Develop Reference

Seeming Discrepancy in Arrays.copyOf

Why this question is not a possible duplication of How Arrays.asList(int[]) can return List<int[]>?.
That question doesn't really answer my particular situation as I am trying to figure out if there is a discrepancy in my use of Arrays.copyOf.
CASE 1: Supposed deep copy of the array
// Creating a integer array, populating its values
int[] src = new int[2];
src[0] = 2;
src[1] = 3;
// Create a copy of the array
int [] dst= Arrays.copyOf(src,src.length);
Assert.assertArrayEquals(src, dst);
// Now change one element in the original
dst[0] = 4;
// Following line throws an exception, (which is expected) if the copy is a deep one
Assert.assertArrayEquals(src, dst);
CASE 2:
Here is where things seem to be weird:
What I am trying to do with the below method (lifted verbatim from a book) is to create an immutable list view of a copy of the input array arguments. That way, if the input array changes, the contents of the returned list don't change.
#SafeVarargs
public static <T> List<T> list(T... t) {
return Collections.unmodifiableList(new ArrayList<>(Arrays.asList(Arrays.copyOf(t, t.length))));
}
int[] arr2 = new int[2];
arr2[0] = 2;
arr2[1] = 3;
// Create an unmodifiable list
List<int[]> list2 = list(arr2);
list2.stream().forEach(s -> System.out.println(Arrays.toString(s)));
// Prints [2, 3] as expected
arr2[0] = 3;
list2.stream().forEach(s -> System.out.println(Arrays.toString(s)));
// Prints [3, 3] which doesn't make sense to me... I would have thought it would print [2, 3] and not be affected by my changing the value of the element.
The contradiction that I see is that in one case (Case 1), Arrays.copyOf seems to be a deep copy, whereas in the other case (Case 2), it seems like a shallow one. The changes to the original array seem to have written through to the list, even though I have copied the array in creating my unmodifiable list.
Would someone be able to help me resolve this discrepancy?

First of all, your list method performs an unnecessary step, you don't need the copyOf operation, so here goes:
#SafeVarargs
public static <T> List<T> list(T... t) {
return Collections.unmodifiableList(
new ArrayList<>(Arrays.asList(t))
);
}
The ArrayList constructor already copies the incoming list, so you're safe there.
Next, when you are calling your list() method with an int[], that array is considered to be a single element of type int[], because the type erasure of your T... is Object..., and int is primitive. There is no way you can make your method do a deep copy inside the list without either changing the parameter types or doing an instanceOf check and performing the copy manually inside the method. I'd say the wisest thing to do is probably to move the Arrays.copyOf() call outside the method:
List<int[]> list2 = list(Arrays.copyOf(arr2));

ArrayList of Arrays to 2d Array in Java

I recently had a problem typecasting/converting an ArrayList of arrays to a 2d array. I found an answer on this site that told me to use
List<Object[]> arrayList;
// initialize and fill the list of arrays
// all arrays have the same length
Object[][] array2d = arrayList.toArray(new Object[][] {});
That worked, but I googled it for a bit and read somewhere that it is conseidered bad practice. I still used it, as it was the only one line variant that actually worked.
What does it actually mean and why is it considered bad? I don't understand the [][]{} bit. I'm guessing that you pass an empty but initialized Object[][] to the .toArray() method?

The only thing I see is that it creates an array but will not use it.
One the Javadoc you can see:
Returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this list.
And it can be verified on the method source code:
#SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}
So what you are creating is an empty array, then the method will create another one.
Object[][] array2d = arrayList.toArray(new Object[arrayList.size()][]);
Concerning the [][]{} your guess is correct.
Some tips
here

I can't find any reason for your example be considered bad practice!

Retrieve a segment of array in another array with a shallow copy and keeping the reference

I'm looking for a way to cut a initial array from a specific index to another index
int[] array = {10,5,4,4,3,5,6};
int[] arrayCopy;
In this example, if I want to segment from index[1] to index[4]. The new array witch I store in arrayCopy should looks like this one arrayCopy[] = {5,4,4,3}. The constante problem is that my objectif is to keep reference of the original array.So if in arrayCopy I change a element il will change too in array
arrayCopy[] = {5,4,4,3}
arrayCopy[0] = 20 // arrayCopy[] = {20,4,4,3} and array = {10,20,4,4,3,5,6};
I tried Arrays.copyOfRange() but it's a deep copy and System.arraycopy() make a shallow copy but it doesn't accomplish what I want.
I tried to make my own method
public E[] fillArray(int[] _array, int begin, int end){
int[] arrayTemp;
for ( int i = begin ; i <= end; i++ ) {
arrayTemp[i] = _array[i];
}
return arrayTemp;
}
Output
arrayCopy = fillArray(array,1,4);
Exception in thread "main" java.lang.NullPointerException
Note: I want to keep the array mechanism and not using an ArrayList or ...

Can I change the second array and the first array will change too?
There is no way for Java arrays to support that feature.
A Java array is an Object, so all arrays are independent objects, with their own memory structure for the array values.
(This is unlike C, where an array is done by pointer manipulation, so a second pointer can point into the memory sequence of an array already referenced by another pointer.)
If you want a sub-array of the first array, that is mapped such that changes to the sub-array is reflected in the main array, and vice-versa, you need something else.
One option is to use a List, and create the sub-list using subList(int fromIndex, int toIndex):
Returns a view of the portion of this list between the specified fromIndex, inclusive, and toIndex, exclusive. (If fromIndex and toIndex are equal, the returned list is empty.) The returned list is backed by this list, so non-structural changes in the returned list are reflected in this list, and vice-versa.
If you already have an array, you can wrap the array with a list using Arrays.asList(T... a), then use the subList() as for any other list.
Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.)
Note that List doesn't work for primitive types, so if your array is a primitive, you should use a Buffer of the appropriate type ( ByteBuffer, CharBuffer, DoubleBuffer, FloatBuffer, IntBuffer, LongBuffer, ShortBuffer).
E.g. for an int[], create the buffer using IntBuffer.wrap(int[] array, int offset, int length):
Wraps an int array into a buffer.
The new buffer will be backed by the given int array; that is, modifications to the buffer will cause the array to be modified and vice versa.
If you always access the array through the IntBuffer, by using IntBuffer.wrap(int[] array) to wrap the full array, your code could use either IntBuffer and not know if it is accessing the full array or the sub-array.

This should work.
public static int[] fillArray(int[] _array, int begin, int end){
int[] arrayTemp = new int[end-begin+1];
for ( int i = begin ; i <= end; i++ ) {
arrayTemp[i-begin] = _array[i];
}
return arrayTemp;
}

Array is given same memory address for every instance of class [duplicate]

I have an array a which is constantly being updated. Let's say a = [1,2,3,4,5]. I need to make an exact duplicate copy of a and call it b. If a were to change to [6,7,8,9,10], b should still be [1,2,3,4,5]. What is the best way to do this? I tried a for loop like:
for(int i=0; i<5; i++) {
b[i]=a[i];
}
but that doesn't seem to work correctly. Please don't use advanced terms like deep copy, etc., because I do not know what that means.

You can try using System.arraycopy()
int[] src = new int[]{1,2,3,4,5};
int[] dest = new int[5];
System.arraycopy( src, 0, dest, 0, src.length );
But, probably better to use clone() in most cases:
int[] src = ...
int[] dest = src.clone();

you can use
int[] a = new int[]{1,2,3,4,5};
int[] b = a.clone();
as well.

If you want to make a copy of:
int[] a = {1,2,3,4,5};
This is the way to go:
int[] b = Arrays.copyOf(a, a.length);
Arrays.copyOf may be faster than a.clone() on small arrays. Both copy elements equally fast but clone() returns Object so the compiler has to insert an implicit cast to int[]. You can see it in the bytecode, something like this:
ALOAD 1
INVOKEVIRTUAL [I.clone ()Ljava/lang/Object;
CHECKCAST [I
ASTORE 2

Nice explanation from http://www.journaldev.com/753/how-to-copy-arrays-in-java
Java Array Copy Methods
Object.clone(): Object class provides clone() method and since array
in java is also an Object, you can use this method to achieve full
array copy. This method will not suit you if you want partial copy of
the array.
System.arraycopy(): System class arraycopy() is the best way to do
partial copy of an array. It provides you an easy way to specify the
total number of elements to copy and the source and destination array
index positions. For example System.arraycopy(source, 3, destination,
2, 5) will copy 5 elements from source to destination, beginning from
3rd index of source to 2nd index of destination.
Arrays.copyOf(): If you want to copy first few elements of an array or
full copy of array, you can use this method. Obviously it’s not
versatile like System.arraycopy() but it’s also not confusing and easy
to use.
Arrays.copyOfRange(): If you want few elements of an array to be
copied, where starting index is not 0, you can use this method to copy
partial array.

I have a feeling that all of these "better ways to copy an array" are not really going to solve your problem.
You say
I tried a for loop like [...] but that doesn't seem to be working correctly?
Looking at that loop, there's no obvious reason for it not to work ... unless:
you somehow have the a and b arrays messed up (e.g. a and b refer to the same array), or
your application is multi-threaded and different threads are reading and updating the a array simultaneously.
In either case, alternative ways of doing the copying won't solve the underlying problem.
The fix for the first scenario is obvious. For the second scenario you will have to figure out some way of synchronizing the threads. Atomic array classes don't help because they have no atomic copy constructors or clone methods, but synchronizing using a primitive mutex will do the trick.
(There are hints in your question that lead me to think that this is indeed thread related; e.g. your statement that a is constantly changing.)

You can try using Arrays.copyOf() in Java
int[] a = new int[5]{1,2,3,4,5};
int[] b = Arrays.copyOf(a, a.length);

All solution that call length from array, add your code redundant null checkersconsider example:
int[] a = {1,2,3,4,5};
int[] b = Arrays.copyOf(a, a.length);
int[] c = a.clone();
//What if array a comes as local parameter? You need to use null check:
public void someMethod(int[] a) {
if (a!=null) {
int[] b = Arrays.copyOf(a, a.length);
int[] c = a.clone();
}
}
I recommend you not inventing the wheel and use utility class where all necessary checks have already performed. Consider ArrayUtils from apache commons. You code become shorter:
public void someMethod(int[] a) {
int[] b = ArrayUtils.clone(a);
}
Apache commons you can find there

You can also use Arrays.copyOfRange.
Example:
public static void main(String[] args) {
int[] a = {1,2,3};
int[] b = Arrays.copyOfRange(a, 0, a.length);
a[0] = 5;
System.out.println(Arrays.toString(a)); // [5,2,3]
System.out.println(Arrays.toString(b)); // [1,2,3]
}
This method is similar to Arrays.copyOf, but it's more flexible. Both of them use System.arraycopy under the hood.
See:
https://docs.oracle.com/javase/8/docs/api/java/util/Arrays.html
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/arrays.html
http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/java/util/Arrays.java?av=f

If you must work with raw arrays and not ArrayList then Arrays has what you need. If you look at the source code, these are the absolutely best ways to get a copy of an array. They do have a good bit of defensive programming because the System.arraycopy() method throws lots of unchecked exceptions if you feed it illogical parameters.
You can use either Arrays.copyOf() which will copy from the first to Nth element to the new shorter array.
public static <T> T[] copyOf(T[] original, int newLength)
Copies the specified array, truncating or padding with nulls (if
necessary) so the copy has the specified length. For all indices that
are valid in both the original array and the copy, the two arrays will
contain identical values. For any indices that are valid in the copy
but not the original, the copy will contain null. Such indices will
exist if and only if the specified length is greater than that of the
original array. The resulting array is of exactly the same class as
the original array.
2770
2771 public static <T,U> T[] More ...copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
2772 T[] copy = ((Object)newType == (Object)Object[].class)
2773 ? (T[]) new Object[newLength]
2774 : (T[]) Array.newInstance(newType.getComponentType(), newLength);
2775 System.arraycopy(original, 0, copy, 0,
2776 Math.min(original.length, newLength));
2777 return copy;
2778 }
or Arrays.copyOfRange() will also do the trick:
public static <T> T[] copyOfRange(T[] original, int from, int to)
Copies the specified range of the specified array into a new array.
The initial index of the range (from) must lie between zero and
original.length, inclusive. The value at original[from] is placed into
the initial element of the copy (unless from == original.length or
from == to). Values from subsequent elements in the original array are
placed into subsequent elements in the copy. The final index of the
range (to), which must be greater than or equal to from, may be
greater than original.length, in which case null is placed in all
elements of the copy whose index is greater than or equal to
original.length - from. The length of the returned array will be to -
from. The resulting array is of exactly the same class as the original
array.
3035 public static <T,U> T[] More ...copyOfRange(U[] original, int from, int to, Class<? extends T[]> newType) {
3036 int newLength = to - from;
3037 if (newLength < 0)
3038 throw new IllegalArgumentException(from + " > " + to);
3039 T[] copy = ((Object)newType == (Object)Object[].class)
3040 ? (T[]) new Object[newLength]
3041 : (T[]) Array.newInstance(newType.getComponentType(), newLength);
3042 System.arraycopy(original, from, copy, 0,
3043 Math.min(original.length - from, newLength));
3044 return copy;
3045 }
As you can see, both of these are just wrapper functions over System.arraycopy with defensive logic that what you are trying to do is valid.
System.arraycopy is the absolute fastest way to copy arrays.

For a null-safe copy of an array, you can also use an optional with the Object.clone() method provided in this answer.
int[] arrayToCopy = {1, 2, 3};
int[] copiedArray = Optional.ofNullable(arrayToCopy).map(int[]::clone).orElse(null);

I had a similar problem with 2D arrays and ended here.
I was copying the main array and changing the inner arrays' values and was surprised when the values changed in both copies. Basically both copies were independent but contained references to the same inner arrays and I had to make an array of copies of the inner arrays to get what I wanted.
This is sometimes called a deep copy. The same term "deep copy" can also have a completely different and arguably more complex meaning, which can be confusing, especially to someone not figuring out why their copied arrays don't behave as they should. It probably isn't the OP's problem, but I hope it can still be helpful.

How to convert an int[] array to a List?

I expected this code to display true:
int[] array = {1, 2};
System.out.println(Arrays.asList(array).contains(1));

The method Arrays.asList(T ...) is, when generics are erased and varargs are transformed, actually equal to a method of type Arrays.ofList(Object[]) (which is the, binary equivalent, JDK 1.4 version of the same Method).
An array of primitives is an Object (see also this question), but not an Object[], so the compiler thinks you are using the varargs version and generates an Object array around your int array. You could illustrate what's happening by adding an extra step:
int[] array = {1, 2};
List<int[]> listOfArrays = Arrays.asList(array);
System.out.println(listOfArrays.contains(1));
This compiles and is equivalent to your code. It also obviously returns false.
The compiler translates varargs calls into calls with a single array, so calling a varargs method that expects parameters T ... with parameters T t1, T t2, T t3 is equivalent to calling it with new T[]{t1, t2, t3} but the special case here is that varargs with primitives will be autoboxed before the array is created if the method needs an object array. So the compiler thinks the int array is passed in as a single Object and creates a single element array of type Object[], which it passes to asList().
So here's the above code once again, the way the compiler implements it internally:
int[] array = {1, 2};
// no generics because of type erasure
List listOfArrays = Arrays.asList(new Object[]{array});
System.out.println(listOfArrays.contains(1));
Here are some good and bad ways to call Arrays.asList() with int values:
// These versions use autoboxing (which is potentially evil),
// but they are simple and readable
// ints are boxed to Integers, then wrapped in an Object[]
List<Integer> good1 = Arrays.asList(1,2,3);
// here we create an Integer[] array, and fill it with boxed ints
List<Integer> good2 = Arrays.asList(new Integer[]{1,2,3});
// These versions don't use autoboxing,
// but they are very verbose and not at all readable:
// this is awful, don't use Integer constructors
List<Integer> ugly1 = Arrays.asList(
new Integer(1),new Integer(2),new Integer(3)
);
// this is slightly better (it uses the cached pool of Integers),
// but it's still much too verbose
List<Integer> ugly2 = Arrays.asList(
Integer.valueOf(1), Integer.valueOf(2), Integer.valueOf(3)
);
// And these versions produce compile errors:
// compile error, type is List<int[]>
List<Integer> bad1 = Arrays.asList(new int[]{1,2,3});
// compile error, type is List<Object>
List<Integer> bad2 = Arrays.asList(new Object[]{1,2,3});
Reference:
Java Tutorial > Classes and Objects > Passing Information to a Method or a Constructor > Varargs
Arrays.asList(T ...)
But to actually solve your problem in a simple way:
There are some library solutions in Apache Commons / Lang (see Bozho's answer) and in Google Guava:
Ints.contains(int[], int) checks whether an array of ints contains a given int
Ints.asList(int ...) creates a List of Integers from an int array

The Arrays.asList(array) will result in a singleton list of an int[].
It works as you expect if you change int[] to Integer[]. Don't know if that helps you though.

Arrays.asList(ArrayUtils.toObjectArray(array))
(ArrayUtils is from commons-lang)
But if you want to just call contains there is no need of that. Simply use Arrays.binarySearch(..) (sort the array first)

This
System.out.println(Arrays.asList(array).contains(array));
returns true.

It seems like your understanding of Arrays.asList(T... a) is wrong. You wouldn't be the first person to make an assumption as to how it works.
Try it with
System.out.println(Arrays.asList(1, 2).contains(1));

Autoboxing just doesn't work the way you want it to in this case. The following code may be a bit verbose, but does the job of converting an int array to a list:
List<Integer> list = new ArrayList<Integer>(array.length);
for (int value : array) {
list.add(value);
}

The following code displays true:
Integer[] array = {1, 2};
System.out.println(Arrays.asList(array).contains(1));
(Your version fails, since Int's not beeing objects, but Int[] is an object. Therefor you will call asList(T... a) with one element beeing a Collection, since it is not possible to have an Collection a.)

When you call
Arrays.asList(array)
on your array of primitives, you get a List instance containing one object: an array of int values! You have to first convert the array of primitives into an array of objects, as #Bozho suggests in his answer.

If you only want to check whether the array contains certain element just iterate over array and search for element. This will take o(n/2). All other solutions are less effective. Any method that copies array to list must iterate over array and therefore this operation only requires n atomic assignments.

I dont think there is a method call you could use. Try it like this
List<Integer> list = new ArrayList<Integer>();
for (int index = 0; index < array.length; index++)
{
list.add(array[index]);
}