Java Reference
In-Depth Information
Checking equality of arrays
Suppose a1 and a2 are of type int [] . They contain the names of array objects
(and not the arrays themselves), so a1 == a2 tests whether a1 , a2 contain the
same name rather than whether the array contents are equal. We write a method
to compare the contents:
/** = " arrays a1 and a2 are equal " */
public static boolean equals( int [] a1, int [] a2)
The method is a function, and it returns true if the arrays are equal and
false otherwise. The arrays are equal if (1) the parameters have exactly the same
value or (2) the array objects have the same length and their corresponding ele-
ments are equal.
If a1 and a2 both are null , or if they contain the same array name, the func-
tion returns true . (In many situations, a method may require its array parameters
to be not null . Here, it makes sense to allow null array arguments.)
If one of a1 and a2 contains null but the other does not, the arrays are not
equal, so the function returns false .
If a1 and a2 have different lengths, the function returns false .
At this point, the arrays have the same length, so their elements have to be
compared for equality. We use the loop schema from Sec. 8.3.2:
See lesson
page 8-3 to get
this program.
// Process elements of c[h..k-1] , in order
// inv: h≤i≤k and c[h..i-1] has been processed
for ( int i= h; i != k; i= i + 1)
{ Process c[i] }
Instead of array c , we have a1 ; instead of h , we have 0 ; and instead of k we have
a1.length . We also update the invariant.
Activity 8-3.5
shows you how
to test it.
/** = " arrays a1 and a2 are equal " */
public static boolean equals( int [] a1, int [] a2) {
if (a1 == a2) { return true ; }
if (a1 == null || a2 == null ) { return false ; }
if (a1.length != a2.length) { return false ; }
// Return false if a1[i] != a2[i] for some i .
// inv: 0 ≤ i ≤ a1.length and a1[0..i-1] = a2[0..i-1]
for ( int i= 0; i != a1.length; i= i + 1) {
if (a1[i] != a2[i])
{ return false ; }
return true ;
Figure 8.3:
Function equals , to check the equality of arrays
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