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Java array initialization second variable type

int[] myArray = new int[5];
Hey guys,
I know there are other ways to initialize an array like:
int[] myArray = {1,2,3,4,5};
But can you tell me what the second int in the top example stands for?
I think the first "int" already specifies the array to have integer values, doesn't it?

The construct in int[] myArray = { 1, 2, 3, 4, 5 }; is handled as a special case. The compiler knows that { 1, 2, 3, 4, 5 } is creating an int[] array because of the declared type.
However, the full array creation syntax for the int array is new int[5] (it can vary), which tells the compiler that a new int[] array is created, regardless of the declared type:
For example, you can declare an Object variable, and assign an int array to it:
Object myArrayObject = new int[5]; //OK: this creates an int array
Whereas this won't work:
Object myArrayObject = { 1, 2, 3, 4, 5 }; //won't compile

With int and other primitive types, you're correct, the double declaration is largely superfluous.
But consider this example:
Object [] array = new String[5];
This is valid Java code (even though it is slightly broken*).
On a more general level it follows the same pattern as when you're initialising any variable with a new object:
Foo foo = new Foo();
The first Foo is the type of the variable (what shape the box is), the second is the type of the actual object (what shape is the thing you put in the box).
Note: Starting with Java 10, you can declare local variables with the keyword var, thus eliminating the need to specify the type twice:
var array = new int[5];
The compiler will infer from this that the type of array will be the same as the object it's initialised with. (int[] in this case).
*It is broken because if you try to put into the array anything other than a String, the code only fails at runtime, not during compilation.

So when you type new, you are specifying you want java to allocate memory to the heap. How much memory? Ah yes enough memory for 5 integers, int[5]. So java will allocated enough consecutive memory on the heap to store 5 integers.

int[] myArray = new int[5];
means you declare an array that can contain integer values called myArray (by int[] myArray) and you define it as an integer-array of size 5 afterwards (= new int[5];);
These steps are made inline, you could alternatively do the same in two lines:
int[] myArray; // declaration
myArray = new int[5]; // definition

First is the data type of the array. And other int is for the intialization of 5 integer object in the array

Java/P5: How to declare an Array of Arraylists holding FloatLists?

If I can declare an Array of FloatLists: FloatList [][] values = new FloatList[3][3];
Why doesn’t it work to declare an Array of ArrayLists holding FloatLists like this: ArrayList<FloatList> [][] values = new ArrayList<FloatList>() [3][3];? OR EVEN: ArrayList<FloatList> [][] values = new ArrayList<FloatList> [3][3]();
How can this be achieved? Will it be hard to refer to the floats buried deep under its crusty texture?

Work from the inner-most type to the outer-most type. You start with FloatList:
FloatList
Then wrap that in an ArrayList:
ArrayList<FloatList>
Then you want an array of that:
ArrayList<FloatList>[]
Or a 2D array:
ArrayList<FloatList>[][]
That gives you the type for the declaration, but then you have to initialize the variable by giving it a value. Start with the array by giving it a size:
ArrayList<FloatList>[] array = new ArrayList[10];
This gives you an array of ArrayList<FloatList> objects, but they start out as null. To give them a value, you'd loop over every index in the array and use the new keyword to set the value of the index to an instance of ArrayList<FloatList>:
for(int i = 0; i < array.length; i++){
array[i] = new ArrayList<FloatList>();
}
For a 2D array, you'd use the same logic, just in a nested for loop.
Then to add a FloatList to an ArrayList at a specific index of the array, you'd do this:
array[i].add(new FloatList());
Finally, to add a float to a FloatList in an ArrayList at an index in the array, you'd do this:
array[x].get(y).append(0.5);
And to get a float out of an index in the FloatList in an ArrayList at an index in the array, you'd do this:
float f = array[x].get(y).get(z);
Putting it all together, it looks like this:
ArrayList<FloatList>[] array = new ArrayList[10];
for(int i = 0; i < array.length; i++){
array[i] = new ArrayList<FloatList>();
}
array[1].add(new FloatList());
array[1].get(0).append(0.25);
array[1].get(0).append(0.5);
array[1].get(0).append(0.75);
float f = array[1].get(0).get(2);
println(f);

ArrayList<FloatList> [][] values = new ArrayList[3][3];
Basically, you're declaring that you want an object that is a 3D array of ArrayLists, and not generating actual ArrayList objects.
Afterwards, you have to instantiate each of them, so for example:
values[0][0] = new ArrayList<>();
And so on.

It doesn't work because of the way the JVM provides Generics. Generics in Java is a front-end compiler feature that becomes raw type usages and casts at execution time.
What is the compiler doing when I use generics?
Here's a terribly-contrived example. Let's say I want to create a List<String> to store command line arguments that I will later use to kick off a new process with, like so:
List<String> cmd = new ArrayList<>();
cmd.add("java");
cmd.add("-jar");
cmd.add("path/to/my.jar");
...
String args = cmd.get(0)+" "+cmd.get(1)+" "+cmd.get(2);
At compile time, the compiler will check to make sure that I am using the String type every time I use a generic List method via cmd and throw an error if I try to use instances of an incompatible type. However, there's a little thing called erasure that happens during compilation, before execution. Effectively, under the hood, the compiler converts the code above into something like this:
List cmd = new ArrayList();
cmd.add("java");
cmd.add("-jar");
cmd.add("path/to/my.jar");
...
String args = (String)cmd.get(0)+" "+(String)cmd.get(1)+" "+(String)cmd.get(2);
So why doesn't my generic array code compile?
In your example, you wanted to create an array of a generic type, like so:
ArrayList<FloatList>[][] array = new ArrayList<FloatList>[n][m]; //Doesn't compile? What gives?
The problem is, because of type erasure, the ArrayList<FloatList> class type doesn't really exist, and now you've asked the JVM to create a 2-dimensional array of that non-existent type.
Okay, so what's the alternative?
Well ... it isn't pretty, but you could do something like this:
class ArrayListOfFloatList extends ArrayList<FloatList>{
...
}
//Somewhere else in your code:
ArrayListOfFloatList[][] myArray = new ArrayListOfFloatList[n][m];
//This will compile because ArrayListOfFloatList is a real class.
The only other way around this would be to not use arrays. Ugly, perhaps, but it's unfortunately a limitation of how Java is currently implemented.

Is there a constructor for arrays that uses arguments other than length?

Search engines just return tons of irrelevant pages when searching for "array constructor", so here I am.
I want to know if there is a constructor for making an array (of primitive types or just objects) that can take arguments other than length.
int[] intArray = new int[length];
or
RandomClass[] otherArray = new RandomClass[length];
is there anything more that can be customized in an array except length at its creation?
for example int[] intArray = new int[](length, somethingElse);

You can construct it and initialize it at the same time:
int[] myArray = {1,2,3};

In a word; no
An array is (conceptually) just a number of boxes in which you can place objects of a certain type. Both the number and type are specified already and there's nothing else to add to such a simple concept.

Java counting array objects

I have some questions regarding arrays in java:
How many objects are created in the following expressions?
(a) new int[0] : There is one object created, an array of size 0.???
(b) new int[2][2] : There is one object created, an array with 2 rows and columns.???
(c) new int[2][] : There is no object created???
I was wondering if my solutions next to the expressions are correct. If not, hopefuly you can help and give me some explanation about them. I don't really get what im supposed to do.
Thanks in advance!

The following is an excerpt from the Java Specification:
In the Java programming language arrays are objects (§4.3.1), are
dynamically created, and may be assigned to variables of type Object
(§4.3.2). All methods of class Object may be invoked on an array.
This means every array is an own object, which makes your answer a) correct.
b) 3 objects are created: 1 array first and two arrays containing each 2 ints. I wouldn't count the int-entries as they are primitive types in Java.
c) 1 object is created: 1 array with two null-entries.

new int[0]
Yes this is an empty array, one object is created.
int[] emptyArray = new int[0];
System.out.println(emptyArray.length); // Outputs 0
new int[2][2]
Yes this creates and array with 2 rows and columns, 3 objects are created.
int[][] bar = new int[2][2];
System.out.println(bar.getClass()); // Outputs class [[I
int[] bar1 = bar[0];
System.out.println(bar1.getClass()); // Outputs class [I
int[] bar2 = bar[1];
System.out.println(bar2.getClass()); // Outputs class [I
new int[2][]
Java supports jagged arrays. This means when you create int[2][] this means you have an array of various sized int[]. Only 1 object is created here.
int[][] foo = new int[2][];
System.out.println(foo[0]); // Outputs null
System.out.println(foo[1]); // Outputs null
foo[0] = new int[10];
foo[1] = new int[5];

Creating an array of objects in Java

I am new to Java and for the time created an array of objects in Java.
I have a class A for example -
A[] arr = new A[4];
But this is only creating pointers (references) to A and not 4 objects. Is this correct? I see that when I try to access functions/variables in the objects created I get a null pointer exception.
To be able to manipulate/access the objects I had to do this:
A[] arr = new A[4];
for (int i = 0; i < 4; i++) {
arr[i] = new A();
}
Is this correct or am I doing something wrong? If this is correct its really odd.
EDIT: I find this odd because in C++ you just say new A[4] and it creates the four objects.

This is correct.
A[] a = new A[4];
...creates 4 A references, similar to doing this:
A a1;
A a2;
A a3;
A a4;
Now you couldn't do a1.someMethod() without allocating a1 like this:
a1 = new A();
Similarly, with the array you need to do this:
a[0] = new A();
...before using it.

This is correct. You can also do :
A[] a = new A[] { new A("args"), new A("other args"), .. };
This syntax can also be used to create and initialize an array anywhere, such as in a method argument:
someMethod( new A[] { new A("args"), new A("other args"), . . } )

Yes, it creates only references, which are set to their default value null. That is why you get a NullPointerException You need to create objects separately and assign the reference. There are 3 steps to create arrays in Java -
Declaration – In this step, we specify the data type and the dimensions of the array that we are going to create. But remember, we don't mention the sizes of dimensions yet. They are left empty.
Instantiation – In this step, we create the array, or allocate memory for the array, using the new keyword. It is in this step that we mention the sizes of the array dimensions.
Initialization – The array is always initialized to the data type’s default value. But we can make our own initializations.
Declaring Arrays In Java
This is how we declare a one-dimensional array in Java –
int[] array;
int array[];
Oracle recommends that you use the former syntax for declaring arrays.
Here are some other examples of legal declarations –
// One Dimensional Arrays
int[] intArray; // Good
double[] doubleArray;
// One Dimensional Arrays
byte byteArray[]; // Ugly!
long longArray[];
// Two Dimensional Arrays
int[][] int2DArray; // Good
double[][] double2DArray;
// Two Dimensional Arrays
byte[] byte2DArray[]; // Ugly
long[] long2DArray[];
And these are some examples of illegal declarations –
int[5] intArray; // Don't mention size!
double{} doubleArray; // Square Brackets please!
Instantiation
This is how we “instantiate”, or allocate memory for an array –
int[] array = new int[5];
When the JVM encounters the new keyword, it understands that it must allocate memory for something. And by specifying int[5], we mean that we want an array of ints, of size 5.
So, the JVM creates the memory and assigns the reference of the newly allocated memory to array which a “reference” of type int[]
Initialization
Using a Loop – Using a for loop to initialize elements of an array is the most common way to get the array going. There’s no need to run a for loop if you are going to assign the default value itself, because JVM does it for you.
All in One..! – We can Declare, Instantiate and Initialize our array in one go. Here’s the syntax –
int[] arr = {1, 2, 3, 4, 5};
Here, we don’t mention the size, because JVM can see that we are giving 5 values.
So, until we instantiate the references remain null. I hope my answer has helped you..! :)
Source - Arrays in Java

You are correct. Aside from that if we want to create array of specific size filled with elements provided by some "factory", since Java 8 (which introduces stream API) we can use this one-liner:
A[] a = Stream.generate(() -> new A()).limit(4).toArray(A[]::new);
Stream.generate(() -> new A()) is like factory for separate A elements created in a way described by lambda, () -> new A() which is implementation of Supplier<A> - it describe how each new A instances should be created.
limit(4) sets amount of elements which stream will generate
toArray(A[]::new) (can also be rewritten as toArray(size -> new A[size])) - it lets us decide/describe type of array which should be returned.
For some primitive types you can use DoubleStream, IntStream, LongStream which additionally provide generators like range rangeClosed and few others.

Here is the clear example of creating array of 10 employee objects, with a constructor that takes parameter:
public class MainClass
{
public static void main(String args[])
{
System.out.println("Hello, World!");
//step1 : first create array of 10 elements that holds object addresses.
Emp[] employees = new Emp[10];
//step2 : now create objects in a loop.
for(int i=0; i<employees.length; i++){
employees[i] = new Emp(i+1);//this will call constructor.
}
}
}
class Emp{
int eno;
public Emp(int no){
eno = no;
System.out.println("emp constructor called..eno is.."+eno);
}
}

The genaral form to declare a new array in java is as follows:
type arrayName[] = new type[numberOfElements];
Where type is a primitive type or Object. numberOfElements is the number of elements you will store into the array and this value can’t change because Java does not support dynamic arrays (if you need a flexible and dynamic structure for holding objects you may want to use some of the Java collections).
Lets initialize an array to store the salaries of all employees in a small company of 5 people:
int salaries[] = new int[5];
The type of the array (in this case int) applies to all values in the array. You can not mix types in one array.
Now that we have our salaries array initialized we want to put some values into it. We can do this either during the initialization like this:
int salaries[] = {50000, 75340, 110500, 98270, 39400};
Or to do it at a later point like this:
salaries[0] = 50000;
salaries[1] = 75340;
salaries[2] = 110500;
salaries[3] = 98270;
salaries[4] = 39400;
More visual example of array creation:
To learn more about Arrays, check out the guide.

Yes it is correct in Java there are several steps to make an array of objects:
Declaring and then Instantiating (Create memory to store '4' objects):
A[ ] arr = new A[4];
Initializing the Objects (In this case you can Initialize 4 objects of class A)
arr[0] = new A();
arr[1] = new A();
arr[2] = new A();
arr[3] = new A();
or
for( int i=0; i<4; i++ )
arr[i] = new A();
Now you can start calling existing methods from the objects you just made etc.
For example:
int x = arr[1].getNumber();
or
arr[1].setNumber(x);

For generic class it is necessary to create a wrapper class.
For Example:
Set<String>[] sets = new HashSet<>[10]
results in: "Cannot create a generic array"
Use instead:
class SetOfS{public Set<String> set = new HashSet<>();}
SetOfS[] sets = new SetOfS[10];

Suppose the class A is such:
class A{
int rollno;
int DOB;
}
and you want to create an array of the objects for the class A. So you do like this,
A[] arr = new A[4]; //Statement 1
for (int i = 0; i < 4; i++) {
arr[i] = new A(); //Statement 2
}
which is absolutely correct.
Here A is the class and in Statement 1 Class A is a datatype of the array. When this statement gets executed because of the new keyword an object is created and dynamically memory is allocated to it which will be equal to the space required for the 4 blocks of datatype A i.e, ( for one block in the array space required is 8 bytes (4+4), I am assuming int takes 4 bytes of space. therefore total space allocated is 4*4 bytes for the array ).
Then the reference of the object is given to the arr variable. Here important point to note is that Statement 1 has nothing to do with creating an object for class A ,no object is created for this class it is only used as a datatype which gives the size of the class A required for the memory allocation of the array.
Then when for loop is run and Statement 2 is executed JVM now allocates the memory for the Class A (i.e creates an object) and gives its reference to the arr[i]. Every time the loop is called an object is created and the reference of it is given to arr[i].
Thus, arr[0] which holds a space of 8 bytes is given the reference of the object of the Class A and everytime loop is run new object is created and reference is given to that object so that it can now access the data in that object .