AP CSA 1.2：变量与数据类型

June 15, 2026

AP CSA 1.2: Variables and Data Types
变量与数据类型

In Lesson 1.1, we learned that a program is an ordered set of instructions.

在 1.1 中，我们学习了：程序是一组按顺序执行的指令。

In Lesson 1.2, we add one important idea:

A program needs memory to store information. Variables are the names we give to those memory locations. 程序需要用内存保存信息。变量就是我们给这些内存位置起的名字。

For example, a game needs to remember your score. The score may start at 0, then become 1, 2, 3, and so on. This changing value can be stored in a variable.

例如，一个游戏需要记录分数。分数可能从 0 开始，然后变成 1、2、3……这个会变化的值，就可以存储在一个变量中。

Core Concepts:
核心知识点

1. What Is a Variable?

A variable is a memory location in the computer that stores a value.

变量就是计算机中的一个内存位置，用来存储一个值。

The key idea:

The value can change while the program is running. 程序运行时，变量中的值可以改变。

Example:

At the beginning: score = 0
After one point:  score = 1
After two points: score = 2

In Java:

int score = 0;
score = 1;

The variable is still called score, but its stored value changes.

变量名仍然是 score，但里面存的值发生了变化。

2. Every Variable Has a Name and a Type

Every variable has two important parts:

每个变量都有两个重要部分：

Part	Meaning	说明
name	what we call the variable	变量名
data type	what kind of value it can store	数据类型

Example:

int score = 0;

Code	Meaning	说明
`int`	data type	数据类型
`score`	variable name	变量名
`0`	value	值

3. Data Types

A data type tells Java what values are allowed and what operations can be used.

数据类型告诉 Java：这个变量可以存哪些值，可以进行哪些操作。

Type	Stores	Examples	Best for	说明
`int`	integers	`3`, `0`, `-76`	counts, scores, ages	整数，没有小数部分
`double`	decimal numbers	`6.3`, `-0.9`, `101.2`	prices, averages, time, temperature	小数，也叫浮点数
`boolean`	true / false	`true`, `false`	yes/no conditions	只有真或假两种值
`String`	text	`"Hello"`, `"Jose"`	names, messages, words	字符串，一串字符

Primitive Types: `int`, `double`, `boolean`

Primitive types store simple values directly.

基本数据类型直接存储简单的值。

AP CSA focuses on these three primitive types:

AP CSA 主要关注这三种基本数据类型：

int
double
boolean

`int`

Use int when the value has no decimal part.

当数值没有小数部分时，用 int。

int age = 16;
int score = 0;
int numStudents = 25;

Good uses: number of students, number of tickets, game score, age.

适合表示：学生人数、票的数量、游戏分数、年龄。

`double`

Use double when the value may have a decimal part.

当数值可能有小数部分时，用 double。

double price = 23.25;
double gpa = 3.5;
double temperature = 70.5;

Good uses: average grade, shoe size, temperature, race time, price.

适合表示：平均分、鞋码、温度、比赛用时、价格。

`boolean`

Use boolean for true/false situations.

当情况只有“真/假”或“是/否”两种可能时，用 boolean。

boolean sunny = true;
boolean won = false;
boolean hasInsurance = false;

Correct:

boolean sunny = true;

Wrong in AP CSA Java:

boolean sunny = 1;

Reference Type: `String`

String is a reference type. It stores text, or a sequence of characters.

String 是一种引用类型，用来存储文本，也就是一串字符。

String name = "Jose";
String message = "Hello";
String color = "blue";

Text values must use double quotes:

文本值必须放在双引号中：

"Hello"
"AP CSA"
"Jose"

4. Choosing the Right Type

Choosing the right type is one of the most important skills in Lesson 1.2.

选择正确的数据类型，是 1.2 最重要的能力之一。

Situation	Best Type	Reason	说明
number of people in a household	`int`	people are counted with whole numbers	人数是整数
average grade	`double`	averages may have decimals	平均值可能有小数
first name	`String`	names are text	名字是文本
raining or not	`boolean`	only true or false	只有是/否
100m race time	`double`	time may be `9.58` seconds	时间可能有小数
number of tickets	`int`	tickets are counted with whole numbers	数量是整数
shoe size like `8.5`	`double`	it has a decimal	有小数

5. Declaring, Initializing, and Assigning

To create and use variables, you need three words:

要创建和使用变量，你需要掌握三个词：

Word	Meaning	Example	说明
declare	create a variable	`int score;`	声明变量
initialize	give the first value	`int score = 4;`	第一次赋值
assign	give or change a value	`score = 4;`	赋值或改值

Declaration

int score;

This creates an int variable named score.

这会创建一个名叫 score 的 int 变量。

Initialization

int score = 4;

This declares the variable and gives it its first value.

这一行同时完成声明和初始化。

Declaration, then Assignment

int score;
score = 4;

First create the variable. Then put the value 4 into it.

先创建变量，再把 4 存进去。

6. Assignment Statements

In Java, the equal sign = means assignment.

在 Java 中，等号 = 表示赋值。

It does not mean the same thing as math equality.

它和数学里的“相等”不是一回事。

Correct:

score = 4;

Meaning:

Put 4 into the variable score.

把 4 存入 score 这个变量。

Wrong:

4 = score;

Rule:

variable = value;
变量 = 值;

The variable must be on the left. The value or expression must be on the right.

变量必须在左边，值或表达式必须在右边。

7. Printing Variables

Use System.out.println to print a variable value.

使用 System.out.println 输出变量的值。

int score = 4;
System.out.println(score);

Output:

Do Not Put Variables Inside Quotes

Correct:

System.out.println(score);

Wrong:

System.out.println("score");

Code	Output	Meaning	说明
`System.out.println(score);`	`4`	prints the variable value	输出变量的值
`System.out.println("score");`	`score`	prints the word `score`	输出 score 这个单词

8. String Concatenation

String concatenation means joining text and values together.

字符串拼接就是把文本和值连接起来。

Use +:

int score = 4;
System.out.println("The score is " + score);

Output:

The score is 4

Spaces Matter

Spaces must be inside quotes.

空格必须写在引号里面。

String name = "Jose";
System.out.println("Hi " + name);  // Hi Jose
System.out.println("Hi" + name);   // HiJose

9. Naming Variables

Variable names should be legal and meaningful.

变量名既要合法，也要有意义。

Basic Rules

A variable name:

变量名：

starts with a letter
以字母开头
can include letters, numbers, and underscores
可以包含字母、数字和下划线
cannot contain spaces
不能有空格
cannot be a Java keyword
不能是 Java 关键字
is case-sensitive
区分大小写

Legal names:

int score;
int gameScore;
double shoeSize;
boolean hasInsurance;
String firstName;

Illegal names:

int 2score;        // starts with a number
int game score;    // contains a space
int class;         // class is a Java keyword

Meaningful Names and camelCase

Good names describe the data.

好的变量名应该能说明数据的含义。

int score;
int numStudents;
double shoeSize;

Poor names are too vague:

int x;
double a;

But names should not be too long either:

int theNumberThatHoldsTheScore;  // too long
int score;                       // better

Java variable names usually use camelCase:

Java 变量名通常使用 驼峰命名法：

int gameScore;
double shoeSize;
int topScore;
boolean hasInsurance;
String firstName;

Java is case-sensitive:

Java 区分大小写：

gameScore
gamescore
GameScore

These are different names.

这些是不同的名字。

10. Common Beginner Mistakes

Mistake	Wrong Code	Fix	说明
wrong data type	`int temperature = 70.5;`	`double temperature = 70.5;`	`int` 不能存小数
wrong boolean value	`boolean sunny = 1;`	`boolean sunny = true;`	`boolean` 只能用 `true` 或 `false`
wrong capitalization	`Int numVisits = 5;`	`int numVisits = 5;`	基本类型要小写
variable inside quotes	`System.out.println("name");`	`System.out.println(name);`	引号中的是文字，不是变量
misspelled variable	`tempurature`	`temperature`	拼写必须完全一致
missing semicolon	`double price = 23.25`	`double price = 23.25;`	语句结尾需要分号

11. Full Example

public class TestVariables
{
    public static void main(String[] args)
    {
        int score = 0;
        System.out.println("The score is " + score);

        double price = 23.25;
        System.out.println("The price is " + price);

        boolean won = false;
        System.out.println("Did the player win? " + won);

        won = true;
        System.out.println("Did the player win now? " + won);

        String name = "Jose";
        System.out.println("Hi " + name);
    }
}

Possible output:

The score is 0
The price is 23.25
Did the player win? false
Did the player win now? true
Hi Jose

12. Debugging Example: Weather Report

Buggy version:

public class Challenge1_2_weather
{
    public static void main(String[] args)
    {
        int temperature = 70.5;
        double tvChannel = 101;
        boolean sunny = 1
        System.out.print("Welcome to the weather report on Channel ")
        System.out.println(TVchannel);
        System.out.print("The temperature today is );
        System.out.println(tempurature);
        System.out.print("Is it sunny today? ");
        System.out.println(sunny);
    }
}

Fixed version:

public class Challenge1_2_weather
{
    public static void main(String[] args)
    {
        double temperature = 70.5;
        int tvChannel = 101;
        boolean sunny = true;

        System.out.print("Welcome to the weather report on Channel ");
        System.out.println(tvChannel);

        System.out.print("The temperature today is ");
        System.out.println(temperature);

        System.out.print("Is it sunny today? ");
        System.out.println(sunny);
    }
}

What was fixed?

Bug	Problem	Fix	说明
`int temperature = 70.5;`	`int` cannot store decimals	use `double`	`int` 不能存小数
`double tvChannel = 101;`	channel number is better as a whole number	use `int`	频道号更适合整数
`boolean sunny = 1`	boolean must be `true` or `false`; missing semicolon	`boolean sunny = true;`	布尔值错误，且少分号
missing semicolon after `print`	statement not ended	add `;`	语句没有结束
`TVchannel`	wrong capitalization	`tvChannel`	大小写不一致
missing quote	string not closed	add `"`	字符串没有闭合
`tempurature`	misspelled variable name	`temperature`	变量名拼错

13. Mini Practice

Practice 1: Choose the Best Type

Situation	Answer
number of people in a household	`int`
average course grade	`double`
first name	`String`
raining or not	`boolean`
shoe size like `8.5`	`double`
number of tickets	`int`

Practice 2: Fix the Code

Buggy code:

public class Practice
{
    public static void main(String[] args)
    {
        Int numTickets = 3;
        double shoeSize = 8.5
        boolean isRaining = "false";
        String name = Jose;
        System.out.println("name");
    }
}

Try to fix it before looking at the answer.

先自己改，再看答案。

Answer:

public class Practice
{
    public static void main(String[] args)
    {
        int numTickets = 3;
        double shoeSize = 8.5;
        boolean isRaining = false;
        String name = "Jose";
        System.out.println(name);
    }
}

14. Quick Checklist

Before you run or submit your code, check:

运行或提交代码前，检查：

Correct type? int, double, boolean, or String?
类型是否正确？
Correct assignment order? variable = value;
赋值顺序是否正确？
Semicolon? Did each statement end with ;?
语句结尾是否有分号？
Same spelling and capitalization?
变量名拼写和大小写是否完全一致？
String text in double quotes?
字符串是否放在双引号里？
Variable not inside quotes when printing its value?
输出变量值时，是否没有把变量放进引号？
Spaces inside string literals?
拼接字符串时，空格是否写在引号里面？

15. Final Summary

A variable is a memory location that stores a value.
变量是一个存储值的内存位置。
A variable has a name and a data type.
变量有变量名和数据类型。
int stores integers.
int 存储整数。
double stores decimal numbers.
double 存储小数。
boolean stores true or false.
boolean 存储 true 或 false。
String stores text.
String 存储文本。
Declaration creates a variable; initialization gives its first value; assignment changes its value.
声明会创建变量；初始化会给第一次赋值；赋值会改变变量中的值。
Good variable names are meaningful, use camelCase, and match exactly every time.
好的变量名有意义，使用驼峰命名，并且每次拼写和大小写都完全一致。

One-sentence takeaway

Variables let a program remember information, and data types tell Java what kind of information each variable can store. 变量让程序能够记住信息，而数据类型告诉 Java 每个变量可以存储哪一类信息。

AP CSA 1.2: Variables and Data Types变量与数据类型

Core Concepts:核心知识点

1. What Is a Variable?

2. Every Variable Has a Name and a Type

3. Data Types

Primitive Types: int, double, boolean

int

double

boolean

Reference Type: String

4. Choosing the Right Type

5. Declaring, Initializing, and Assigning

Declaration

Initialization

Declaration, then Assignment

6. Assignment Statements

7. Printing Variables

Do Not Put Variables Inside Quotes

8. String Concatenation

Spaces Matter

9. Naming Variables

Basic Rules

Meaningful Names and camelCase

10. Common Beginner Mistakes

11. Full Example

12. Debugging Example: Weather Report

13. Mini Practice

Practice 1: Choose the Best Type

Practice 2: Fix the Code

14. Quick Checklist

15. Final Summary

One-sentence takeaway

AP CSA 1.2: Variables and Data Types
变量与数据类型

Core Concepts:
核心知识点

Primitive Types: `int`, `double`, `boolean`

`int`

`double`

`boolean`

Reference Type: `String`