Objects and Variables Local variables – Confined to single context: allocated on stack – Primitive types such as int or object references – Must be initialized before use (or fail compilation) Objects – Shared among multiple procedure contexts – Allocated on heap using new operator – Can be initialized after creation (by constructor) 1

Variables and Objects: Example int i = 6; int j; int [] a = {1, 3, 5, 7, 9}; int [] b = new int[3]; String s = “abcdef”; String t = null; j = i; b = a; t = s; Stack i = 6 j a b s t = null heap “abcdef” j = 6 t 2

Object references All object references are uninitialized initially – Can be initialized to null, but not necessary Need to explicitly allocate the object on the heap or assign it to (the start of ) an existing object – No pointer arithmetic possible once assigned – No need to explicitly de-allocate the reference (garbage collection frees it when not in use) Can be passed to procedures and copied around 3

Example of Objects and References { Object b = null; { Object a = new Object(); b = a; } c = b.hashCode(); } 4 Reference b is allocated on stack and initialized to null Reference a is allocated on stack Object is allocated on the heap and reference a points to it b and a both point to the same object a goes out of scope, so only b points to object b goes out of scope too, so nobody points to the object. Object is automatically reclaimed by garbage collector

Object Mutability By default, Java objects are mutable – Modifications made through one reference will be visible when the object is accessed through another reference to the object Example: Arrays – int [] a = {1, 3, 5, 7, 9}; – a[3] = -1; – b = a; – b[4] = -2;

Exception: Immutable objects State of immutable object never changes once it is assigned Example: String object String s1 = “abcdef”; String s2 = “ghij”; String s3 = s1; s3 = s1 + s2; String s4 = s3; s4 = s2 + s1; “abcdef” “ghij” “abcdefghij” “ghijabcdef” 6 Heap

Group Activity: Try it yourself What happens after these ? int[ ] a = {1, 2, 3}; int[ ] b = new int[2]; int[] c = a; int x = c[0]; b[0] = x; a[1] = 6; x = b[1]; int y = a[1]; What happens after these ? String s1 = “ace”; String s2 = “f”; String s3 = s1; String s4 = s3 + s2; s1 = s4; s4 = s1 + s2; 7