What is "(something else)" in jol GraphLayout output?
When using jol's GraphLayout class to print the graph of objects referenced from an object instance, some of the output entries say "(something else)" instead of a type and reference path. For example, consider the following code that prints the graph of a list of 20 random Integer objects:
List<Integer> foo = new Random().ints(20).boxed().collect(Collectors.toList());
System.out.println(GraphLayout.parseInstance(foo).toPrintable());
This code prints:
java.util.ArrayList object externals:
ADDRESS SIZE TYPE PATH VALUE
d642ecc8 24 java.util.ArrayList (object)
d642ece0 16 java.lang.Integer .elementData[0] 212716192
d642ecf0 56 (something else) (somewhere else) (something else)
d642ed28 16 java.lang.Integer .elementData[1] 1503736768
d642ed38 16 java.lang.Integer .elementData[2] -2099759732
d642ed48 16 java.lang.Integer .elementData[3] 445566433
d642ed58 16 java.lang.Integer .elementData[4] -1528625708
d642ed68 16 java.lang.Integer .elementData[5] -555424299
d642ed78 16 java.lang.Integer .elementData[6] 1607595284
d642ed88 16 java.lang.Integer .elementData[7] 763466772
d642ed98 16 java.lang.Integer .elementData[8] 638331919
d642eda8 16 java.lang.Integer .elementData[9] -1742026575
d642edb8 16 java.lang.Integer .elementData[10] 1920101909
d642edc8 80 (something else) (somewhere else) (something else)
d642ee18 16 java.lang.Integer .elementData[11] 2001035318
d642ee28 16 java.lang.Integer .elementData[12] -1920666937
d642ee38 16 java.lang.Integer .elementData[13] -991335829
d642ee48 16 java.lang.Integer .elementData[14] -47760298
d642ee58 16 java.lang.Integer .elementData[15] 855824902
d642ee68 104 [Ljava.lang.Object; .elementData [212716192, 1503736768, -2099759732, 445566433, -1528625708, -555424299, 1607595284, 763466772, 638331919, -1742026575, 1920101909, 2001035318, -1920666937, -991335829, -47760298, 855824902, 2137884845, -226328690, 1472718384, 890105604, null, null]
d642eed0 16 java.lang.Integer .elementData[16] 2137884845
d642eee0 16 java.lang.Integer .elementData[17] -226328690
d642eef0 16 java.lang.Integer .elementData[18] 1472718384
d642ef00 16 java.lang.Integer .elementData[19] 890105604
Searching for jol "something else" on DuckDuckGo and Google did not return any useful hits.
What do the "(something else)" entries represent?
"(something else)" means just that -- something not part of this object graph. There may be other (live or garbage) objects there, or just a gap in the heap for some VM-internal reason.
Note that the table of objects in the graph is sorted by address. The ArrayList's elementData array is lazily initialized when the first element is added, and when the tenth element is added to the list, elementData is reallocated to make room for more elements. This creates gaps in the heap area corresponding to the (now-garbage) arrays, and jol prints them as "(something else)" entries. The code responsible for printing these entries is GraphLayout line 246 (at least as of this posting).
jol prints this information about heap gaps to aid in understanding garbage collector behavior. Some of the later jol examples demonstrate this, such as the compaction example which shows how the objects referenced from an ArrayList are initially sparse but are compacted by the garbage collector.
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