How Java Runs in Summary — How Java Runs

record Bag(int[] counts) {
  int[] doubled() {
    int[] result = new int[counts.length];
    int index = 0;
    for(int n: counts) {
      result[index] = n * 2;
      index += 1;
    }
    return result;
  }
  void addOne() {
    int index = 0;
    for(int n: counts) {
      counts[index] = n + 1;
      index += 1;
    }
  }
}
int[] start = { 1, 2 };
Bag x = new Bag(start);
Bag y = new Bag(x.counts);
int[] more = y.doubled();
void main() {
  x.addOne();
}

Java hands out references in the order objects are created, starting at @1001. Fill in the memory picture for when main finishes.

Names
top level: start = @1001, x = @1002, y = @1003, more =
y.doubled() on , returns @1004
main()
x.addOne() on

Heap
@1001:
@1002: Bag[counts=@1001]
@1003: Bag[counts=]
@1004:

Still at the end of main, what does each expression produce?

x == y
x.equals(y)
start == y.counts
Arrays.equals(start, more)

This test fails. Why?

record Bag(int[] counts) { }
void main() {
  assertEquals(new Bag(new int[]{ 1, 2 }), new Bag(new int[]{ 1, 2 }));
}

The two counts fields hold different arrays, and a record’s equals compares an array field with the array’s own equals, which checks the reference.
assertEquals compares the two Bags with ==.
assertEquals uses .equals(), not ==. That is why two Cars with only String and int fields passed.
The two arrays have different contents.
Both arrays are { 1, 2 }. Their contents are the same; they are just two separate arrays.
Records always compare by reference.
A record with only String and int fields passes this kind of test, like the first Car. It’s the array field that compares by reference.