Control Flow & switch

if — Always Use Braces

Even for one-liners, Java style guides recommend braces:

if (x > 0) {
    System.out.println("positive");
}

Skipping braces can hide bugs:

if (x > 0)
    System.out.println("a");
    System.out.println("b");   // ALWAYS runs — not part of the if!

Boolean Operators

&&  // logical AND, short-circuits
||  // logical OR,  short-circuits
!   // NOT
==  // equality (reference for objects, value for primitives)
!=  // inequality
&   
^   // bitwise — also works on booleans (no short-circuit)
> Use && and  for control flow — they short-circuit, avoiding NPE / divide-by-zero.
if (str != null && str.length() > 0) /* safe */;
if (x != 0 && 100 / x > 1)            /* safe */;
Ternary
String label = (score >= 60) ? "pass" : "fail";
int max = a > b ? a : b;
Don't nest ternaries deeply — they get unreadable fast.
while and do-while
int n = 10;
while (n > 0) {
    System.out.println(n);
    n--;
}
int x;
do {
    x = readUserInput();
} while (x < 0);    // loop body runs at least once
Classic for Loop
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i]);
}
// Multiple init / update
for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
    /* ... */
}
// Infinite loop — the canonical idiom
for (;;) {
    if (done) break;
}
Enhanced for (foreach)
Works on arrays and anything implementing Iterable:
int[] arr = {1, 2, 3};
for (int x : arr) System.out.println(x);
List<String> names = List.of("a", "b", "c");
for (String n : names) System.out.println(n);
Map<String, Integer> ages = Map.of("Asha", 19, "Bishan", 21);
for (var entry : ages.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}
> Foreach gives you the value, not the index. If you need the index, use a classic for.
break and continue
for (int i = 0; i < 10; i++) {
    if (i == 5) break;        // exit the loop
    if (i % 2 == 0) continue; // skip to next iteration
    System.out.println(i);
}
Labeled Break (rare but useful)
outer:
for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
        if (matrix[i][j] == target) {
            System.out.println("found at " + i + "," + j);
            break outer;       // break BOTH loops
        }
    }
}
switch — Three Generations
1. Classic Statement (1.0)
switch (day) {
    case 1: case 2: case 3: case 4: case 5:
        System.out.println("weekday"); break;
    case 6: case 7:
        System.out.println("weekend"); break;
    default:
        System.out.println("invalid");
}
⚠️ Forget break and execution falls through to the next case. This is the source of countless bugs.
2. Switch Expression (Java 14)
String label = switch (day) {
    case 1, 2, 3, 4, 5 -> "weekday";
    case 6, 7          -> "weekend";
    default            -> "invalid";
};
Returns a value.
No fallthrough.
Multiple labels per arm (1, 2, 3).
Compiler enforces exhaustiveness (every value covered).

For multi-statement branches, use a block + yield:
int rate = switch (level) {
    case "premium" -> 100;
    case "basic"   -> 50;
    case "free"    -> {
        log("free user");
        yield 0;
    }
    default        -> throw new IllegalArgumentException(level);
};
3. Pattern Matching for switch (Java 21)
Type-tested with binding:
sealed interface Shape permits Circle, Square, Rect {}
record Circle(double r) implements Shape {}
record Square(double side) implements Shape {}
record Rect(double w, double h) implements Shape {}
double area(Shape s) {
    return switch (s) {
        case Circle c -> Math.PI * c.r() * c.r();
        case Square sq -> sq.side() * sq.side();
        case Rect r   -> r.w() * r.h();
    };
}
Add guards:
String describe(Object o) {
    return switch (o) {
        case Integer i when i < 0 -> "negative int";
        case Integer i            -> "int " + i;
        case String s             -> "string \"" + s + "\"";
        case null                  -> "null";
        default                    -> "other";
    };
}
> Pattern matching for switch only switches on what the JVM can statically know — generally objects and primitives that fit in 32 bits (int, String, enum), and reference types under sealed hierarchies.
When to Use Each
Construct
Use case

if/else
Conditions on continuous values, multiple unrelated checks
switch (expression)
Mapping a discrete value to another value/string
switch (pattern)
Type-dispatch over sealed hierarchy / objects
while
Loop with unknown count
do-while
Loop that must run at least once (input validation)
for
Counted loop with index
Enhanced for
Iterate every element — clearest, fewest bugs
Common Mistakes
Bug
Fix

Missing break in classic switch
Use switch expression with ->
if (a = b) instead of a == b
Java forbids non-boolean conditions, so this is a compile error for ints — but watch for boolean variables
Off-by-one on <= length
Use < arr.length or enhanced for
Modifying a list during enhanced for
Use Iterator.remove() or stream filter
Comparing strings with == in a switch
switch on Strings uses .equals() automatically — that's fine
Cheat-Sheet
Need
Code

Branch
if (cond) { ... } else { ... }
Map a value
var x = switch(k) { case A -> 1; default -> 0; };
Counted loop
for (int i = 0; i < n; i++)
Iterate elements
for (T x : iterable)
At-least-once
do { ... } while (cond);
Exit nested loop
labelled break
Type dispatch
pattern-matching switch (Java 21)
You've got branching down. Phase 2 next: methods, strings, arrays, and your first classes.