Scope in loops, traced like a compiler

☕ Java 17 · Managing Variable Scope · 2/3

Braces define scope. Loops follow the same rule, with a detail that catches many people: the loop counter.

The loop variable belongs to the loop

public void loadEpisodes(int count) {
    for (int i = 0; i < count; i++) {
        String title = "Ep-" + i;
        System.out.println(title);
    }
    System.out.println(i);         // ❌ i is out of scope
    System.out.println(title);     // ❌ title too
}

• i is declared in the for header, so its scope is the loop, not the method.
• title is declared in the loop body: a fresh title is created on each iteration, and none survives the loop.

while loops work the same way for variables declared in their body. And var changes nothing: a var declared in a block dies with that block.

If you need a value after the loop, declare the variable before the loop.

Tracing scope line by line

The safest way to reason about scope is to note where each variable is born and where it dies:

1  public void recommend(boolean premium) {
2      int suggestions = 5;
3      while (suggestions > 0) {
4          int watched = 1;
5          if (premium) {
6              var bonus = 2;
7              watched += bonus;
8          }
9          suggestions -= watched;
10     }
11     System.out.println(suggestions);
12 }

Variable	Lives from → until
premium	line 1 → end of the method
suggestions	line 2 → end of the method
watched	line 4 → line 10, recreated on each iteration
bonus	line 6 → line 8, recreated each time the if runs

At line 11, only premium and suggestions exist. Using watched or bonus there would not compile.

Declared but unused is fine

int episodes = 10;
for (int i = 0; i < 2; i++) {
    int bonus = i * 3;
}
System.out.println(episodes);      // ✅ prints 10

bonus goes out of scope after the loop, but nothing tries to use it there, so the code compiles. Scope errors happen only when a variable is used outside its region.

The lab puts all three levels — field, method and loop — in one runnable class.

💻 Code: Java17_Part_1_Fundamentals_Core_Syntax on GitHub

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