Java objects & references (recap)

Variables hold references, not objects

This is the one mental model the rest of COMP 308 is built on, so re-anchor it before the advanced material.

In Java there are exactly two kinds of value:

• Primitives — byte short int long float double boolean char. The variable is the value. Assigning copies the value.
• Reference types — everything else (objects, arrays, String). The variable holds a reference (a handle) to an object that lives elsewhere. Assigning copies the reference, not the object.

The slogan from Thinking in Java is: "you manipulate objects with references." A reference is like a TV remote — the remote is not the TV, and several remotes can drive one TV.

Stack vs heap

Local variables and method parameters live on the stack (per call, short-lived). Objects created with new live on the heap and are reclaimed by the garbage collector once nothing references them.

int x = 10;                 // primitive: the int 10 sits on the stack
String s = new String("hi");// reference s (stack) -> String object (heap)

Aliasing: two references, one object

Because assignment copies the reference, two variables can point at the same object. Mutating through one is visible through the other:

StringBuilder a = new StringBuilder("cat");
StringBuilder b = a;        // b now refers to the SAME object as a
b.append("s");              // mutate through b
System.out.println(a);      // prints "cats" -- a sees the change

Contrast with primitives, where each variable is independent:

int p = 5;
int q = p;                  // q gets a COPY of the value 5
q = 99;
System.out.println(p);      // prints 5 -- p is unaffected

null and the default

A reference that points at no object holds null. Fields of reference type default to null; calling a method on a null reference throws NullPointerException at runtime.

Pass-by-value (the careful version)

Java is always pass-by-value. For a reference type the reference is passed by value, so the method can mutate the pointed-to object but cannot make the caller's variable point somewhere new:

static void rename(StringBuilder sb) {
    sb.append("!");          // visible to caller (same object)
    sb = new StringBuilder("x"); // reassigns the LOCAL copy only
}

After rename, the caller's builder ends in ! but is not the new "x" object.

Identity vs equality (preview)

a == b for references asks "are these the same object?" (identity). a.equals(b) asks "are these objects logically equal?" (value). For String, always use .equals — == can lie. We return to this in the Strings and Collections modules.