Java-Arrays

How to initialize an Array in Java

Code Snippet:

	//initialize primitive one dimensional array
	int[] arrInt = new int[5];
	
	//initialize Object one dimensional array
	String[] strArr; //declaration
	strArr = new String[4]; //initialization

	//initialize multidimensional array
	int[][] twoArrInt = new int[4][5];

	//multidimensional array initialization with only leftmost dimension
	int[][] twoIntArr = new int[2][];
	twoIntArr[0] = new int[2];
	twoIntArr[1] = new int[3]; //complete initialization is required before we use the array


	//array initialization using shortcut syntax
	int[] arrI = {1,2,3};
	int[][] arrI2 = {{1,2}, {1,2,3}};


	/invalid because dimension is not provided
	int[] a = new int[];

	//invalid because leftmost dimension value is not provided
	int[][] aa = new int[][5];

Two Dimensional Array in Java

Code Snippet:
	
	public class TwoDimensional {
		
		public static void main(String[] args) {
			
			String [][] array = new String[2][4];
			for(int i=0; i<array.length; i++) {
				for(int j = 0; j < array[i].length; j++) {
					array[i][j] = "Str" + j;
					System.out.print(array[i][j]+" ");
				}
				System.out.println();
			}
		
		
			int [][] iArray = new int[2][];
			
			iArray[0] = new int[2];
			iArray[1] = new int[3];
			
			for(int row=0; row < iArray.length; row++) {
				for(int column=0; column < iArray[row].length;column++) {
					iArray[row][column] = column;
					System.out.print(iArray[row][column]+" ");
				}
				System.out.println();
			}
		}
	}


	public class MultidimensionalArrayExample {

		public static void main(String[] args) {

			// creating and initializing two dimensional array with shortcut syntax
			int[][] arrInt = { { 1, 2 }, { 3, 4, 5 } };
			for (int i = 0; i < arrInt.length; i++) {
				for (int j = 0; j < arrInt[i].length; j++) {
					System.out.print(arrInt[i][j] + " ");
				}
				System.out.println("");
			}

			// showing multidimensional arrays initializing
			int[][] arrMulti = new int[2][]; // yes it's valid

			arrMulti[0] = new int[2];
			arrMulti[1] = new int[3];

			arrMulti[0][0] = 1;
			arrMulti[0][1] = 2;
			arrMulti[1][0] = 3;
			arrMulti[1][1] = 4;
			arrMulti[1][2] = 5;
			for (int i = 0; i < arrInt.length; i++) {
				for (int j = 0; j < arrInt[i].length; j++) {
					System.out.print(arrInt[i][j] + " ");
				}
				System.out.println("");
			}
		}
	}

Java Array of ArrayList, ArrayList of Array

Java Array of ArrayList

Code Snippet 1:

	public class JavaArrayOfArrayList {

		public static void main(String[] args) {
			List<String> l1 = new ArrayList<>();
			l1.add("1");
			l1.add("2");

			List<String> l2 = new ArrayList<>();
			l2.add("3");
			l2.add("4");
			l2.add("5");

			List<String>[] arrayOfList = new List[2];
			arrayOfList[0] = l1;
			arrayOfList[1] = l2;

			for (int i = 0; i < arrayOfList.length; i++) {
				List<String> l = arrayOfList[i];
				System.out.println(l);
			}
		}
	}
	
	
Code Snippet of ArrayStoreException 2:

	public static void main(String[] args) {
	
		List<String>[] list = new ArrayList[2];
		
		list[0] = new ArrayList<>();
		list[1] = new LinkedList<>();
		
		System.out.println(list);
	}

Output:
	
	Exception in thread "main" java.lang.ArrayStoreException: java.util.LinkedList
		at twodimensional.ListArray.main(ListArray.java:15)

		
Code Snippet of Array with Generics:

	List<String>[] arrayOfList = new List<String>[2];

Java doesn’t support generic array. It will produce compile time error as “Cannot create a generic array of List”.

Java ArrayList of Array

Code Snippet:

	public class JavaArrayListOfStringArray {

		public static void main(String[] args) {
			// List of String arrays
			List<String[]> list = new ArrayList<String[]>();
			
			String[] arr1 = { "a", "b", "c" };
			String[] arr2 = { "1", "2", "3", "4" };
			list.add(arr1);
			list.add(arr2);
			// printing list of String arrays in the ArrayList
			for (String[] strArr : list) {
				System.out.println(Arrays.toString(strArr));
			}
		}
	}

Java String to String Array Example

Code Snippet:
	
	public class StringToArrayExample {

		/**
		 * This class shows how to convert String to String Array in Java
		 * @param args
		 **/
		public static void main(String[] args) {
			String line = "My name is Bharath";
			//using String split function
			String[] words = line.split(" ");
			System.out.println(Arrays.toString(words));
			//using java.util.regex Pattern
			Pattern pattern = Pattern.compile("\\s+");
			words = pattern.split(line);
			System.out.println(Arrays.toString(words));
		}
	}

Java provides a legacy class StringTokenizer
But we should not use it because it doesn’t have option for regular expression and using it is confusing
We can use Java regular expressions also to split String into String array in java, learn more about Java regular expression

Java varargs

Java varargs was introduced in Java 1.5
Java varargs is also known as java variable arguments

Code Snippet:
```
public static int sum(int i, int...js ){
	//do something
}
```

Important points about varargs in java

Few points to know about varargs in java are:
- We can have only one varargs in the method
- Only the last argument of a method can be varargs
- According to java documentation, we should not overload a varargs method

Why we should not overload varargs method:

Code Snippet:

	public class VarargsExample {

		public static void main(String[] args) {
			System.out.println(sum(1));
			System.out.println(sum(1,2)); //compiler error, ambiguous method

		}

		public static int sum(int i, int...js ){
			System.out.println("sum1 called");
			int sum = i;
			for(int x : js){
				sum+=x;
			}
			return sum;
		}
		public static int sum(int i, int k, Object...js ){
			System.out.println("sum2 called");
			int sum = i+k;
			for(Object x : js){
				sum+=1;
			}
			return sum;
		}	
	}
	
Code Snippet:

	public class OverloadingVarArgs {	
		public static void main(String[] args) {
			new OverloadingVarArgs().method(3, 2);
		}
		
		void method(int i, int ... j) {
		}
		
		void method(int i, Integer ... j) {
		}
	}

In above example, we will notice that compiler will not complain when we overload methods with varargs
But when we try to use it, compiler get’s confused which method to use when mapping the second argument

Add Elements to Array

In java array add elements is not supported because size of an Array in java is fixed
We have to provide size of the array when we initialize array in java
We can write a utility method that we can use to add elements to an array
Create a temporary array, whose size will be the addition of length of array and number of elements to add in the array

Then I will copy input array to the temporary array and add the elements and then return it

Code Snippet:

public class AddToArray {

	public static void main(String[] args) {
		Object[] objArr1 = {"1","2","3"};
		Object[] objArr2 = {"4","5","6"};
		//adding an element to array
		Object[] objArr = add(objArr1, "4");
		System.out.println(Arrays.toString(objArr));
		//adding two arrays
		objArr = add(objArr1, objArr2);
		System.out.println(Arrays.toString(objArr));
		
	}
	
	/**
	 * This method will add elements to an array and return the resulting array
	 * @param arr
	 * @param elements
	 * @return
	 **/
	public static Object[] add(Object[] arr, Object... elements){
		Object[] tempArr = new Object[arr.length+elements.length];
		System.arraycopy(arr, 0, tempArr, 0, arr.length);
		
		for(int i=0; i < elements.length; i++)
			tempArr[arr.length+i] = elements[i];
		return tempArr;
		
	}
}

Arrays.sort();

Code Snippet:

	public static void main(String[] args) {
		int[] intArr = {1, 4, 2, 6, 3};
		String[] strArr = {"E", "A", "U","O","I"};
		//sort int array
		Arrays.sort(intArr);
		Arrays.sort(strArr);
		
		System.out.println(Arrays.toString(intArr));
		System.out.println(Arrays.toString(strArr));
	}

Java String Array to String

Code Snippet:

	String string = Arrays.stream(stringArray).collect(Collectors.joining(""));

Java ArrayList to Array

Code Snippet:	
	
	List<String> stringList = new ArrayList<String>();
	
	stringList.add("abc");
	stringList.add("def");
	stringList.add("mno");
	String[] array = stringList.stream().toArray(String[]::new);	
	
	Person[] array = personList.stream().toArray(value -> new Person[value]);
	Person[] array = personList.stream().toArray(Person[]::new);
	
	
Code Snippet of Shallow Copy:

	public static void main(String[] args) {
		Person p1 = new Person("Bharath");
		Person p2 = new Person("Lisa");
		
		List<Person> pList = new ArrayList<>();
		pList.add(p1); 
		pList.add(p2);
		
		Person[] pArray = pList.toArray(new Person[0]);
		
		System.out.println("Original List = "+pList);
		System.out.println("Created Array from ArrayList = "+Arrays.toString(pArray));
		
		//let's change the list and array
		pList.get(0).setName("David");
		pArray[1].setName("Ram");
		
		System.out.println("Modified List = "+pList);
		System.out.println("Modified Array = "+Arrays.toString(pArray));
		
	}
	
Code Snippet of Deep Copy:

	public static void main(String[] args) {
		Person p1 = new Person("Bharath");
		Person p2 = new Person("Lisa");
		
		List<Person> pList = new ArrayList<>();
		pList.add(p1); pList.add(p2);

		//convert ArrayList to Array using deep copy
		Person[] pArray = new Person[pList.size()];
		
		for(int i =0; i<pList.size(); i++) {
			Person p = pList.get(i);
			Person temp = new Person(p.getName());
			pArray[i] = temp;
		}
		
		System.out.println("Original List = "+pList);
		System.out.println("Created Array from ArrayList = "+Arrays.toString(pArray));
		
		//let's change the list and array
		pList.get(0).setName("David");
		pArray[1].setName("Ram");
		
		System.out.println("Modified List = "+pList);
		System.out.println("Modified Array = "+Arrays.toString(pArray));

	}

How to convert Array to ArrayList in Java

Arrays.asList(T… a):

This method returns the underlying representation of the array in the form of ArrayList
The returned ArrayList is fixed-sized and any attempt to modify that will result in UnsupportedOperationException at runtime

Also any change in the array will change the elements in ArrayList also

Code Snippet:

 String[] strArr = {"1", "2", "3", "4"};
 List<String> strList = new ArrayList<String>();
 strList = Arrays.asList(strArr);
 strArr[0] = "5";
 // Arrays.asList()
//strList.add("5");// code will throw java.lang.UnsupportedOperationException because
//  returns a fixed-size list backed by the specified array

Collection.addAll(ArrayList list, T[] t):

The array data is copied to the list and both are independent object

Once the array is copied, we can modify both the objects independently

Code Snippet:

strList = new ArrayList<String>();
Collections.addAll(strList, strArr);
strList.add("5");
strArr[0] = "1";

Primitive Array to ArrayList

Code Snippet:

	int[] iArray = { 1, 3, 5, 6, 7, 8 };
	List<int[]> aList = Arrays.asList(iArray);
	
	IntStream flatMapToInt = aList.stream().flatMapToInt(x -> Arrays.stream(x));
	List<Integer> collect = flatMapToInt.boxed().collect(Collectors.toList());
	collect.add(10);
	System.out.println(collect);

UnsupportedOperationException:

Code Snippet:

	String[] sArray = {"a","d","q","e","y"};
	List<String> sList = Arrays.asList(sArray);
	
	sList.add("bharath");
	System.out.println(sList);
	
	

Output:

	Exception in thread "main" java.lang.UnsupportedOperationException
		at java.util.AbstractList.add(AbstractList.java:148)
		at java.util.AbstractList.add(AbstractList.java:108)
		at arrays.copy.ArrayToArrayList.main(ArrayToArrayList.java:24)

Java Copy Array – Array Copy in Java

4 Ways of copying Array:

Object.clone():
- Since array in java is also an Object, we can use this method to achieve full array copy
- This method will not if we want partial copy of the array
- New separate Array will be created and elements will be copied from existing Array to new Array
System.arraycopy():
- System class arraycopy() is the best way to do partial copy of an array
- It provides 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(sourceArray, noOfElementsToBeCopied):
- If we want to copy first few elements of an array or full copy of array, we can use this method
- It’s not versatile like System.arraycopy() but easy to use
- This method internally use Syste.arraycopy() method

Arrays.Arrays.copyOfRange(source, biginIndex, endIndex):

If we want few elements of an array to be copied, where starting index is not 0
We can use this method to copy partial Array
Again this method is also using System.arraycopy() method itself

Code Snippet:

Object[] oA = { 1, 4, 5, "ddfd", "ddd", new ArrayList<>(), 2, 34, 343, 34, 35, 4, 5, 4, };
System.out.println("Origianl Array: "+ Arrays.toString(oA));

Object[] copyOf = Arrays.copyOf(oA, 3);
System.out.println("Arrays.copyOf 3 elements: "+Arrays.toString(copyOf));

Object[] copyOfBE = Arrays.copyOfRange(oA, 3, 6);
System.out.println("Arrays.copyOfRange 3 from 3 to 6 elements: "+Arrays.toString(copyOfBE));

Object[] oA2 = new Object[90];
System.arraycopy(oA, 0, oA2, 5, 8);
System.out.println("System.arraycopy: "+Arrays.toString(oA2));

Object[] clone = oA.clone();
clone[0]="abc";
System.out.println("Original Array: "+Arrays.toString(oA));
System.out.println("Cloned Array: "+Arrays.toString(clone));

Output:

Origianl Array: [1, 4, 5, ddfd, ddd, [], 2, 34, 343, 34, 35, 4, 5, 4]
Arrays.copyOf 3 elements: [1, 4, 5]
Arrays.copyOfRange 3 from 3 to 6 elements: [ddfd, ddd, []]
System.arraycopy: [null, null, null, null, null, 1, 4, 5, ddfd, ddd, [], 2, 34, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null, null]
Original Array: [1, 4, 5, ddfd, ddd, [], 2, 34, 343, 34, 35, 4, 5, 4]
Cloned Array: [abc, 4, 5, ddfd, ddd, [], 2, 34, 343, 34, 35, 4, 5, 4]

Summary:

System.arraycopy(sourceArray, srcIndex, destArray, dstIndex, src.length);
Arrays.toString();
Arrays.sort(T[] t);
Arrays.sort(T[] t, Comparator c);
list.stream().toArray(String[]::new);
Arrays.stream();
Arrays.asList(T[] t);
Collection.addAll(ArrayList<T> list, T[] t);
public static <T> boolean addAll(Collection<? super T> c, T... elements) 

Copy Array:

	clone()
	System.arraycopy()
	Arrays.copyOf()
	Arrays.copyOfRange()
	
compare Two Arrays:

	Arrays.compare(T[] t1, TG []t2) 
	
Fill:

	Arrays.fill(srcArray, element);
	Arrays.fill(srcArray, BInd, EInd, element);

deepToString(Object[] o);

How do you compare two arrays?

int[] numbers1 = { 1, 2, 3 }; int[] numbers2 = { 4, 5, 6 };

System.out.println(Arrays.equals(numbers1, numbers2)); //false

int[] numbers3 = { 1, 2, 3 }; System.out.println(Arrays.equals(numbers1, numbers3)); //true

What is Arrays.deepToString() method

Arrays.deepToString() method is used to get the string representation of multidimensional arrays
This method returns the deep contents of the specified array

deepToString() method will only work for type Object[] and its Child

Code Snippet:

String[] oneDArray = new String[] {"ONE", "TWO", "THREE", "FOUR", "FIVE"};
System.out.println("One Dimensional Array : ");
//Printing one dimensional array contents using deepToString() method
System.out.println(Arrays.deepToString(oneDArray));

String[][] twoDArray = new String[][] {
    {"ONE", "TWO", "THREE", "FOUR"},
    {"FIVE", "SIX", "SEVEN"},
    {"EIGHT", "NINE", "TEN", "ELEVEN", "TWELVE"}
};
	System.out.println("Two Dimensional Array : ");
	System.out.println(Arrays.deepToString(twoDArray));

	String[][][] threeDArray = new String[][][] {
	    {
	        {"ONE", "TWO", "THREE"},
	        {"FOUR", "FIVE", "SIX", "SEVEN"}
	    },
	    {
	        {"EIGHT", "NINE", "TEN", "ELEVEN"},
	        {"TWELVE", "THIRTEEN", "FOURTEEN"}
	    },
	    {
	        {"FIFTEEN", "SIXTEEN"} ,
	        {"SEVENTEEN", "EIGHTEEN", "NINETEEN"},
	        {"TWENTY", "TWENTY ONE"}
	    }
	};
System.out.println("Three Dimensional Array : ");
//Printing three dimensional array contents using deepToString() method
System.out.println(Arrays.deepToString(threeDArray));

What are Jagged Arrays:

Jagged Arrays are Arrays with different dimensions

Code Snippet:

int [][] iArray = new int[3][];

iArray[0] = {1,2,4,5,6};
iArray[1] = {2, 5, 6};
iArray[2] = {8, 9, 10, 11, 12};

Can size of the Array can be Negative

No, we will get NegativeArraySizeException

Is auto-widening, auto-boxing and auto-unboxing is allowed in Java

No, only Upcasting is allowed

auto-widening and auto-boxing is not allowed

Code Snippet:

Integer[] I = new int[5];   //Compile Time Error : Auto-Boxing not allowed
int[] i = new Integer[10];   //Compile Time Error : Auto-UnBoxing not allowed
long[] l = new byte[10];    //Compile Time Error : Auto-widening not allowed
Object[] o = new String[10];    //No Compile Time Error : Auto-Upcasting is allowed, String[] is upcasted to Object[]

Anonymous array i.e an array without reference

Code Snippet:

	 public static void main(String[] args) {
		//Creating anonymous array
		System.out.println(new int[]{1, 2, 3}.length);    //Output : 3
		System.out.println(new int[]{47, 21, 58, 98}[1]);   //Output : 21
	}

Way of declaring multi dimensional arrays

Code Snippet:

	public static void main(String[] args) {
		int[][] twoDArray;    //Normal way of declaring two-dimensional array
		int[] TwoDArray [];   //Another way of declaring two-dimensional array
		int[][][] threeDArray;  //Normal way of declaring three-dimensional array
		int[] ThreeDArray [][];    //This is also legal
	}

Declaring the contents to the array without new operator

Code Snippet:

	public static void main(String[] args) {
		int[] i = {1, 2, 3, 4};   //This is the correct way
		i = {1, 2, 3 , 4};     //Compile time error
		i = new int[]{1, 2, 3, 4};  //This is also correct way
	}


	A ab[] = new A[]{new A(), new A()};
	System.out.println(Arrays.deepToString(ab));

Name		Name	Last commit message	Last commit date
Latest commit History 9 Commits
ArraysPrograms.md		ArraysPrograms.md
JavaArrayIntQue.md		JavaArrayIntQue.md
README.md		README.md

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Java-Arrays

How to initialize an Array in Java

Two Dimensional Array in Java

Java Array of ArrayList, ArrayList of Array

Java Array of ArrayList

Java ArrayList of Array

Java String to String Array Example

Java varargs

Important points about varargs in java

Why we should not overload varargs method:

Add Elements to Array

Arrays.sort();

Java String Array to String

Java ArrayList to Array

How to convert Array to ArrayList in Java

Arrays.asList(T… a):

Collection.addAll(ArrayList list, T[] t):

Primitive Array to ArrayList

UnsupportedOperationException:

Java Copy Array – Array Copy in Java

4 Ways of copying Array:

Summary:

How do you compare two arrays?

What is Arrays.deepToString() method

What are Jagged Arrays:

Can size of the Array can be Negative

Is auto-widening, auto-boxing and auto-unboxing is allowed in Java

Anonymous array i.e an array without reference

Way of declaring multi dimensional arrays

Declaring the contents to the array without new operator

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Java-Arrays

How to initialize an Array in Java

Two Dimensional Array in Java

Java Array of ArrayList, ArrayList of Array

Java Array of ArrayList

Java ArrayList of Array

Java String to String Array Example

Java varargs

Important points about varargs in java

Why we should not overload varargs method:

Add Elements to Array

Arrays.sort();

Java String Array to String

Java ArrayList to Array

How to convert Array to ArrayList in Java

Arrays.asList(T… a):

Collection.addAll(ArrayList list, T[] t):

Primitive Array to ArrayList

UnsupportedOperationException:

Java Copy Array – Array Copy in Java

4 Ways of copying Array:

Summary:

How do you compare two arrays?

What is Arrays.deepToString() method

What are Jagged Arrays:

Can size of the Array can be Negative

Is auto-widening, auto-boxing and auto-unboxing is allowed in Java

Anonymous array i.e an array without reference

Way of declaring multi dimensional arrays

Declaring the contents to the array without new operator

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