AP CSA 1.1：算法、Java 程序结构与调试入门

June 11, 2026

Core Concepts:
核心知识点

Introduction

This lesson is the first step into AP Computer Science A.

这一课是 AP CSA 的第一步。你不需要一开始就完全理解 Java 的每一个单词，但你需要先建立一个重要的基本观念：

A program is an ordered set of instructions, written with strict syntax, checked by the compiler, and improved through debugging. 程序是一组按顺序执行的指令，需要遵守严格语法，由编译器检查，并通过调试不断改进。

1. Big Picture

What you should understand after this lesson

Core idea	What it means	中文理解
Algorithm	A step-by-step process for solving a problem	算法就是解决问题的一组步骤
Sequencing	Steps happen in a specific order, one at a time	程序会按照顺序一步一步执行
Java program structure	Java code is written inside a class and usually starts from `main`	Java 程序写在类里面，通常从 `main` 方法开始运行
Compiler	A tool that checks and translates Java code	编译器会检查并翻译 Java 代码
Syntax error	A grammar mistake in code	语法错误，编译器可以发现
Run-time error	An error that happens while the program is running	运行时错误，程序运行时才出现
Logic error	The program runs, but the result is wrong	逻辑错误，程序能跑，但结果不对
Debugging	Finding and fixing errors	调试，也就是找 bug、改 bug

2. Algorithm

What is an algorithm?

An algorithm is a step-by-step process used to complete a task or solve a problem.

算法就是完成一个任务或解决一个问题的步骤。

Example 1: Making instant noodles

Boil water.
Put noodles into the bowl.
Add hot water.
Wait three minutes.
Add seasoning.
Eat the noodles.

This is an algorithm because it gives clear steps.

这就是一个算法，因为它一步一步告诉你该怎么做。

Example 2: Finding a student’s name in a class list

Start from the first name.
Compare the name with the target name.
If it matches, stop.
If it does not match, move to the next name.
Repeat until the name is found or the list ends.

This is also an algorithm.

这也是一个算法。以后我们会在 Java 里用循环和判断来实现类似的过程。

3. Sequencing

What is sequencing?

Sequencing means steps are completed in order.

顺序执行指的是步骤按照一定顺序一个一个完成。

In Java, code usually runs from top to bottom.

在 Java 中，程序通常从上往下一行一行执行。

Example 1: Daily-life sequencing

Correct order:

Wake up.
Brush your teeth.
Put on your clothes.
Pack your bag.
Go to school.

Wrong order:

Go to school.
Wake up.
Brush your teeth.

The steps are similar, but the order is wrong.

步骤差不多，但顺序错了，结果就不合理。

Example 2: Java sequencing

System.out.println("Good morning!");
System.out.println("Good afternoon!");
System.out.println("Good evening!");

Output:

Good morning!
Good afternoon!
Good evening!

Java prints the lines in order.

Java 会按照顺序依次输出。

4. First Java Program

A basic Java program looks like this:

public class MyClass
{
    public static void main(String[] args)
    {
        System.out.println("Hi there!");
    }
}

What students should notice first

Code	Meaning	中文理解
`public class MyClass`	Defines a class named `MyClass`	定义一个叫 `MyClass` 的类
`{ }`	Groups code into blocks	大括号用来包住代码块
`main`	The starting point of the program	程序开始运行的地方
`System.out.println(...)`	Prints output to the screen	在屏幕上输出内容
`;`	Ends a statement	分号表示一条语句结束

Important classroom note

At this stage, you do not need to fully understand every word in:

public static void main(String[] args)

For now, remember this:

class → main → statement → output

中文记忆：

类 → main 方法 → 语句 → 输出

5. Class and File Name

In Java, code is written inside a class.

Java 程序写在 class（类） 里面。

Example:

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hello!");
    }
}

If the class is named Hello, the file is usually named:

Hello.java

Common mistake

Wrong:

public class Hello
{
}

but the file name is:

MyClass.java

This can cause problems because the class name and file name should match.

这可能会出问题，因为类名和文件名应该一致。

6. The `main` Method

What is `main`?

The main method is where the Java program starts running.

main 方法是 Java 程序开始运行的地方。

public static void main(String[] args)
{
    System.out.println("Program starts here!");
}

When the program runs, Java looks for main.

程序运行时，Java 会从 main 方法开始执行。

Simple way to remember

No main, no starting point.

中文：

没有 main，程序就没有正常的起点。

7. Statements and Semicolons

What is a statement?

A statement is a complete action in Java.

语句就是 Java 中一个完整的动作。

Example:

System.out.println("Hi!");

This line prints something, so it is a statement.

这一行完成了“输出”这个动作，所以它是一条语句。

Semicolon rule

Most Java statements end with a semicolon:

System.out.println("Hello");

Wrong:

System.out.println("Hello")

The second example is missing a semicolon.

第二个例子少了分号。

Easy memory trick

A semicolon in Java is like a period in English.

中文：

Java 里的分号，就像英文句子的句号。

But remember: not every line needs a semicolon.

但是注意：不是每一行都要加分号。

For example:

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hi!");
    }
}

These lines do not end with semicolons:

public class Hello
public static void main(String[] args)
{
}

8. Curly Braces `{ }`

Curly braces group code blocks.

大括号 { } 用来包住代码块。

Example

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hi!");
    }
}

The outer braces belong to the class:

public class Hello
{
    ...
}

The inner braces belong to the main method:

public static void main(String[] args)
{
    ...
}

Common mistake

Wrong:

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hi!");
}

This code is missing one closing brace }.

这段代码少了一个右大括号。

Quick check

Every { should have a matching }.

每一个 { 都应该有一个对应的 }。

9. `System.out.println()`

System.out.println() displays information on the screen.

System.out.println() 用来在屏幕上输出内容。

Example 1: Print text

System.out.println("Hello, AP CSA!");

Output:

Hello, AP CSA!

Example 2: Print a calculation

System.out.println(3 + 4);

Output:

Java calculates 3 + 4 first, then prints the result.

Java 会先计算 3 + 4，再输出结果。

10. IDE

An IDE is an Integrated Development Environment.

IDE 是集成开发环境。

It helps programmers:

write code
compile code
run code
find errors

中文理解：

写代码
编译代码
运行代码
查找错误

Examples of IDEs:

CSAwesome Active Code
BlueJ
IntelliJ IDEA
VS Code
Eclipse

A simple way to understand it:

An IDE is like a coding notebook, checker, and run button in one place. IDE 就像一个集“代码本、检查器、运行按钮”于一体的工具。

11. Compiler

Computers do not directly understand Java source code.

计算机不能直接理解我们写的 Java 源代码。

So Java code must be compiled.

所以 Java 代码需要被编译。

What does a compiler do?

A compiler:

1. Checks your code for syntax errors.
2. Translates your code into a form the computer can run.

中文：

1. 检查代码有没有语法错误。
2. 把代码翻译成计算机可以运行的形式。

Important idea

If the compiler finds a syntax error, the program will not run.

如果编译器发现语法错误，程序就不能运行。

12. Java Keywords

Keywords are reserved words with special meanings in Java.

关键字是 Java 中有特殊含义的保留词。

Examples:

public
class
static
void

You cannot use them randomly as names.

你不能随便把这些词当作变量名或类名来用。

Capitalization matters

Correct:

public class Hello

Wrong:

Public Class Hello

Java is case-sensitive.

Java 区分大小写。

13. Capitalization in Java

Capitalization is very important in Java.

Java 中大小写非常重要。

Example

Correct:

System.out.println("Hi!");

Wrong:

system.out.println("Hi!");

System must start with a capital S.

System 必须大写 S。

14. Syntax Errors

A syntax error is a grammar mistake in code.

语法错误就是代码没有遵守 Java 的语法规则。

The compiler can detect syntax errors.

编译器可以发现语法错误。

Example 1: Missing semicolon

Wrong:

System.out.println("Hi!")

Correct:

System.out.println("Hi!");

Example 2: Missing quote

Wrong:

System.out.println("Hi!);

Correct:

System.out.println("Hi!");

Example 3: Missing parenthesis

Wrong:

System.out.println("Hi!";

Correct:

System.out.println("Hi!");

15. Reading Error Messages

When the compiler reports an error, do not panic.

当编译器报错时，不要慌。

Read the error message carefully.

认真读错误信息。

Example error message

FirstClass.java:5: error: unclosed string literal
System.out.println("Hi there!);
                  ^
1 error

How to read it

Part	Meaning	中文理解
`FirstClass.java`	File name	文件名
`5`	Line number where the compiler found the error	编译器发现错误的位置
`unclosed string literal`	Something with quotation marks is not closed	字符串没有正确闭合
`^`	Where the compiler thinks the problem is	编译器认为出问题的位置

Important reminder

The error message is helpful, but it is not always perfect.

错误信息很有帮助，但不一定百分之百准确。

Sometimes the real mistake is:

on the line shown
one line before
one line after

中文：

可能就在提示的那一行
也可能在上一行
也可能在下一行

16. Debugging

Debugging means finding and fixing bugs.

调试就是找出并修复 bug。

All programmers make mistakes.

所有程序员都会犯错。

Debugging is a normal part of coding.

调试是写代码的正常过程。

Example

Buggy code:

public class Hello
{
    public static void main(String[] args)
    {
        system.out.println("Hello, AP CSA!")
    }
}

Fixed code:

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hello, AP CSA!");
    }
}

Bugs fixed

Bug	Fix	中文
`system`	Change to `System`	`System` 首字母要大写
Missing `;`	Add semicolon	语句结尾要加分号

17. Run-time Errors

A run-time error happens while the program is running.

运行时错误是在程序运行过程中发生的错误。

The compiler may not catch it.

编译器不一定能提前发现它。

Example: Divide by zero

public class DivideByZero
{
    public static void main(String[] args)
    {
        System.out.println(3 / 0);
    }
}

This code may compile, but it will crash when it runs.

这段代码可能能通过编译，但运行时会出错。

Why?

Because dividing by zero is impossible.

因为除以 0 是不允许的。

18. Exception

An exception is a type of run-time error.

异常是一种运行时错误。

Example:

System.out.println(3 / 0);

This causes an:

ArithmeticException

Student-friendly meaning:

The program starts running, hits an impossible action, and Java reports an exception. 程序开始运行，但遇到了一个无法完成的操作，于是 Java 报出异常。

19. Logic Errors

A logic error happens when the program runs, but the result is wrong.

逻辑错误指的是程序能运行，但结果不正确。

Example 1: Wrong calculation

Suppose you want to calculate:

10 + 2

But you write:

System.out.println(10 - 2);

Output:

The program runs, but the answer is wrong.

程序能运行，但答案错了。

Example 2: Wrong percent

You want to print a score as a percentage:

But your program prints:

0.85

The program does not crash, but the logic is wrong.

程序没有崩溃，但逻辑错了。

How to find logic errors

Use test cases.

使用测试数据。

Ask yourself:

What output do I expect?
What output did I actually get?
Why are they different?

中文：

我期待的输出是什么？
实际输出是什么？
为什么不一样？

20. Comments

Comments are notes for humans.

注释是写给人看的说明。

The compiler skips comments.

编译器会跳过注释，不会执行它们。

Single-line comment

// Print a greeting
System.out.println("Hello!");

Multi-line comment

/*
Programmer: Roy
Date: June 11
Purpose: Print a greeting
*/

Good comments explain why

Less useful:

// print hello
System.out.println("Hello!");

More useful:

// Show the first greeting message to the user
System.out.println("Hello!");

21. Pair Programming and Rubber Duck Debugging

Pair programming

Pair programming means two programmers work together.

结对编程指两个程序员一起写代码。

Usually:

Role	Job	中文
Driver	Types the code	负责打代码
Navigator	Checks ideas, logic, and mistakes	负责看思路、逻辑和错误

They switch roles often.

两个人会经常交换角色。

Rubber duck debugging

Rubber duck debugging means explaining your code line by line to a rubber duck, a toy, or another person.

小黄鸭调试法就是把代码一行一行讲给小黄鸭、玩具或同学听。

Why does it work?

Because when you explain your code slowly, you often notice your own mistake.

因为当你慢慢解释代码时，经常会突然发现自己的错误。

22. Best First Examples to Remember

Example 1: Algorithm

Goal: Get ready for school

Wake up.
Brush your teeth.
Put on your uniform.
Pack your bag.
Go to school.

Key idea:

Order matters.

中文：

顺序很重要。

Example 2: First Java program

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hello, AP CSA!");
    }
}

Key idea:

class → main → statement → output

中文：

类 → main 方法 → 语句 → 输出

Example 3: Debugging

Buggy version:

public class Hello
{
    public static void main(String[] args)
    {
        system.out.println("Hello, AP CSA!")
    }
}

Fixed version:

public class Hello
{
    public static void main(String[] args)
    {
        System.out.println("Hello, AP CSA!");
    }
}

Two bugs:

1. system should be System.
2. The statement needs a semicolon.

中文：

1. system 应该改成 System。
2. 语句结尾需要分号。

23. Quick Error Checklist

Before you ask for help, check these first:

在求助之前，先检查这些地方：

Did I forget a semicolon ;? 我有没有漏掉分号？
Did I match every { with a }? 每个左大括号有没有对应的右大括号？
Did I match every ( with a )? 每个左括号有没有对应的右括号？
Did I close every quote "? 字符串的引号有没有闭合？
Did I write System with a capital S? System 的 S 有没有大写？
Is my code inside the main method? 我的代码是不是写在 main 方法里面？
Did I read the error message carefully? 我有没有认真读错误信息？

24. Mini Practice

Practice 1: Find the syntax errors

public class FirstClass
{
    public static void main(String[] args)
    {
        system.out.println("Hi there!")
    }
}

Questions:

1. What is wrong with system?
2. What is missing at the end of the statement?

Answers:

1. system should be System.
2. The statement is missing a semicolon.

中文答案：

1. system 应该改成 System。
2. 语句结尾少了分号。

Practice 2: Predict the output

public class Greeting
{
    public static void main(String[] args)
    {
        System.out.println("Good morning!");
        System.out.println("Good afternoon!");
        System.out.println("Good evening!");
    }
}

Output:

Good morning!
Good afternoon!
Good evening!

Practice 3: Runtime error or syntax error?

System.out.println(3 / 0);

Answer:

Run-time error

Why?

The code can be written in correct Java syntax, but dividing by zero fails when the program runs.

中文：

这是运行时错误。代码语法上可能没问题，但程序运行时不能除以 0。

25. Final Summary

The most important ideas

Algorithms are step-by-step processes. 算法是一步一步解决问题的过程。
Sequencing means steps run in order. 顺序执行表示步骤按照顺序运行。
A Java program is written inside a class. Java 程序写在类里面。
The program usually starts from the main method. 程序通常从 main 方法开始。
System.out.println() prints output. System.out.println() 用来输出内容。
Java statements usually end with a semicolon. Java 语句通常用分号结尾。
A compiler checks and translates your code. 编译器检查并翻译代码。
Syntax errors are caught by the compiler. 语法错误会被编译器发现。
Run-time errors happen while the program runs. 运行时错误发生在程序运行过程中。
Logic errors produce wrong results even if the program runs. 逻辑错误会让程序能运行但结果错误。
Debugging is part of programming. 调试是编程的一部分。

One-sentence takeaway

Code is not magic. It is a carefully ordered set of instructions. When something goes wrong, read the error message, check the structure, and debug step by step. 代码不是魔法，而是一组按顺序执行的精确指令。出错时，先读错误信息，再检查结构，然后一步一步调试。

Core Concepts:核心知识点

Introduction

1. Big Picture

What you should understand after this lesson

2. Algorithm

What is an algorithm?

Example 1: Making instant noodles

Example 2: Finding a student’s name in a class list

3. Sequencing

What is sequencing?

Example 1: Daily-life sequencing

Example 2: Java sequencing

4. First Java Program

What students should notice first

Important classroom note

5. Class and File Name

Common mistake

6. The main Method

What is main?

Simple way to remember

7. Statements and Semicolons

What is a statement?

Semicolon rule

Easy memory trick

8. Curly Braces { }

Example

Common mistake

Quick check

9. System.out.println()

Example 1: Print text

Example 2: Print a calculation

10. IDE

11. Compiler

What does a compiler do?

Important idea

12. Java Keywords

Capitalization matters

13. Capitalization in Java

Example

14. Syntax Errors

Example 1: Missing semicolon

Example 2: Missing quote

Example 3: Missing parenthesis

15. Reading Error Messages

Example error message

How to read it

Important reminder

16. Debugging

Example

Bugs fixed

17. Run-time Errors

Example: Divide by zero

18. Exception

19. Logic Errors

Example 1: Wrong calculation

Example 2: Wrong percent

How to find logic errors

20. Comments

Single-line comment

Multi-line comment

Good comments explain why

21. Pair Programming and Rubber Duck Debugging

Pair programming

Rubber duck debugging

22. Best First Examples to Remember

Example 1: Algorithm

Example 2: First Java program

Example 3: Debugging

23. Quick Error Checklist

24. Mini Practice

Practice 1: Find the syntax errors

Practice 2: Predict the output

Practice 3: Runtime error or syntax error?

25. Final Summary

The most important ideas

One-sentence takeaway

Core Concepts:
核心知识点

6. The `main` Method

What is `main`?

8. Curly Braces `{ }`

9. `System.out.println()`