Eric C Chiruka

Array variable assignment

Case 1:

import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[] x = {10,20,30,40};
        char[] ch = {'a','b','c','d'};
        int[] b = x; // VALID
        int[] c = ch; // INVALID: Compile time error: Incompatible types, found char[], required int[]
    }
}

Element level type promotions are not applicable at array level. For example, char element can be promoted to int type, whereas char[] cannot be promoted to int[]. This is because char[] is a object of type [C and int[] is a object of type [I, and there is no relation between the two classes.

import java.lang.*;

class Test {
    public static void main(String[] args) {
        String[] s = {"A", "B", "C"};
        Object[] a = s; // VALID
    }
}

The only exception to this is when promoting to an Object[]. In the case of Object[], child class array can be promoted to parent class type aray.

Case 2:

import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[] a = {10,20,30,40,50,60};
        int[] b = {70,80};
        a = b;
    }
}

When assigning one array to another array, internally, elements won't be copied, just the reference variables will be re-assgined. Hence it does not matter if the arrays are different sizes as the number of elements are not re-assigned.

import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[] a = new int[3][];

        a[0] = new int[4][3]; // INVALID: Compile time Error: Incompatible types: Found: int[][], Required: int[]

        a[0] = 10; // INVALID: Compile time Error: Incompatible types: Found: int, Required: int[]

        a[0] = new int[2]; // VALID
    }
}

Whenever assigning one array to another array, both dimensions and data-type should be matched. E.g. in place of 1 dimensional int array, we should only provide a 1 dimensional array. If any other dimension is provided, then compile-time error will be thrown.

import java.lang.*;

class Test {
    public static void main(String[] args) {
        for(int i = 0; i <= args.length; i++) {
          System.out.println(args[i]);
        }
    }
}

java Test.java a b c
a
b
c
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 3 out of bounds for length 3
  at Test.main(Test.java:6)

java Test.java a b
a
b
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 2 out of bounds for length 2
  at Test.main(Test.java:6)

java Test.java
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
at Test.main(Test.java:6)

import java.lang.*;

class Test {
    public static void main(String[] args) {
        String[] argh = {"x","y","z"};
        args = argh;
        for(String s : args) {
          System.out.println(s);
        }
    }
}

java Test.java a b c
x
y
z

java Test.java a b
x
y
z

java Test.java
x
y
z

import java.lang.*;

class Test {
    public static void main(String[] args) {
        int[][] a = new int[4][3]; // 5 objects created: base array 'a' plus 4 arrays of size 3
        a[0] = new int[4]; // 1 object created 'a[0]'
        a[1] = new int[2]; // 1 object created 'a[1]'
        a = new int[3][2]; // 4 objects created: base array 'a' plus 3 arrays of size 2: all 7 objects created before are garbage collected
    }
}