So, lets say I have 2 arrays defined like this:
int[] a = {1,2,3};
int[] b = a;
a[1] = 35; // b[1] becomes 35
b[0] = -5; // why would a[0] become -5?
So, I can reason that changing a value in a will automatically change the value in b (b is pointing to a). But, if I change value in b, why would it affect a? Wouldn't that be as if a is pointing to b (they are interchangeable)?
I was confused with this concept and would like some clarification. Any advice would be appreciated. Thanks.
When you create the variable int[] a = {1,2,3};, by using the brackets { } just as by saying new int[n] java allocates enough memory for those values and now a references that address in memory.
Then, when you create the new variable int[] b = a;, you aren't allocating memory to that variable b, rather b references the same address which a references.
Now, when you change something in a or b the change occurs at the address referenced by both those variables and will be reflected by both.
Let's do an analogy here. The objects (such as arrays) that you create are like balloons. And the variables (a and b) are like children. And the children can hold a single balloon with a string. And that string is what we call a reference. On the other hand, one balloon can be held by multiple children.
In your code, you created a single balloon i.e. an array {1, 2, 3}. And let a child called a hold it. Now you can tell child a to modify items in the array because the child a is holding it.
In the second line, you tell another child b to hold the balloon that a is holding, like this:
int[] b = a;
Now a and b are actually holding the same balloon!
When you do this:
a[1] = 35;
Not only is the balloon that a is holding been changed, but also is the balloon held by b been changed. Because, well, a and b are holding the same balloon.
The same thing happens when you do:
b[0] = -5;
That's how reference types work in Java. But please note that this doesn't apply to primitive types, such as int or float.
For example,
int a = 10;
int b = a;
b = 20; //a is not equal to 20!!!
Because Java Array is considered Reference Object.
int[] b = a; // variable 'b' points same memory location with 'a'
If you want to copy all value of a, write code below
@Test
public void arrayCopy1() {
int[] a = {1, 2, 3};
int[] b = a;
System.out.println(a); //Point [I@64a294a6
System.out.println(b); //Point [I@64a294a6
printArrayValues(a); //[1,2,3]
printArrayValues(b); //[1,2,3]
a[0] = 10;
printArrayValues(a); //[10,2,3]
printArrayValues(b); //[10,2,3]
}
@Test
public void arrayCopy2() {
int[] a = {1, 2, 3};
int[] b = Arrays.copyOf(a, a.length);
System.out.println(a); //Point [I@64a294a6
System.out.println(b); //Point [I@7e0b37bc
printArrayValues(a); //[1,2,3]
printArrayValues(b); //[1,2,3]
a[0] = 10;
printArrayValues(a); //[10,2,3]
printArrayValues(b); //[1,2,3]
}
private void printArrayValues(int[] arr) {
System.out.print("[");
for(int i = 0 ; i < arr.length ; i++) {
System.out.print(arr[i]);
if (i != arr.length-1)
System.out.print(",");
}
System.out.println("]");
}
Because when you set int[] b = a you didn't simply copied the values of a and gave them to b you gave to b the memory location of a, therefore if you change one, you change the other
