Found the answer
public class App {
public static void main(String[] args) throws Exception {
Car car = new Car();
Bicycle bicycle = new Bicycle();
Van van = new Van();
Object[] racers = {car, bicycle, van};
for(Object x : racers) {
System.out.println(x.getClass());
((Vehicle) x).go(); // this is the only change I made
}
}
}
Found the answer
public class App {
public static void main(String[] args) throws Exception {
Car car = new Car();
Bicycle bicycle = new Bicycle();
Van van = new Van();
Object[] racers = {car, bicycle, van};
for(Object x : racers) {
System.out.println(x.getClass());
((Vehicle) x).go(); // this is the only change I made
}
}
}
The following would have worked
Vehicle[] racers = {car, bicycle, van};
for (Vehicle x : racers) { ... x.go();
Dynamic detection does works too. You could use the modern Stream<?>.
Object[] racers = {car, bicycle, van};
Arrays.stream(racers)
.filter(r -> r instanceOf(Vehicle)
.map(Vehicle.class::cast)
.forEach(r -> {
r.go(); ...
};
If you're looping through an array, it shouldn't matter - the enhanced for loop uses array accesses anyway.
For example, consider this code:
public static void main(String[] args)
{
for (String x : args)
{
System.out.println(x);
}
}
When decompiled with javap -c Test we get (for the main method):
public static void main(java.lang.String[]);
Code:
0: aload_0
1: astore_1
2: aload_1
3: arraylength
4: istore_2
5: iconst_0
6: istore_3
7: iload_3
8: iload_2
9: if_icmpge 31
12: aload_1
13: iload_3
14: aaload
15: astore 4
17: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
20: aload 4
22: invokevirtual #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
25: iinc 3, 1
28: goto 7
31: return
Now change it to use an explicit array access:
public static void main(String[] args)
{
for (int i = 0; i < args.length; i++)
{
System.out.println(args[i]);
}
}
This decompiles to:
public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iload_1
3: aload_0
4: arraylength
5: if_icmpge 23
8: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
11: aload_0
12: iload_1
13: aaload
14: invokevirtual #3; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
17: iinc 1, 1
20: goto 2
23: return
There's a bit more setup code in the enhanced for loop, but they're basically doing the same thing. No iterators are involved. Furthermore, I'd expect them to get JITted to even more similar code.
Suggestion: if you really think it might make a significant difference (which it would only ever do if the body of the loop is absolutely miniscule) then you should benchmark it with your real application. That's the only situation which matters.
This falls squarely in the arena of micro-optimization. It really doesn't matter. Stylistically I always prefer the second because it's more concise, unless you need the loop counter for something else. And that's far more important than this kind of micro-optimization: readability.
That being said, For an ArrayList there won't be much difference but a LinkedList will be much more efficient with the second.
The better question might be: Why wouldn't you want to use a FOR loop to iterate through an array? There are many ways to iterate through an Array or a collection and there is no law that states you have to use the FOR loop. In a lot of cases, it's simply the best to use for speed, ease of use, and readability. And yet, in other cases it is not:
The Array:
int[] array = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
Display Array with the typical for loop:
for (int i = 0; i < array.length; i++) {
System.out.println(array[i]);
}
Display Array with the enhanced for loop:
for(Integer num : array) {
System.out.println(num);
}
Display Array with the do/while loop:
int i = 0;
do {
System.out.println(array[i++]);
} while (i < array.length);
Display Array with the while loop:
int j = 0;
while (j < array.length) {
System.out.println(array[j++]);
}
Display Array through Recursive Iteration:
iterateArray(array, 0); // 0 is the start index.
// The 'iterateArray()' method:
private static int iterateArray(int[] array, int index) {
System.out.println(array[index]);
index++;
if (index == array.length) {
return 0;
}
return iterateArray(array,index);
}
Display Array using Arrays.stream() (Java8+):
Arrays.stream(array).forEach(e->System.out.print(e + System.lineSeparator()));
Display Array using IntStream (Java8+):
IntStream.range(0, array.length).mapToObj(index -> array[index]).forEach(System.out::println);
Choose your desired weapon....
Considering you have an array like :
int[] array = {1,2,4,5,6};
You can use stream to iterate over it, apart from printing you can perform lot many thing over this array.
Arrays.stream(array).forEach(System.out::println);
Similarly you can do lot many action over collections as well:
List<String> myList = new ArrayList<>(); List
myList.add("A");
myList.add("B");
Stream.of(myList).forEach(System.out::println);
myList.forEach(System.out::println);
Since atleast Java 1.5.0 (Java 5) the code can be cleaned up a bit. Arrays and anything that implements Iterator (e.g. Collections) can be looped as such:
public static boolean inArray(int[] array, int check) {
for (int o : array){
if (o == check) {
return true;
}
}
return false;
}
In Java 8 you can also do something like:
// import java.util.stream.IntStream;
public static boolean inArray(int[] array, int check) {
return IntStream.of(array).anyMatch(val -> val == check);
}
Although converting to a stream for this is probably overkill.
You should definitely encapsulate this logic into a method.
There is no benefit to repeating identical code multiple times.
Also, if you place the logic in a method and it changes, you only need to modify your code in one place.
Whether or not you want to use a 3rd party library is an entirely different decision.
I am going to use a slightly different analogy, since a lot of answers are providing you with the correct information.
An array is just a list of sequential boxes containing values. Imagine it is a list of houses, each numbered from 0 (arrays in Java like many other programming languages start at 0 not 1) to n (in your case it is 2).
So if you were someone collecting donations from each house, you would start from the first house, 0, then proceed to the next house 1, then to the next house 2. You would keep track of which house you are visiting now, so that you also know which is the next one, and that is what the integer variable i is doing.
Printing numbers[i] does not make the loop cycle, in the same way that knocking on the first house does not mean you are going to knock at the second house.
The for loop is keeping track of which house numbers you need to visit next. It does it by the 3 statements it has in the same line:
for(int i = 0; i < number.length; i++)
The first statement is saying start from the first house (int i = 0). You could start wherever you like.
The second statement is saying, you want to go up to the last house, irrespective of how many houses there are. (i < number.length). This literally translates to keep looping as long as i is less than the size of the array. You might ask why isn't the condition <=. It is because we start from 0, so the length (size of the array) will be the number of the last house + 1. In your case, length = 2 + 1.
The third statement, is what is making you move to the next house. i++ is shorthand for saying i = i + 1. This statement takes place after every iteration of the loop.
So essentially your for loop first initialises i to 0, checks the condition (the second statement) and if the condition is satisfied does the first iteration, then it finally increments i, to start another iteration if the second condition is still satisfied. This loop is repeated until the second statement of your for expression is not true any more.
Finally all numbers[i] means is, since numbers is an array, the [ ] operator accesses the value at the specified index. So it is like knocking at the house with number i to see who lives there. Feeding the value to System.out.println() prints that value on screen.
Here's a breakdown of how the for-loop is executed (derived from this certification study guide)
Names as referred to below:
for(initialization code;
boolean expression;
update statements){
body
}.
The for-loop's parts are executed in this sequence:
- Initialization code
- If boolean expression is true, execute body, else exit the loop
- Body executes
- Execute update statements
- Return to Step 2
So when applied to your example, we can say:
Run initialization:
int i = 0
Iteration 1:
if(i < number.length) // -> if(0 < 3)
//run body
i++ //i becomes 1
//return to step 2 (new iteration)
Iteration 2:
if(i < number.length) // -> if(1 < 3) //i was incremented to 1
//run body
i++ //i becomes 2
//return to step 2 (new iteration)
Iteration 3:
if(i < number.length) // -> if(2 < 3) //i was incremented to 2
//run body
i++ //i becomes 3
Iteration 4 (not run):
//return to step 2 (new iteration)
if(i < number.length) // -> if(3 < 3) -> condition not met
//condition not met, stop the loop
More specifically, I do not understand how i comes to represent the contents of the array
As you can see above, every time the loop body finishes executing, the update statements are executed, which makes the i variable increment, thus allowing to access a different index of the array.
This is done advisedly, if you change your update statements to i += 2, then some indexes of the array will be skipped.
You can do an enhanced for loop (for java 5 and higher) for iteration on array's elements:
String[] elements = {"a", "a", "a", "a"};
for (String s: elements) {
//Do your stuff here
System.out.println(s);
}
String[] elements = { "a", "a", "a", "a" };
for( int i = 0; i < elements.length - 1; i++)
{
String element = elements[i];
String nextElement = elements[i+1];
}
Note that in this case, elements.length is 4, so you want to iterate from [0,2] to get elements 0,1, 1,2 and 2,3.