1.1 Introduction to Algorithms, Programming, and Compilers

1. Algorithms and sequencing

An algorithm is a step-by-step process for completing a task or solving a problem. You can represent an algorithm in written language (plain words) or as a diagram, for example a flowchart 1.1.A.1.

Here is an everyday algorithm in written language:

1. Boil water.
2. Put a tea bag in a cup.
3. Pour in the water.
4. Wait 3 minutes.
5. Remove the bag.

As a flowchart, each step is a box, and arrows join the boxes in this order: boil water, then tea bag in cup, then pour, then wait, then remove the bag.

Sequencing defines the order in which the steps happen. Steps are completed one at a time 1.1.A.2. Change the order and you may change the result: swap steps 1 and 3 and you get cold, wet tea leaves.

A Java program is an algorithm too. The statements in main run in sequence, top to bottom, one at a time:

int score1 = 80;
int score2 = 90;
int score3 = 85;
int total = score1 + score2 + score3;
System.out.println("Total: " + total);
System.out.println("Average: " + total / 3);

Output:

Total: 255
Average: 85

Watch out: the total must be computed before the average. If the average line came first, total would not exist yet and the code would not compile.

2. Writing, compiling and running code

Code can be written in any text editor. However, programmers often use an integrated development environment (IDE), because it gives tools to write, compile and run code in one place 1.1.B.1. Without an IDE, you run the compiler and then the program yourself.

A compiler checks your code for some errors. Errors the compiler can detect must be fixed before the program can run 1.1.B.2.

The path from code to a working program has four stages. Each type of error shows up at a different stage:

Stage	What happens	Error that shows up here
1. Write code	You type the program in a text editor or IDE.
2. Compile	The compiler checks some errors.	Syntax error: rules broken; fix it, then compile again.
3. Run	The statements execute.	Run-time error: for example an exception; the program stops abnormally.
4. Test	You try known inputs and compare with the expected outcome.	Logic error: the program runs, but gives a wrong result.

Here the programmer forgot a semicolon:

// does not compile
public class Lives
{
    public static void main(String[] args)
    {
        int lives = 3
        System.out.println(lives);
    }
}

Real view: the compiler stops and reports the file, the line number (6) and the problem. Nothing runs.

Lives.java:6: error: ';' expected
        int lives = 3
                     ^
1 error

3. The three types of programming error

Error	What it is	Who or what finds it	Example
Syntax error	The rules of Java are not followed 1.1.C.1.	The compiler	A missing ; or )
Logic error	The algorithm or program is wrong, so it behaves incorrectly or unexpectedly 1.1.C.2.	Testing with specific data and comparing to the expected outcome	Area computed as length + width
Run-time error	Happens while the program runs (during execution); usually makes it stop abnormally 1.1.C.3.	Shows up during execution	Dividing an int by 0

A logic error compiles and runs without complaint. This program should print the area of a 5 by 3 rectangle, which is 15:

int length = 5;
int width = 3;
int area = length + width;
System.out.println("Area: " + area);

Output:

Area: 8

The output is wrong because the formula is wrong. Testing with data where you already know the answer (5 and 3 give 15) reveals the error. The fix is length * width.

Watch out: “it compiles” does not mean “it is correct”.

4. Exceptions

An exception is a type of run-time error. It happens because of an unexpected problem that the compiler did not detect, and it interrupts the normal flow of the program’s execution 1.1.C.4. In this course, an exception stops the program, so the statements after it do not run.

int total = 12;
int count = 0;
System.out.println("Start");
System.out.println(total / count);
System.out.println("End");

Output:

Start
Exception in thread "main" java.lang.ArithmeticException: / by zero

The code compiles, because the compiler does not know that count will be 0. The program prints Start, then stops at the division. End is never printed.

Unit 1 names three exceptions:

• ArithmeticException: an integer divided by the integer zero (topic 1.3).
• NullPointerException: a method called on null (topic 1.14).
• StringIndexOutOfBoundsException: an index outside a String (topic 1.15).

Background (not tested): what the compiler actually makes. The Java compiler turns your .java file into bytecode (a .class file). The Java Virtual Machine (JVM) then runs the bytecode, which is why the same Java program runs on different operating systems. Every program starts running at its main method: public static void main(String[] args). Programmers use these words often; the exam does not test them.

5. Unit 1 on the AP® exam

• The College Board weights Unit 1 at 15 to 25 percent of the multiple-choice section. That section has 42 questions in 90 minutes and is 55 percent of the exam score.
• Unit 1 skills also appear in the free-response section (4 questions, 90 minutes, 45 percent). Question 1, Methods and Control Structures, has a part (3 points) that requires calling String methods, and the sample free-response Questions 1, 3 and 4 in the official Course and Exam Description (CED) all use Unit 1 objectives.
• The unit develops 9 skills: 1.A (design), 2.A, 2.B, 2.C (develop code), 3.A, 3.C, 3.D (analyze code) and 4.A, 4.B (document code).
• Topic 1.15 (String Manipulation) is the largest topic in the unit.
• You get the Java Quick Reference on the exam. Learn the Math and String methods on it now (topics 1.11 and 1.15).
• Tags like 1.3.C on these pages point to the College Board learning objective each note covers. Background boxes help you understand a topic; their wording is not tested.

Official practice: do the CED sample multiple-choice Questions 1 to 4 (Exam Information section of the AP® Computer Science A CED, effective Fall 2025; Questions 1 and 2 are on page 149, Questions 3 and 4 on page 150, answer key on page 170). Questions 5 to 20 test later units. Sample free-response Question 1, Methods and Control Structures (pages 161 to 163): part (b) uses Unit 1 String methods, and part (a) needs Unit 2 loops, so try the full question after Unit 2. The same goes for the 2026 Free-Response Question 1 (released exam, pages 4 to 6) on the AP® Central past exam questions page. Score yourself with the College Board’s own scoring guidelines.

Background (not tested): words you will hear. Programmers use these words every day. They are useful, but they are not the official course wording, so on the exam look for the course term.

You may hear	Course term or meaning
static method	class method (1.10)
field, property	attribute (1.7); Unit 3 adds the term instance variable
pass by value	call by value (1.9)
actual parameter / formal parameter	argument / parameter (1.9)
build, make an object	instantiation: create an object with new (1.13)
type casting, type conversion	casting (1.5)
bytecode, JVM, main method	not in the course framework; see section 4 above
operand; unary / binary operator	the value an operator works on; an operator with one / two operands (1.3)
crash; bug	run-time error; bug means any error, often a logic error (1.1)
string joining	concatenation (1.15)
object address, pointer	object reference (1.12)