Control Flow

1Not every line runs next

Without control, Java runs the next statement, then the next. Control statements change that: skip a block, pick one of several, or repeat.

The machine still only does what you wrote. It does not guess which seats are free.

Figure. The top node is the default: run the next line. The right branch is what if, switch and loops are for — skip, pick, or repeat. Not a full flowchart of a program, only that fork.

How it works

1. Not every line runs nextDefault is the next line. Control is how you skip, pick, or repeat.

2if picks one block

if (condition) { ... } else { ... } runs one block or the other. The condition must be boolean, not an int.

An else-if chain tests in order and takes the first true. Forgetting braces is legal for one statement and is how people attach the wrong line to the if.

Figure. if (student) runs one block or the other. The condition is boolean. Here student is true, so price is 150; the else sets 200.

Student fare or list price

boolean student = true;
int price;
if (student) {
    price = 150;
} else {
    price = 200;
}

Coding lab. Choose Meera's fare runs in the app, with checks on your output.

3switch matches a value; break stays put

switch picks a case from a value — a day name, a seat type — instead of a long else-if. default runs when no case matches.

Each case should break, or it falls through into the next case. Use switch when you match one value against known labels, not when you test ranges.

Figure. Sat and Sun share one body: extra is 30, then break. default is 0. Each case should break, or it falls through into the next case.

Weekend surcharge; Sat and Sun share a body

String day = "Sat";
int extra = 0;
switch (day) {
    case "Sat":
    case "Sun":
        extra = 30;
        break;
    default:
        extra = 0;
}

4while may run zero times; do-while runs once

while (condition) { ... } repeats as long as the condition is true — maybe zero times if it starts false.

do { ... } while (condition); runs the body once, then tests — at least once. You must change something the condition reads, or the loop does not stop.

No diagram — the idea is carried by the prose or the Java listing.

Count down remaining seats

int seatsLeft = 3;
while (seatsLeft > 0) {
    System.out.println(seatsLeft);
    seatsLeft = seatsLeft - 1;
}

5Counted for versus walk each value

for (int i = 0; i < n; i++) is a counted loop: start, test, step. The index i is how you name a position.

The enhanced for (for (String seat : seats)) walks each element. Use the counted for when you need the index; use for-each when you only need each value.

Figure. The counted for names a position: 0 A12, 1 A13, 2 A14. The enhanced for walks each seat value with no index. Use the counted for when you need the index.

Index when you need it; for-each when you do not

String[] seats = {"A12", "A13", "A14"};
for (int i = 0; i < seats.length; i++) {
    System.out.println(i + " " + seats[i]);
}
for (String seat : seats) {
    System.out.println(seat);
}

6break leaves; continue skips this pass

break leaves the innermost loop or switch immediately. continue skips the rest of this pass and goes to the next test or step.

Neither is a substitute for a clear condition. They are exits from a pass you already started.

No diagram — the idea is carried by the prose or the Java listing.

Skip a blocked mark; take the first real seat

String[] seats = {"A12", "X", "A14"};
for (String seat : seats) {
    if (seat.equals("X")) {
        continue;
    }
    System.out.println(seat);
    break;
}