AP CSA 1.3：表达式与输出

June 17, 2026

In Lesson 1.2, we learned how variables store values. In this lesson, we learn how Java prints values, calculates expressions, and decides the final output.

在 1.2 中，我们学习了变量如何储存值。本节课我们学习 Java 如何输出信息、计算表达式，以及判断程序最终会打印什么。

Expressions calculate values. Output statements display values. 表达式负责计算，输出语句负责显示。

Core Concepts:
核心知识点

1. Main Idea
核心理解

This lesson is mainly about predicting output.

本节课的核心能力是：判断 Java 代码会输出什么结果。

When reading output code, check:

Check	Why it matters	中文理解
`print` or `println`?	Controls line breaks.	判断是否换行
Text or expression?	Text is printed; expressions are calculated.	文字直接输出，表达式先计算
`int` or `double`?	Division behaves differently.	整数除法和小数除法不同
Operator order?	Java follows precedence rules.	Java 按运算优先级计算

2. Output Statements
输出语句

Java commonly uses two output methods.

Java 常用两个输出方法：

Code	Meaning	中文解释
`System.out.print(value);`	Prints without moving to a new line.	输出后不换行
`System.out.println(value);`	Prints and then moves to a new line.	输出后换行

Example:

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

Output:

Hi there
!

print("Hi ") stays on the same line. println("there") prints there and then moves to the next line.

print("Hi ") 不换行。 println("there") 输出后换行，所以 ! 在下一行。

3. String Literals and Escape Sequences
字符串字面量与转义序列

A string literal is fixed text inside double quotes.

字符串字面量 String Literal 是双引号里的固定文本。

System.out.println("Hello");

Useful escape sequences:

Code	Prints	中文解释
`\"`	`"`	输出双引号
`\\`	`\`	输出反斜杠
`\n`	new line	换行

Example:

System.out.println("She said, \"Hi!\"");
System.out.println("C:\\Users\\Student");
System.out.println("Line 1\nLine 2");

Output:

She said, "Hi!"
C:\Users\Student
Line 1
Line 2

4. Arithmetic Expressions
算术表达式

An arithmetic expression uses numbers, variables, and operators to calculate one value.

算术表达式 Arithmetic Expression 使用数字、变量和运算符，最后算出一个值。

Operator	Meaning	Example	Result
`+`	addition	`2 + 3`	`5`
`-`	subtraction	`2 - 3`	`-1`
`*`	multiplication	`2 * 3`	`6`
`/`	division	`7 / 2`	`3`
`%`	remainder	`7 % 2`	`1`

Java uses * and /, not × or ÷.

Java 中乘法用 *，除法用 /，不能写 × 或 ÷。

5. `int` Division vs. `double` Division
整数除法 vs 小数除法

This is the most important trap in this lesson.

这是本节课最重要的易错点。

System.out.println(2 / 3);
System.out.println(2.0 / 3);

Output:

0
0.6666666666666666

Expression	Type	Result	中文解释
`2 / 3`	`int / int`	`0`	整数除法会截掉小数部分
`2.0 / 3`	`double / int`	`0.666...`	有一个 `double`，结果就是小数

Important:

int division truncates. It does not round.

整数除法是截断 truncation，不是四舍五入。

System.out.println(7 / 2);    // 3
System.out.println(7.0 / 2);  // 3.5

6. Remainder Operator `%`
余数运算符

The % operator gives the remainder after division.

% 是余数运算符 Remainder Operator，表示除法后的余数。

System.out.println(5 % 2);  // 1
System.out.println(8 % 2);  // 0
System.out.println(9 % 2);  // 1
System.out.println(3 % 8);  // 3

Key rule:

If x < y, then x % y is x.

如果 x < y，那么 x % y 的结果就是 x。

Example:

3 % 8 is 3, because 8 cannot go into 3 even once.

3 % 8 等于 3，因为 8 一次都除不进 3。

7. Order of Operations
运算顺序

Java follows normal math precedence.

Java 遵守常见数学运算优先级。

Priority	Operators	中文解释
1	`()`	括号优先
2	`*`, `/`, `%`	乘、除、余数
3	`+`, `-`	加、减
4	left to right	同级从左到右

Example:

System.out.println(2 + 3 * 2);
System.out.println((2 + 3) * 2);
System.out.println(2 + (3 * 2));

Output:

8
10
8

When the expression is hard to read, use parentheses.

表达式不好判断时，使用括号会更清楚。

8. Common Beginner Mistakes
常见初学者错误

Mistake	Wrong code	Why wrong	Fixed code	中文解释
Expecting decimal from integer division	`2 / 3`	Both values are `int`, so result is `0`.	`2.0 / 3`	想要小数，至少一个数要是 `double`
Using math symbols	`2 × 3`	Java does not use `×`.	`2 * 3`	Java 乘法用 `*`
Forgetting quotes for text	`System.out.println(Hi);`	Java thinks `Hi` is a variable.	`System.out.println("Hi");`	普通文字要加双引号
Putting a variable inside quotes	`System.out.println("age");`	Prints the word `age`.	`System.out.println(age);`	变量名放进引号就变成文字
Integer division by zero	`10 / 0`	Causes `ArithmeticException`.	Do not divide by zero.	整数除以 0 会运行时报错

9. Debugging Example
调试例子

Buggy code:

public class CmToInches
{
    public static void main(String[] args)
    {
        System.out.print("100 centimeters in inches is: ");
        System.out.println(100 * 2.54);
    }
}

The code can run, but the formula is wrong.

这段代码可以运行，但公式错了。

Since:

1 inch = 2.54 centimeters

To convert centimeters to inches, divide by 2.54.

厘米转换成英寸，要除以 2.54。

Fixed code:

public class CmToInches
{
    public static void main(String[] args)
    {
        System.out.print("100 centimeters in inches is: ");
        System.out.println(100 / 2.54);
    }
}

Bug	Fix	中文解释
`100 * 2.54`	`100 / 2.54`	厘米转英寸要除以 2.54
No error message	Test the formula	逻辑错误不一定会报错

10. Mini Practice
小练习

Practice 1: Choose the output
练习 1：选择输出

What is printed?

System.out.print("Java is ");
System.out.println("fun ");
System.out.print("and cool!");

Java is fun and cool!

Java isfun
and cool!

Java is
fun
and cool!

Java is fun
and cool!

Answer: D

print("Java is ") does not move to a new line. println("fun ") prints and then moves to the next line.

第一句不换行，第二句输出后换行，所以 and cool! 在下一行。

Practice 2: Predict the result
练习 2：判断计算结果

What is printed?

System.out.println(5 + 5 / 2 * 3 - 1);

Answer: 10

+ 5 / 2 * 3 - 1
+ 2 * 3 - 1
+ 6 - 1
10

5 / 2 is 2, not 2.5, because both values are int.

5 / 2 是 2，不是 2.5，因为这是整数除法。

Practice 3: Fix the code
练习 3：修改代码

Buggy code:

System.out.println("Pay for 12 hours at 7.50 dollars per hour:");
System.out.println(12 7.50);

Fixed code:

System.out.println("Pay for 12 hours at 7.50 dollars per hour:");
System.out.println(12 * 7.50);

两个数字之间必须有运算符。这里要计算工资，所以用乘法 *。

11. Quick Checklist
快速检查清单

Before answering an output question, check:

做输出题前，检查：

Is it print or println?
Did I track line breaks?
Is the content inside quotes just text?
Am I printing a variable or a string literal?
Did I use Java operators: +, -, *, /, %?
Is the division int / int or does it include a double?
Did I apply parentheses first?
Did I calculate *, /, % before +, -?
Did I avoid integer division by zero?

Core Concepts:核心知识点

1. Main Idea核心理解

2. Output Statements输出语句

3. String Literals and Escape Sequences字符串字面量与转义序列

4. Arithmetic Expressions算术表达式

5. int Division vs. double Division整数除法 vs 小数除法

6. Remainder Operator %余数运算符

7. Order of Operations运算顺序

8. Common Beginner Mistakes常见初学者错误

9. Debugging Example调试例子

10. Mini Practice小练习

Practice 1: Choose the output练习 1：选择输出

Practice 2: Predict the result练习 2：判断计算结果

Practice 3: Fix the code练习 3：修改代码

11. Quick Checklist快速检查清单

Core Concepts:
核心知识点

1. Main Idea
核心理解

2. Output Statements
输出语句

3. String Literals and Escape Sequences
字符串字面量与转义序列

4. Arithmetic Expressions
算术表达式

5. `int` Division vs. `double` Division
整数除法 vs 小数除法

6. Remainder Operator `%`
余数运算符

7. Order of Operations
运算顺序

8. Common Beginner Mistakes
常见初学者错误

9. Debugging Example
调试例子

10. Mini Practice
小练习

Practice 1: Choose the output
练习 1：选择输出

Practice 2: Predict the result
练习 2：判断计算结果

Practice 3: Fix the code
练习 3：修改代码

11. Quick Checklist
快速检查清单