SizeOfAndStringPool.java

public class SizeOfAndStringPool {
  public static void main(String[] args) throws Exception  {
    long    m0, m1, m2, m3, m4, m5;
    String  s1, s2, s3, s4, s5; 
    
    Runtime rt = Runtime.getRuntime();
    rt.gc();
    
    m0 = rt.freeMemory();
    
    // Very long string
    s1 = "abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ2abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ3abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ4abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ5abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ6abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ7abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ8abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ9abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ10"; 
    m1 = rt.freeMemory();
    
    s2 = s1; // no new obj created
    m2 = rt.freeMemory();
    
    // add an even longer one, 2 new objs created
    s3 = s2 + "add_abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ2abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ3abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ4abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ5abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ6abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ7abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ8abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ9abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ10"; 
    m3 = rt.freeMemory();
    
    s4 = s3; // no new obj created
    m4 = rt.freeMemory();
    
    // Assign the same string of s1 to s5
    // since string pooling, no new obj created
    s5 = "abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ2abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ3abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ4abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ5abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ6abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ7abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ8abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ9abcdefghijkmnlopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ10"; 
    m5 = rt.freeMemory();
    
    // The results are in the reasonable error range
    // since new String obj has mem-overhead too
    System.out.println("s1 took " + (m0-m1) + " bytes."); // 2456
    System.out.println("s2 took " + (m1-m2) + " bytes."); // 0
    System.out.println("s3 took " + (m2-m3) + " bytes."); // 4408
    System.out.println("s4 took " + (m3-m4) + " bytes."); // 0
    System.out.println("s5 took " + (m4-m5) + " bytes."); // 0
  }
}