An array is an ordered list with fixed size.
The most common ways to create an array are:
type[] variableName = new type[size];
type variableName[] = new type[size];
Array have fixed size, so you have to specify array size or give initial values (which specifies size implicitly). When you don't give initial values, the default values are given. Though Array's size can not change, array's contents can change, so array is mutable.
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OCAJP>cat test.java
class test {
public static void main(String[] args) {
String[] stringArr = new String[9];
int[] intArr = new int[3]; //initial value is 0,0,0, size is 3
//boolean[] booArr = new boolean[]; //array dimension missing
//String foo[] = new foo[2]; //array name is not a type
//String foo[] = new foo[2]; //array name is not a type
boolean[] booleanArr = new boolean[5];
Boolean [] booArr1 = new Boolean[2]; //null, null, size is 2
Boolean booArr2 [] = new Boolean[4];
Boolean booArr3[] = new Boolean[2]; //false, false, size is 2
//initialize
int intArr2[] = new int[] {1,2,3}; //initial value is 1, 2, 3, size is 3
int[] intArr4 = new int[]{0,9,2};
int[] intArr1 = {1,3,5,7};
//char[] charArr = new Char[] {'a', 'b'}; //Char can not find symbol
//int intArr3 [] = new int[2]{1,2}; //not a statement
for(int i: intArr4) {
System.out.print(i+" ");
}
System.out.println();
}
}
OCAJP>javac test.java
OCAJP>java test
0 9 2
Array is an object (although weird looking). That means array can be initialized with new method, it extends object class, and has methods.
For example, while int i; is a primitive type variable stores a number, int[] b; is an array reference variables that points to an array of int object or null.
OCAJP>cat test.java
class test {
public static void main(String[] args) {
Integer[] a = new Integer[]{1, 3, 4};
for(int i:a)System.out.print(i+" ");
Integer[] b = a; //now both a, b refer to the same array object
b[0] = 10; //change object through b
System.out.println();
for(int i:a)System.out.print(i+" ");
System.out.println(a.equals(b)); //true
System.out.println(a == b); //true
Integer c[] = b.clone(); //c refer to a different array object
for(int i:c)System.out.print(i+" ");
for(int i:b)System.out.print(i+" ");
System.out.print("c==b: ");
System.out.println(c==b); //false
System.out.print("c.equals(b): ");
System.out.println(c.equals(b)); //false
c[2] = 9;
for(int i:c)System.out.print(i+" ");
for(int i:b)System.out.print(i+" "); //b didn't change
System.out.println(c.equals(b)); //false
b[2] = 9;
for(int i:a)System.out.print(i+" ");
System.out.print("equals: ");System.out.println(c.equals(a)); //false
System.out.print("== compare referenced object:"); System.out.println(c==a); //false
//Long[] c = b; //incompatible types: Integer[] cannot be converted to Long[]
//Short d[] = b; //incompatible types: Integer[] cannot be converted to Short[]
//int[] c = b; //incompatible types: Integer[] cannot be converted to int[]
}
}
OCAJP>javac test.java
OCAJP>java test
1 3 4
10 3 4 true
true
10 3 4 10 3 4 c==b: false
c.equals(b): false
10 3 9 10 3 4 false
10 3 9 equals: false
== compare referenced object:false
Besides methods inherited from object class, array class only have one method: length, which returns the size of the array. We can copy, compare, sort, search etc. using static methods of java.util.Arrays class.
OCAJP>cat test.java
import java.util.*;
class test {
public static void main(String[] args) {
String[] a = new String[] {"cat", "dog", "bird", "9", "10"};
String[] b = new String[] {"cat", "dog", "bird", "9","10"};
//String c [] = Arrays.copyOf(b); //no suitable method found for copyOf(String[])
System.out.println(Arrays.equals(a,b)); //compare contents
System.out.println(a.equals(b)); //array didn't override Object class's equals method
for(String s: a) System.out.println(s);
Arrays.sort(a); //sort by alphabetic order
for(String s: a) System.out.println(s);
System.out.println(Arrays.binarySearch(a, "dog")); //sorted
System.out.println(Arrays.binarySearch(b, "dog")); //unsorted, unpredicatable result
System.out.println(Arrays.binarySearch(a, "nosuchthing")); //sorted
System.out.println(Arrays.binarySearch(b, "nosuchthing")); //unsorted, unpredicatable result
}
}
OCAJP>javac test.java
OCAJP>java test
true
false
cat
dog
bird
9
10
10
9
bird
cat
dog
4
-6
-6
-6
In the above example,
System.out.println(Arrays.binarySearch(a, "nosuchthing"));
print out -6.
How it is calculated?
Arrays.sort(a); //sort by alphabetic order
a became {"10", "9", "bird", "cat", "dog"}.
if nosuchthing is added into, the array will be {"10", "9", "bird", "cat", "dog", "nosuchthing"}.
The index of "nosuchthing" is 5.
negate the index then minus 1 we got -6.
Finally, let's take a look at some OCA tricks.
OCAJP>cat test.java
import java.util.*;
class test {
public static void main(String... args) {
int a, b = 3, c[];
c = new int[3];
//a = new int[3]; //not compile
a = 2;
int[] d, e, f = {2,3}, g;
d = e = f;
List<Integer> m = Arrays.asList(f);
//List<Integer> m = f.asList(); //not comile
List<Integer> m = Arrays.asList(f);
//List<Integer> m = f.asList(); //not comile
//d = 2; //not compile
//g = args; //not compile
String[] h = args;
h[0] = "change parameter";
//System.out.println(h.capacity); //not compile
//System.out.println(h.capacity()); //not compile
//System.out.println(h.capacity); //not compile
//System.out.println(h.capacity()); //not compile
for(String arg: h) System.out.println(arg);
args = null;
for(String arg: h) System.out.println(arg);
}
}
OCAJP>javac test.java
OCAJP>java test a b c
change parameter
b
c
change parameter
b
c
OCAJP>java test
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
at test.main(test.java:14)
