Using string vs byte[] as value, Memory usage in map

Question

I read it in multiple places that using byte[] instead of string would save you memory. I wanted to test it out using jol .

Here my test:

public static void main (String[] args) throws java.lang.Exception{
    System.out.println(VMSupport.vmDetails());
    String StrByte   = GraphLayout.parseInstance(sizeOfStrByteMap(100000)).toFootprint();
    String ByteByte = GraphLayout.parseInstance(sizeOfByteByteMap(100000)).toFootprint();
    String StrStr     = GraphLayout.parseInstance(sizeOfStrStrMap(100000)).toFootprint();
    System.out.println(StrByte);
    System.out.println(ByteByte);
    System.out.println(StrStr);
}

public static HashMap<String, String> sizeOfStrStrMap(int size) {
    String value = "this is the sample value";

    HashMap<String, String> map = new HashMap<>();
    for (int i = 0; i < size; i++) {
        map.putIfAbsent(Integer.toString(i), value);
    }
    return map;
}

public static HashMap<String, byte[]> sizeOfStrByteMap(int size) {
    byte[] value = "this is the sample value".getBytes();

    HashMap<String, byte[]> map = new HashMap<>();
    for (int i = 0; i < size; i++) {
        map.putIfAbsent(Integer.toString(i), value);
    }
    return map;
}

public static HashMap<byte[], byte[]> sizeOfByteByteMap(int size) {
    byte[] value = "this is the sample value".getBytes();

    HashMap<byte[], byte[]> map = new HashMap<>();
    for (int i = 0; i < size; i++) {
        map.putIfAbsent(Integer.toString(i).getBytes(), value);
    }
    return map;
}

Here is my result:

Running 64-bit HotSpot VM.
Using compressed oop with 3-bit shift.
Using compressed klass with 3-bit shift.
Objects are 8 bytes aligned.
Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

// StrByteMap
java.util.HashMap@15327b79d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1       184       184   [B
    100000        31   3120000   [C
         1   1048592   1048592   [Ljava.util.HashMap$Node;
    100000        24   2400000   java.lang.String
         1        48        48   java.util.HashMap
    100000        32   3200000   java.util.HashMap$Node
    300003             9768824   (total)

// ByteByteMap
java.util.HashMap@a9d12ad footprint:
     COUNT       AVG       SUM   DESCRIPTION
    100001        24   2400184   [B
         1   1048592   1048592   [Ljava.util.HashMap$Node;
         1        48        48   java.util.HashMap
    100000        32   3200000   java.util.HashMap$Node
    200003             6648824   (total)


// StrStrMap
java.util.HashMap@716d90fad footprint:
     COUNT       AVG       SUM   DESCRIPTION
    100001        31   3120344   [C
         1   1048592   1048592   [Ljava.util.HashMap$Node;
    100001        24   2400024   java.lang.String
         1        48        48   java.util.HashMap
    100000        32   3200000   java.util.HashMap$Node
    300004             9769008   (total)

As you can see, memory usage between StrByteMap and StrStrMap is almost identical. Am I testing it wrong here?

UPDATE : Please see @Amod Pandey's question below, I would also like to know why.

Answer 1

For Map test you are putting in the same value reference so it is not going to use much space. In the same way that you have a different key you need to make the values different or as you see, the choice of value type doesn't make much difference.

Answer 2

I find it strange.

// StrByteMap
java.util.HashMap@15327b79d footprint:
     COUNT       AVG       SUM   DESCRIPTION
         1       184       184   [B
    100000        31   3120000   [C

There is one instance of byte arrary and 100000 instances of char array. Which is 100001 char array in the StrStr case

// StrStrMap
java.util.HashMap@716d90fad footprint:
     COUNT       AVG       SUM   DESCRIPTION
    100001        31   3120344   [C

So even if you have stored byte array the memory footprint is of char array!!

Another point is for ByteByte the the Avg size of byte array object is 24 which is less than byte array size of 184 (StrByteMap) and shouldn't the total count in all three cases be the same.