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)

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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

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heap

0 0 0

“abcdef”

j = 6

t

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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 around3

Example of Objects and References

{ Object b = null; {

Object a = new Object();b = a;

} c = b.hashCode();}

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Reference b is allocated on stackand 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

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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;

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Exception: Immutable objects

• State of immutable object never changes once it is assigned

• Example: String objectString s1 = “abcdef”;String s2 = “ghij”;String s3 = s1;s3 = s1 + s2;String s4 = s3;s4 = s2 + s1;

“abcdef”

“ghij”

“abcdefghij”

“ghijabcdef”

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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;

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