Making Decisions with if and else

New words in this topic, in plain English. Come back here whenever one feels fuzzy.

Control flow

The order in which the statements of a program run. if, switch and loops change that order.

Condition

An expression that is either true or false, written in the round brackets after if.

Block

A group of statements inside curly braces { }. Variables declared inside a block exist only inside it.

Branch

One of the possible paths through an if/else. Exactly one branch of an if/else runs.

else-if ladder

A chain of if ... else if ... else if ... else tests, checked from top to bottom.

Nested if

An if statement placed inside another if or else block.

Definite assignment

The compiler rule that a local variable must surely have a value on every path before you read it.

Guard clause

An early check at the top of a piece of code that handles a special case (often by returning) so the main logic stays flat.

1. 1

   Predict, then run the ladder

   Open 'Grades with an else-if ladder'. Before running, write down the grade for 89, 90 and 59. Add three println lines for them and run. Were the boundary values (90 exactly, 59) what you expected?

2. 2

   Break the order on purpose

   In the grade method, swap the score >= 90 and score >= 60 rungs. Predict which grades change, then run. Only scores of 60 and above are affected, and they all become D.

3. 3

   Remove the braces

   In 'The dangling else' example, set temp = 20 and raining = false. Predict both outputs before you run. (Without braces you'll now see Cold! even though it's 20 degrees, because the else belongs to if (raining).)

Errors are the best teachers. Make each change, read the error, guess what went wrong, then reveal the answer.

public class Main {
    public static void main(String[] args) {
        int x = 3;
        if (x = 5) {
            System.out.println("five");
        }
    }
}

public class Main {
    public static void main(String[] args) {
        int x = 5;
        String s;
        if (x > 2) { s = "big"; }
        System.out.println(s);
    }
}

public class Main {
    public static void main(String[] args) {
        int x = 5;
        if (x > 10);
        {
            System.out.println("big");
        }
    }
}

1. 1

   An if statement has a condition in round brackets and a block in curly braces: if (age >= 18) { ... }. The condition must be an expression of type boolean (true or false). Java does not treat numbers as truth values the way C does: if (1) and if (count) are compile errors, which rules out a whole family of C bugs.

2. 2

   else gives the 'otherwise' path. else if lets you test more conditions in a row, forming a ladder. Java checks the conditions from top to bottom and runs only the first branch whose condition is true; the rest are skipped, even if they would also be true. That means the order of the tests matters: put the most specific (narrowest) condition first.

3. 3

   Braces are optional when a branch has a single statement, but then only that one statement belongs to the if. Anything after it runs every time. An else always pairs with the nearest unmatched if (the 'dangling else' rule), whatever the indentation suggests. Always using braces removes both traps.

4. 4

   The compiler tracks definite assignment: if you declare a variable and give it a value only inside some branches, Java refuses to let you read it afterwards (variable s might not have been initialized). Assign it in every branch (an else is often the missing piece) or give it a starting value.

5. 5

   Conditions are often built with &&, || and ! from Topic 1.8. Because && and || short-circuit (they stop as soon as the answer is known), you can write a guard: if (name != null && name.length() > 3) never calls length() on null.

6. 6

   Under the hood, an if becomes a conditional jump in bytecode. The compiler usually flips the test: if (age >= 18) compiles to 'if age < 18, jump over the then-block'. There is no runtime cost to else if beyond one comparison per rung that is actually tested.

• ↔

  An else-if ladder handles ranges and mixed conditions; a switch (Topics 2.3 and 2.4) reads better when you compare one value against many fixed constants.

• ↔

  Guard clauses keep code flat, but many early exits can hide the main path in long methods. Use them for genuine special cases (bad input, nothing to do), not for every branch.

• ↔

  Omitting braces saves two characters and invites the dangling-else and 'second line always runs' bugs. Most teams require braces always.

• Done when you can write if, if/else and an else-if ladder from memory, with braces.

• Done when you can explain why the order of an else-if ladder matters and fix a wrongly ordered one.

• Done when you can spot the dangling else and the stray-semicolon bug in someone else's code.

• Done when you can read the 'might not have been initialized' error and fix it by covering every path.

• Done when you can explain that an if compiles to a conditional jump, and why the test appears inverted in bytecode.