10 Interesting Observations about arrays in Java

The followings are 10 interesting observations about arrays in java. You may be asked in the interviews or Java certification exams about these observations.

1) The size of an array can not be negative. If you give size of an array as negative, you don’t get any errors while compliing. But, you will get NegativeArraySizeException at run time.

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public class ArraysInJava {     public static void main(String[] args)     {         int[] i = new int[-5];   //No Compile Time Error         //You will get java.lang.NegativeArraySizeException at run time     } }

2) The size of an array must be an integer or an expression which results an integer. Auto-widening is also allowed.

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public class ArraysInJava {     public static void main(String[] args)     {         int[] i = new int[10+5];    //Array size can be an expression resulting an integer         int[] i1 = new int[(byte)10];    //byte is auto-widened to int         int i3 = new int[10.25];     //Compile Time Error : size can not be double     } }

3) Declaration and instantiating of an array strictly must be of same type. No auto-widening, auto-boxing and auto-unboxing is allowed. But only auto-upcasting is allowed.

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public class ArraysInJava {     public static void main(String[] args)     {         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[]     } }

4) The type of elements of an array must be compatible with type of the array object. If you try to store non-compatible element in an array object, you will get ArrayStoreException at run time.

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public class ArraysInJava {     public static void main(String[] args)     {         Object[] o = new String[10];    //No Compile Time Error : String[] is auto-upcasted to Object[]         //i.e array object of strings can be referred by array reference variable of Object type         o[2] = "java";         o[5] = 20;   //No Compile time error,         //but you will get java.lang.ArrayStoreException at run time.     } }

5) If you are supplying the contents to the array without new operator, then it should be at the time of declaration only. Not at any other places.

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public class ArraysInJava {     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     } }

6) Another way of declaring multi dimensional arrays.

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public class ArraysInJava {     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     } }

7) While creating multi dimensional arrays, you can not specify an array dimension after an empty dimension.

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public class ArraysInJava {     public static void main(String[] args)     {         int[][][] threeDArray = new int[10][][10];    //Compile Time Error         int[][][] threeDArray1 = new int[][10][];     //Compile Time Error         int[][][] threeDArray2 = new int[][][10];     //Compile Time Error     } }

8) You can create an anonymous array i.e an array without reference.

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public class ArraysInJava {     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     } }

9) While assigning one array reference variable to another, compiler checks only type of the array not the size.

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public class ArraysInJava {     public static void main(String[] args)     {         int[] a = new int[10];         int[] b = new int[100];         double[] c = new double[20];         a = b;         b = c;     //Compile Time Error : can not convert from double[] to int[]     } }

10) The size of an array can not be changed once you define it. You can not insert or delete array elements after creating an array. Only you can change is the value of the elements. This is the main drawback of arrays.