Object Creation with new, and Aliasing

The new keyword builds an object out on the heap and gives you back its address, a reference. That reference is what actually lives in your variable — not the object itself. An int or double variable holds its value directly, but an object variable holds a pointer to where the real data sits. This is why object variables behave so differently from primitive ones.

When you write b = a for two array or object variables, Java copies the reference, not the array or object it points to. After that line, a and b are two separate names for the exact same thing on the heap. There is no second object anywhere. Change the object through b, using b[0] = 99 for example, and that change is visible through a too, because a was never a separate array — it was always this one, now sitting with a 99 in slot zero.

This is called aliasing, and it is the single most important mental model for the rest of the course. Method calls that pass objects as arguments hand over a copy of the reference the same way, which is why a method can reach into an object and mutate it permanently, but cannot make the caller's variable point somewhere new. The one exception that trips people up is immutable objects like String: operations such as concatenation never change the original object, they build a new one and only the reassigned variable points at it, so aliasing has no visible effect.