Variable Scope in Java

As we saw with conditional statements, blocks of code in Java are a set of statements. They are enclosed by braces:

if (...) {
	// a block of code
}

A block of code is essentially a "mini" program inside a bigger program. And because they are effectively sub-programs, there are rules about what we can and cannot do inside or outside blocks of code. In particular: Variables declared inside the block are not visible outside the block. Once we leave the block of code, Java can no longer see that variableā€”the variable is no longer available for us to access. It is only accessible inside the block.

Inversely, if a variable is declared outside a block, then the variable is visible inside the block. If we declare a variable outside the block, then Java can see that variable inside the block.

int outsideBlock = 1
if (true) {
	int insideBlock = 10;
	System.out.println(outsideBlock);
}
System.out.println(insideBlock);

Line 6: error: cannot find symbol
System.out.println(inside);
						^
symbol:   variable inside
1 error

We get an error because we are telling Java to print the value assigned to a variable it cannot see. Once we left the if statement's blockā€”insideBlock's enclosing block (also called enclosing scope)ā€”insideBlock no longer exists for all intents and purposes. However, we can still access outsideBlock because it exists outside the if statement's block:

int outsideBlock = 1
if (true) {
	int insideBlock = 10;
	System.out.println(outsideBlock);
}

1

This inside-outside-block affair is an implication of scope. The scope of a variable refers to the areas of a program where we can refer to the variable by name. Outside of the variable's scope, we cannot refer to the variable by name. Scope is precisely why programming languages require, or encourage, indentation. Assuming we follow proper indentation like the examples above, the general rule of thumb is: We can access variables "to the left" or on the same level; we cannot access variables "to the right." (Where the directions are relative to the start of the line).