1. Java Array Object Chuen-Liang Chen Department of Computer Science
& Information Engineering
National Taiwan University

2. Fortran Array & C Array Fortran – a contiguous space column major
declaration & allocation – INTEGER a(2,3)
memory
a(i, j) is located at – S + (j -1) * 2 + (i - 1)
C, C++ – a contiguous space row major
declaration & allocation – int a[2][3];
memory
a[i][j] is located at – S + i * 3 + j
a(1,1)
a(2,1)
a(1,2)
a(2,2)
a(1,3)
a(2,3) S
a[0][0]
a[0][1]
a[0][2]
a[1][0]
a[1][1]
a[1][2] S

3. Java Array
an object; a special instance of subclass of java.lang.Object
an array of 3 int’s
an array of 3 shorts
an array of 3 (array of int)’s
an array of 3 (array of short)’s
int[ ]
length 3
?[0]
?[1]
?[2]
int[ ][ ]
length 3
?[0]
?[1]
?[2]
short[ ]
length 3
?[0]
?[1]
?[2]
short[ ][ ]
length 3
?[0]
?[1]
?[2]

4. Array Variable Declaration
class variable declaration
class X { int i; short j; } X x1;
32 bits
a pointer; a reference
to an object of X, to an object of subclass of X
array variable declaration
int[ ] a; OR int a[ ];
32 bits
a pointer; a reference
to an arbitrary length array of int
note:
both of int[ ] and int[ ][ ] are direct subclasses of java.lang.Object
int[ ][ ] is not the subclass of int[ ], and vise versa

5. Array Creation Expression
ordinary instance creation
class X { int i; short j; } x1 = new X();
array instance creation
int[ ] a; a = new int[3]; x1 X i j
int[ ]
length 3
?[0]
?[1]
?[2] a

6. Array Access Expression
access of instance variable
instance variables
x1.i, x1.j
initial value = 0
access of array element
instance variables
a.length (read only)
initial value = 3
a[0], a[1], a[2]
initial value = 0 x1 X i j
int[ ]
length 3
?[0]
?[1]
?[2] a

7. Operation of Array Variable
assignment of class variable
x1.i = 10; X x2 = x1; // now, x2.i == 10
assignment of array variable
a[0] = 10; int[ ] b = a; // now, b[0] == 10 x1 X i 10 j x2
int[ ]
length 3
?[0] 10
?[1]
?[2] a b

8. Array Initializer int[ ] a = {10, 20, 30}; 4 steps
array variable declaration
empty array instance creation (length = 3)
array elements initialization
assignment of array variable

9. Array of Object (1/3)
class Y { int i; Y(int p) { i = p;} void m() { … } } class Z extends Y { int j; Z(int p, int q) { super(p); j = q;} }
Y[ ] c;
declare an array variable, named c
32 bits; a pointer, a reference
to an arbitrary length array
element type: Y c

10. Array of Object (2/3) c = new Y[3]; create a 3 elements array instance
each element
32 bits; a pointer, a reference
to an object of Y, or an object of subclass of Y
initial value = null
Y[ ]
length 3
?[0]
null
?[1]
?[2] c

11. Array of Object (3/3) c[0] = new Y(10); c[1] = new Z(20, 30);
create a Y instance and a Z instance
c[1].i c[1].m()
access of field and method
precedence of field selection, array access, method invocation are the same (15L)
Y[ ]
length 3
?[0]
?[1]
?[2]
null c Y i 10 Z i 20 j 30

12. Array of Array (1/3) int[ ][ ] d; declare an array variable, named d
32 bits; a pointer, a reference
to an arbitrary length array
element type: int[ ]
Y[ ][ ] e;
declare an array variable, named e
element type: Y[ ] d e

13. Array of Array (2/3) d = new int[2][1]; d[1][0] = 10;
d = new int[2][ ]; d[1] = new int[2];
int[ ][ ]
length 2
?[0]
?[1] d
int[ ] 1 10
int[ ][ ]
length 2
?[0]
null
?[1] d
int[ ]

14. Array of Array (3/3) e = new Y[2][1]; e[1][0] = new Z(20,30);
e = new Y[2][ ]; e[1] = new Y[2];
Y[ ][ ]
length 2
?[0]
?[1] e
Y[ ] 1
null Z i 20 j 30
Y[ ][ ]
length 2
?[0]
null
?[1] e
Y[ ]