Selection and Iteration in Java

Starter: think before typing

Before running this control flow example, find the line where the main idea becomes active. Write a prediction: what must already be true for that line to work, and what should be different after it runs? The checked run ends with `Week 1: start carefully Week 2: repeat, test, improve Week 3: repeat, test, improve`; predict how the focus line helps produce that evidence.

Learning Objectives

• Use if/else for decisions.
• Use for loops when the repeat count is known.
• Use while loops when repetition depends on a condition.
• Trace loop variables to avoid off-by-one errors.

Learning Outcomes

• By the end of the lesson, you can use if/else for decisions.
• By the end of the lesson, you can use for loops when the repeat count is known.
• By the end of the lesson, you can use while loops when repetition depends on a condition.
• By the end of the lesson, you can trace loop variables to avoid off-by-one errors.

Why this idea exists

Sequence, selection and iteration are the core control structures behind structured programming, a movement that pushed programmers away from tangled jumps and towards readable blocks of logic.

Selection lets a program respond to state. Iteration lets it scale effort by repeating a reliable process. Together they turn a fixed list of instructions into a general solution for many inputs.

Degree-level work expects more than knowing the syntax. You should be able to justify why a `for` loop, `while` loop, `if` chain or `switch` fits the shape of the problem and how you know it terminates correctly.

Deep dive

Lesson visual

Type this and run it

Create SelectionAndIterationDemo.java, type the program, and run it before changing anything. This section is about reproducing the checked baseline.

public class SelectionAndIterationDemo {
  public static void main(String[] args) {
    for (int week = 1; week <= 3; week++) {
      if (week == 1) {
        System.out.println("Week " + week + ": start carefully");
      } else {
        System.out.println("Week " + week + ": repeat, test, improve");
      }
    }
  }
}

Build and run it with:

javac SelectionAndIterationDemo.java && java SelectionAndIterationDemo

Expected baseline: Week 1: start carefully Week 2: repeat, test, improve Week 3: repeat, test, improve

Line-by-line explanation

Read the code as a sequence of responsibilities. The focus line for this lesson is for (int week = 1; week <= 3; week++) {; the surrounding lines prepare it, use its result or make the behaviour observable.

Worked example

Trace the program

Common misconception

A common mistake in selection and iteration in java is treating the example as a finished answer. For control flow, the important question is narrower: which operation carries the idea, what does it make possible, and what would break if you changed it carelessly?

Quick checks

How to study this lesson

For Selection and Iteration in Java, predict how control flow changes the run before you press Run.

Use the first portfolio task as your main edit: Write a menu with three choices.

Use the second task as your variation: Loop until the user chooses quit.

Finish with evidence, not a diary entry: Create a trace table for one loop.

Portfolio Practice

1. Write a menu with three choices.
2. Loop until the user chooses quit.
3. Create a trace table for one loop.

Final self-check

Study route

Practise control flow by predicting the Java example, typing it, running it in the browser, tracing the result and saving portfolio evidence.

Next, move into If Statements, Determinism and Safer Control Flow and carry forward one improvement from this lesson into the next program.