows 330mb

for this java process. How can this be explained?