☕ Java
Control Statements
Control statements determine the flow of execution in a Java program. Without them, code executes line by line from top to bottom. Control statements allow the program to make decisions, repeat actions, and jump to different parts of the code based on conditions. Java provides three categories: selection statements (if, if-else, switch), iteration statements (for, while, do-while), and jump statements (break, continue, return).
Categories of Control Statements
Java control statements fall into three categories. Selection statements choose which block of code to execute based on a condition. Iteration statements repeat a block of code multiple times. Jump statements transfer control to a different part of the program unconditionally.
Java
// ── 1. SELECTION STATEMENTS — choose a path: ────────────────────────
//
// if → execute a block if condition is true
// if-else → choose between two blocks
// if-else if → choose among multiple conditions
// switch → choose among multiple constant values
// ── 2. ITERATION STATEMENTS — repeat a block: ────────────────────────
//
// for → repeat a known number of times
// while → repeat while condition is true (check first)
// do-while → repeat while condition is true (execute first)
// for-each → iterate over arrays and collections
// ── 3. JUMP STATEMENTS — transfer control: ───────────────────────────
//
// break → exit a loop or switch immediately
// continue → skip current iteration, jump to next
// return → exit the current method, optionally return a value
// ── Without control statements — always the same path: ───────────────
int score = 85;
System.out.println("Calculating grade...");
System.out.println("Grade: A"); // always prints A regardless
System.out.println("Done.");
// ── With control statements — path depends on data: ──────────────────
int score2 = 85;
System.out.println("Calculating grade...");
if (score2 >= 90) {
System.out.println("Grade: A");
} else if (score2 >= 80) {
System.out.println("Grade: B"); // this executes for score=85
} else {
System.out.println("Grade: C or below");
}
System.out.println("Done.");Selection Statements Overview
Selection statements evaluate a boolean expression and execute different code blocks based on the result. Java provides if, if-else, if-else-if chains, and switch statements. Every selection statement evaluates to true or false — Java does not allow non-boolean types (such as integers) as conditions, unlike C or JavaScript.
Java
// ── Boolean expressions in conditions: ───────────────────────────────
// Comparison operators:
int age = 20;
boolean isAdult = age >= 18; // true
boolean isMinor = age < 18; // false
boolean isExact = age == 20; // true
boolean isNot20 = age != 20; // false
// Logical operators — combine conditions:
boolean a = true, b = false;
boolean andResult = a && b; // false — both must be true
boolean orResult = a || b; // true — at least one must be true
boolean notResult = !a; // false — negation
// ── Java requires a boolean — NOT an integer: ─────────────────────────
int x = 1;
// if (x) { } // COMPILE ERROR — not a boolean
if (x == 1) { } // CORRECT — explicit comparison
// ── Common boolean expressions used in conditions: ────────────────────
String name = "Alice";
int score = 75;
Object obj = null;
// Null check:
if (name != null) { }
// String comparison — use .equals(), never ==:
if (name.equals("Alice")) { }
if ("Alice".equals(name)) { } // null-safe — preferred
// Range check:
if (score >= 60 && score <= 100) { }
// Null-safe check before method call:
if (obj != null && obj.toString().isEmpty()) { }
// instanceof check:
Object value = "Hello";
if (value instanceof String s) { // Java 16+ pattern matching
System.out.println(s.toUpperCase());
}Iteration Statements Overview
Iteration statements (loops) repeat a block of code. Java provides four loop constructs. The for loop is used when the number of iterations is known in advance. The while loop is used when the condition is evaluated before each iteration. The do-while loop guarantees at least one execution. The enhanced for-each loop iterates over arrays and Iterable collections.
Java
// ── for loop — known iteration count: ────────────────────────────────
for (int i = 0; i < 5; i++) {
System.out.print(i + " "); // 0 1 2 3 4
}
// ── while loop — condition checked before each iteration: ─────────────
int count = 0;
while (count < 5) {
System.out.print(count + " "); // 0 1 2 3 4
count++;
}
// ── do-while loop — executes at least once: ───────────────────────────
int n = 10;
do {
System.out.println("Executed once even though n >= 5");
n++;
} while (n < 5); // condition false — but ran once
// ── for-each loop — iterate over array or collection: ─────────────────
int[] numbers = {10, 20, 30, 40, 50};
for (int num : numbers) {
System.out.print(num + " "); // 10 20 30 40 50
}
List<String> names = List.of("Alice", "Bob", "Charlie");
for (String name : names) {
System.out.println("Hello, " + name);
}
// ── Jump statements inside loops: ────────────────────────────────────
// break — exit loop immediately:
for (int i = 0; i < 10; i++) {
if (i == 5) break;
System.out.print(i + " "); // 0 1 2 3 4
}
// continue — skip current iteration, continue to next:
for (int i = 0; i < 10; i++) {
if (i % 2 == 0) continue; // skip even numbers
System.out.print(i + " "); // 1 3 5 7 9
}Related Topics in Control Statements
while Loop
The while loop is a condition-controlled loop — it continues executing as long as its boolean condition remains true, without specifying in advance how many iterations will occur. It is the natural choice for reading input until end-of-stream, polling until a condition changes, retrying an operation, or any scenario where the termination condition depends on data or events that cannot be predetermined. This entry covers syntax, execution flow, common patterns, infinite loops with controlled exit, and the differences between while and for.
do-while Loop
The do-while loop is Java's only post-test loop — the condition is evaluated after the body executes, which guarantees the body runs at least once regardless of the condition's initial value. This property makes do-while the natural fit for scenarios where the first execution must happen before the first check: menu-driven programs, input validation, digest computation, and game turns. This entry covers syntax, when to use do-while over while, input validation patterns, and the subtle differences that matter in practice.
Enhanced for Loop
The enhanced for loop — also called the for-each loop — was introduced in Java 5 to provide a clean, readable syntax for iterating over arrays and any class that implements Iterable. It eliminates index management and iterator boilerplate entirely. The developer declares what each element is and what to do with it, without concerning themselves with how the iteration mechanism works. This entry covers syntax, the Iterable contract, limitations compared to the indexed loop, modification restrictions, and how for-each maps to iterators under the hood.
break Statement
The break statement immediately terminates the nearest enclosing loop or switch block and transfers control to the first statement after that block. It is one of Java's three jump statements alongside continue and return. break is essential for exiting loops early when a search condition is met, preventing infinite loops from running forever, and stopping switch execution after a matched case.