How do I know the memory size in virtual machine?

If you want to know the free memory, and the total memory available in the Java runtime environment then you can use the following code snippet to check.

package org.kodejava.lang;

public class MemoryExample {
    public static void main(String[] args) {
        long freeMemory = Runtime.getRuntime().freeMemory();
        long totalMemory = Runtime.getRuntime().totalMemory();

        System.out.println("Free Memory  = " + freeMemory);
        System.out.println("Total Memory = " + totalMemory);
    }
}

Here is the result of the code snippet above:

Free Memory  = 1018439896
Total Memory = 1029177344