Need to figure out what is the difference between ... and arrays in JAVA, also an array list.
It seems we can use both as unlimited, but ... is rarely used.
Really appreciate any help you can.
The three dots can only be used in a method argument, and are called 'varargs'. It means you can pass in an array of parameters without explicitly creating the array.
private void method(String[] args) {} is called like method(new String[]{"first", "second"});
private void method(String... args) {} is called like method("first", "second");
An array is a fixed length collection of objects. e.g. new int[5];
An ArrayList is a variable length collection of objects. e.g. new ArrayList<Integer>();
The ... in variadic functions is a part of a method signature denoting an array of parameters. e.g. public void printLines(String... lines)
In other words, method(String...) means passing a variable number of parameters to the method.
Related
My question is in java we know we can't create dynamic arrays , because when ever we are going to initialize values for array indexes before that we need to define the array size. But we all know that there is a java feature called variable length arguments ,which will create a dynamic array.
Best Ex:public static void main (String... args)
So using this variable length arguments we can actually insert any amount of elements for the args array. What is this contradiction ,basic rules of java saying you can't and but another feature enables to do so.
But we all know that there is a java feature called variable length arguments ,which will create a dynamic array.
No, it won't. It will create an array of a static size, and that static size is defined (in your example) by the number of command line arguments. There's no way to increase or decrease the size of this array after it's created. This is really no different than creating a fixed array of a size you calculate programmatically.
If you want an array of a dynamic (i.e. changing size) then Java doesn't offer that. You'll need to use a list, or another collection, instead.
When it comes to vararg methods, you can think of them as methods that accept an array:
public void f(String... args)
is the same as
public void f(String [] args)
This is merely a syntactic sugar that allows the method defined with varags to be called as:
f("a","b","c")
Instead of creating an array first
But if so, you are supposed to specify the length of array when you create it (outside) this specific function f will accepts arrays of any length in runtime
There is no contradiction. What doesn't change is the size of an array that has been created.
So using this variable length arguments we can actually insert any amount of elements for the args array
Yes, (String... args) allows you to specify any size for the array, just as:
main(String[] args) //called with (new String[]{"a", "b", "c"})
Lets you choose any size for args. You do the same thing by calling main(String...) with
main("a", "b", "c")
What you don't realize is that neither of the resulting arrays can change in size. The size of the array is only dynamic as far as it does not need to be known at compile time, but once created (at runtime), the size of the array cannot be changed.
I have numbers[x][y] and int pm2 = 0;. Is there a way to pass on this Mult-Array onto public static boolean checkNumber(int[] list, int num)? <- the parameters has to be used this way.
I invoked checkNumber(numbers[x][y], pm2);
I need to use the checkNumber method to check if a number has already been entered and returns true if the number is present and false if number is absent.
I am allowed to use multiple methods thought so I did have an idea of doing numbers[x][0] , numbers[x][1] etc, etc and invoking them into multiple checkNumber() methods. I was just wondering if there's a shorter way.
You have single dimensional array as parameter.
So you have to pass one at a time probably in loop.
I was just wondering if there's a shorter way.
No there isn't. The Java language doesn't support any kind of array "slicing", and you can't subvert the type system to allow you to refer use an array with a different type to what it really has.
You need to implement your idea of iterating the int[] component array of the int[][], passing each one to checkNumber(int[], int). Something like this:
for (int[] subarray : numbers) {
checkNumbers(subarray, pm2);
}
I want to get some views on the behavior of the following program :
package main;
import java.util.Arrays;
public class StringAnagram {
public static void main(String args[]) {
String a = "aabbaabb";
char[] aArr = a.toCharArray();
Arrays.sort(aArr); //1
Arrays.sort(a.toCharArray()); //2
System.out.println(aArr); // Sorted
System.out.println(a.toCharArray()); // UnSorted
}
}
According to me statement 1 sorts the character array referenced by aArr but when it comes to statement 2 the sorting of character array is taking place but somehow the sorted array is not referenced by any variable, so the behavior is lost. Could someone please help me with the same.
Yes. You are right.
Each call to toCharArray() actually creates a new array instance with the characters in the string.
In the case of aArr, you actually refer to the new array instance, you use aArr to sort. Its the array instances referred by the variable aArr which gets sorted.
But when you pass a.toCharArray() to Array.sort() method, you are passing array instance which you don't have a variable referring to. The array instance gets sorted but you don't have any reference.
When you you call println using a.toCharArray() again a new array instance is created and passed to println which is obviously unsorted.
Well, let's see what happens.
First, a.toCharArray() is called. This returns a new char array containing the chars "aabbaabb".
Then, this array is sorted.
You didn't give yourself a way to access the array, so you can't. This is what "lost" means here. Nothing special or magic happened, you just made an array you can't access. It's not going to waste memory - the garbage collector will detect that it can't be accessed, and destroy it.
It's similar to if you just did this without the sorting:
a.toCharArray();
Again, toCharArray goes and makes an array for you, and you don't give yourself a way to use it, so it's also "lost". The sorting was a red herring.
Arrays.sort(a.toCharArray());
This sorts the array returned by a.toCharArray() and not the String a. So whenever you do a.toCharArray() you get a separate(new) Character array of the String. Hence it is unsorted.
Essentially, I want to have a method:
public void setArgs(String... vals)
{
for (String s: vals)
// Here I can add s every string passed to an array list
The thing I'm wondering is: can I simply store all of the arguments passed to setArgs in an Array, instead of bothering with an ArrayList? I know arrays are immutable in java, so I was wondering if you can somehow there's an easy way to extract the arguments without using a loop.
Thanks.
Yes, Using Arrays class asList() method
List<String> argsList= new ArrayList<String>(Arrays.asList(vals));
How do you set a variable equal to an array in Java. For example I have a class for input and a class for calculations which hold arrays. when I accept the user input from the input class how do I pass that variable into the array of my calculation class?
You should look into varargs. Code sample below:
public MyClass method(String ...arg);
You can call this method as :
method("test1", "test2", "test3"); // with arbitrary number of values.
Or as
String[] test = something;
method(test);
Unless you have strict requirements to use arrays, you should probably be using a Collection, like a List.
For example, if you're trying to manage an array of ints, you could instead do:
List<int> intList = new ArrayList<int>();
Then, if you really need the data in the form of an array, you can do:
intList.toArray();
Which would return an array holding the integer values in your list. Lists are easier to read and use.