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For teachersTopic 2.4 · Lesson kit

Nested if Statements

  • 2 class periods
  • 45 minutes each
  • Objective 2.4.A
  • 6 downloads with keys

Downloads

At a glance

Pacing
2 class periods of 45 minutes
Learning objectives
2.4.A
Essential knowledge
2.4.A.12.4.A.22.4.A.3
Materials
Projector and the Branches on Branches slide deckWhiteboard and markersMini whiteboards or scrap paperIDE on every computer (optional demonstrations)Printed Branches on Branches puzzle worksheet (answer key for you) and exit ticket

45 minutes a day

Agenda

Day 145 minutes

  • Warm-up
  • Direct instruction
  • Guided practice
  • Closure
  1. 0:00–0:055 minWarm-upa question inside a question (slide 2)
  2. 0:05–0:2015 minDirect instructionnested if and the guard (slides 3–7)
  3. 0:20–0:3515 minDirect instructionnesting in else, the inner else, the dangling else (slides 8–11)
  4. 0:35–0:438 minGuided practicewhat prints for each player (slide 12)
  5. 0:43–0:452 minClosurewhich else?

Day 245 minutes

  • Warm-up
  • Direct instruction
  • Guided practice
  • Closure
  • Activity
  1. 0:00–0:055 minWarm-upnested or chained?
  2. 0:05–0:2015 minDirect instructionmultiway selection, order, trailing else (slides 13–17)
  3. 0:20–0:3212 minGuided practiceshipping tiers and write the chain (slides 18–19)
  4. 0:32–0:353 minClosurevocabulary and sentence starter (slides 20–21)
  5. 0:35–0:4510 minActivityDay 2 independent practice and exit ticket

Overview

Day 1 is nesting: an if or if-else inside the body of another one, and why the inner condition is checked only when the program reaches it (2.4.A.1, 2.4.A.2). Day 2 is multiway selection: an if, else if, else chain, in which at most one body runs, the order of the conditions matters, and the trailing else catches everything left over (2.4.A.3).

Students know the relational operators (Topic 2.2) and if and if-else statements (Topic 2.3). Stay inside the topic boundary: conditions are single comparisons or boolean variables, with no &&, ||, or ! (that is Topic 2.5), and no while or for loops. Strings appear only in a few guard examples, with length and substring from Topic 1.15.

Teacher note

The slides are the Branches on Branches deck (21 slides, with speaker notes); slide numbers below refer to it. Day 1 uses slides 1–12 and Day 2 uses slides 13–21. The Branches on Branches puzzle worksheet (10 questions, about 35 minutes) is split across the two nights: questions 2, 3, and 6 after Day 1; questions 1, 4, 5, 7, 8, 9, and 10 after Day 2.

Files whose names end in .snippet.java hold only the statements that go inside main. All files are in the topic’s examples/ folder. Everything here is optional to run; the slides carry the examples.

  • Day 1 demonstrations: Parking, Shipping, PagesGuard, GuardWrongOrder, StringGuard, StringNoGuard, InnerElseMisread, InnerElseFixed, DanglingElse, DanglingElseFixed.java.
  • Day 2 demonstrations: SpeedCheck, GradeChain, GradeWrongOrder, GradeLowFirst, SeparateIfs, HotMildChain, HotMildSeparate, ElseFirstBug.java, ElseInMiddle.java, ElseWithCondition.java.
  • Printed only (exit ticket): ExitPool, ExitChainOrder, and ExitNoBraces.

Day 1 · Warm-up: A question inside a question

Show slide 2, Hopper’s day plan. Students answer in pairs: what does the plan say on a sunny Tuesday, and on a rainy Sunday? Take answers: school on the Tuesday, a movie on the Sunday. Then ask: on which days does the plan ask about the weather at all? Only on the weekend, because the weather question sits inside the weekend branch. Name it: a question that is asked only after an earlier question was answered.

Day 1 · Direct instruction: Nested if and the guard

  1. Goals (slide 3). Goals 1–3 are Day 1; goals 4 and 5 are Day 2.
  2. A nested if (slide 5; 2.4.A.1). A nested if is an if inside the body of another if or else. Trace the library check: with age 15, the outer condition is true, so Java reaches if (hasCard); with age 10, the whole outer body is skipped and hasCard is never looked at. Then Welcome is displayed either way, because it is outside both bodies.
  3. The guard (slide 6; 2.4.A.1). An outer condition that makes an inner check safe is a guard. With players 0, players > 0 is false, so Java never evaluates points / players. Trace it step by step with the table on the slide.
  4. Guard on the inside (slide 7). Swap the two checks and the division is now the outer condition, so it runs first and throws an ArithmeticException (Topic 1.3). Nothing is displayed, not even Done.

Misconception: The guard can go inside the risky check

Students write both checks but put the safety check inside, so the risky one runs first. A guard only protects what is inside its body. The outer condition is always evaluated first, so the check that can crash must be the inner one. Have students say, for each snippet, “Which condition does Java evaluate first?”

Teacher note

If computers are open, run PagesGuard and then GuardWrongOrder. Students see Report finished for one and an exception with no output for the other. StringGuard and StringNoGuard show the same idea with substring.

Day 1 · Direct instruction: Nesting in else, the inner else, the dangling else

  1. Nesting in the else branch (slide 9; 2.4.A.2). An if can sit in an else body as well. Trace temp 40, raining true: temp >= 60 is false, so Java runs the else body and reaches if (raining), which displays Raincoat. One path runs, with three possible results. Draw the decision tree from the slide on the board.
  2. An inner else answers only its own if (slide 10; 2.4.A.2). With age 14, age >= 16 is false, so the whole body is skipped, inner else included; only Done is displayed.
  3. Braces and the nearest if (slide 11; 2.4.A.2). Without braces, an else pairs with the nearest earlier if that does not already have an else, whatever the indentation says. Compare the two columns for n 3: the left prints not positive?, because the else belongs to if (n > 5); the braced version prints only end because n > 0 is true and the inner condition is false.

Misconception: The inner else runs whenever the outer condition is false

Students read an inner else as “otherwise, for anything that failed.” Each else answers exactly one if. The inner else runs only when the outer condition is true and the inner one is false. This is the Think First question on worksheet question 2.

Misconception: The indentation shows which if the else belongs to

Students trust the layout. The compiler never looks at indentation, and code with a dangling else compiles without any warning. Teach the routine: ignore indentation, find each else, and pair it with the nearest unpaired if above it; then add braces if you want a different pairing.

Day 1 · Guided practice: What prints for each player

Show slide 12 and give students two minutes with mini whiteboards, one row at a time: hours 9 with rested true displays Ready; 9 with false displays Nap first; 6 with true displays Sleep more, because the inner if-else is skipped. For the third row ask: is rested ever checked? No. If time remains, students draw the decision tree on their whiteboard.

Teacher note

Hold up a whiteboard check after each row. If many students write Nap first for the third row, return to slide 10 and the inner else.

Day 1 independent practice and closure

Puzzle worksheet questions 2, 3, and 6 are homework: the ride sign, the code tiles that guard the division, and two truths and a lie about the else with no braces. Question 3 settles the guard-order idea from today.

For closure, write one sentence on the board: “An inner else belongs to __.” Students finish it with their partner: the nearest if without an else, which is not always the outer one.

Day 2 · Warm-up: Nested or chained?

Show this on the board: a movie ticket costs one price for children and a different price for everyone else. Ask students whether they need a nested if or an if-else. Then add a third group, seniors. Take two answers on how the code changes. This introduces the else if chain.

Day 2 · Direct instruction: Multiway selection, order, trailing else

  1. The first true condition wins (slide 14; 2.4.A.3). A multiway selection is an if, else if, else chain. Trace speed 72: 72 > 80 is false, so Java goes on; 72 > 65 is true, so Warning is displayed and the rest is skipped. The else is never checked.
  2. Order matters (slide 15). Reverse the first two conditions: speed 90 is over 65, so it displays Warning and Ticket is unreachable, because every value over 80 is also over 65. Rule: put the more specific or narrower condition first, or order the tests so each one needs only a single comparison.
  3. The trailing else (slide 16). With no trailing else, coins 3 matches neither condition and nothing is displayed from the chain; the next line, Coins: 3, still runs. With the trailing else, No chest is displayed first.
  4. The plain else goes last (slide 17). An else never has a condition, and nothing can follow it in the chain, so else followed by else if does not compile ('else' without 'if').
  5. Vocabulary (slide 20) can wait until closure.

Misconception: Every true condition runs its body

For score 95, students see that >= 90, >= 80, and >= 70 are all true and predict several lines. Each later condition is reached only through an else, so only the first true one runs. This is the question in worksheet question 1 (answer A, not B).

Misconception: A row of separate ifs is the same as a chain

Leave out the word else and Java checks every if, so several bodies can run, or none. Show SeparateIfs next to GradeChain, or HotMildSeparate next to HotMildChain. Worksheet question 7 (one token: add else) and question 5 (the face-off) cover it.

Misconception: A plain else can go before an else if

Once an if has its plain else, the statement is complete, so another else if cannot attach. Students need to see the compiler message once, on slide 17 or with ElseFirstBug.java.

Day 2 · Guided practice: Shipping tiers and write the chain

  1. Shipping tiers (slide 18). Students write the value printed for each weight on mini whiteboards: 25 displays 25.0; 20 displays 12.0; 5 displays 6.0; 1 displays 6.0. Ask about 20 and 5: neither is above 20 or above 5 (the boundaries), so each falls to the next branch.
  2. Write the chain (slide 19). Students write their own if, else if, else chain that prints Freezing, Cold, Mild, or Hot for an int temp. A model: if (temp < 0) Freezing, else if (temp <= 15) Cold, else if (temp <= 25) Mild, else Hot. Follow the hint on the slide: each condition needs one comparison because the order does the work. Have two students share different valid orders (for example, starting from temp > 25).

Teacher note

Check the boundaries, 0, 15, and 25, with the class. They are the usual off-by-one errors, and each lands in a different branch.

Day 2 · Independent practice: Independent practice

Puzzle worksheet questions 1, 4, 5, 7, 8, 9, and 10 are homework: the points chain, the grade snippet pool, the chain versus separate if face-off, the one-token fix (add else), who am I, you’re the compiler (snippets 2 and 3 return to the guard order), and the wrap-up sentence.

Day 2 · Closure: Closure and exit ticket

Show slide 20 (vocabulary), then slide 21 (closure) and have students finish the sentence starter with a partner. Give the exit ticket (about five minutes). Question 1 checks tracing a nested if-else (2.4.A.1, 2.4.A.2), question 2 checks that only the first true branch of a chain runs, in order (2.4.A.3), and question 3 checks pairing an else without braces (2.4.A.2). Re-teach the weakest one at the start of Topic 2.5.

Exit ticket

Question 1 · Tracing

Write the exact output.

int minutes = 40;
boolean member = false;
if (minutes >= 30)
{
    if (member)
    {
        System.out.println("Free");
    }
    else
    {
        System.out.println("Pay");
    }
}
else
{
    System.out.println("Short");
}
System.out.println("Done");
Open in IDE
Check your reasoning
Pay
Done

minutes >= 30 is true (40), so Java enters the outer body and checks the inner if (member). member is false, so the inner else runs and displays Pay. The outer else is skipped, and Done is displayed after the whole statement.

A complete answer has both lines. Short is wrong because the outer condition is true, and Free is wrong because member is false.

Question 2 · Multiple choice

What does this code display?

int score = 95;
if (score >= 60)
{
    System.out.println("Pass");
}
else if (score >= 90)
{
    System.out.println("Honors");
}
else
{
    System.out.println("Fail");
}
Open in IDE
  1. Honors
  2. Pass
  3. Pass and then Honors
  4. Nothing
Check your reasoning

B. Java checks the chain in order and runs only the first true condition. 95 >= 60 is true, so Pass is displayed and the rest of the chain, including score >= 90, is skipped. A assumes the most specific condition wins; the order decides, not the size of the number. C treats the else if like a separate if; at most one body of a chain runs. D would need every condition to be false, but the first is true.

Question 3 · Tracing

This code has no braces. Which if does the else belong to, and what is the exact output?

int n = 8;
if (n > 0)
    if (n > 10)
        System.out.println("A");
else
    System.out.println("B");
System.out.println("C");
Open in IDE
Check your reasoning
B
C

Java ignores indentation. An else pairs with the nearest earlier if that has no else, which is if (n > 10). The outer condition n > 0 is true (8), so Java enters the inner if-else; n > 10 is false, so the else runs and displays B. The last line is not part of any if, so C is always displayed.

A complete answer names if (n > 10) as the partner of the else and gives both lines.