AP CSA 1.10：调用类方法

June 27, 2026

In the previous lesson, we learned method signatures, parameters, arguments, and how method calls change the flow of control. In this lesson, we focus on static class methods and how non-void methods return values back to the calling code.

上一课我们学习了方法签名、参数、实参，以及方法调用如何改变执行流程。这一课，我们重点学习 static 类方法，以及非 void 方法如何把结果返回给调用它的代码。

A non-void method returns a value; the calling code should store it, print it, or use it in an expression. 非 void 方法会返回一个值；调用它的代码应该保存、打印或在表达式中使用这个返回值。

Core Concepts:
核心知识点

1. Main Idea
核心理解

A static method, also called a class method, is associated with the class.

static 方法，也叫 类方法（class method），是和类相关的方法。

A static method header includes the keyword static.

static 方法的方法头中包含关键字 static。

public static int square(int number)
{
    return number * number;
}

For AP CSA 1.10, the most important question is:

对于 AP CSA 1.10，最重要的问题是：

What value does the method return, and how is that value used?

也就是：

这个方法返回什么值？这个返回值被怎样使用？

2. Static Class Methods
static 类方法

A class method is usually called using the class name and the dot operator.

类方法通常用类名加点运算符调用。

ClassName.methodName(arguments);

Example:

Math.sqrt(9);

Here, Math is the class name, and sqrt is a static method in the Math class.

这里 Math 是类名，sqrt 是 Math 类中的 static 方法。

If the method is in the same class, the class name is usually optional.

如果方法就在同一个类里，类名通常可以省略。

public class Example
{
    public static int square(int number)
    {
        return number * number;
    }

    public static void main(String[] args)
    {
        int result = square(5);
        System.out.println(result);
    }
}

Output:

3. Void vs. Non-void Methods
void 与非 void 方法

A void method does not return a value.

void 方法不返回值。

public static void printHello()
{
    System.out.println("Hello");
}

A non-void method returns a value.

非 void 方法会返回一个值。

public static int square(int number)
{
    return number * number;
}

The return type in the method header tells you what type of value comes back.

方法头中的返回类型告诉你会返回什么类型的值。

Method Header	Return Type	Meaning
`public static void printHello()`	`void`	returns nothing
`public static int square(int number)`	`int`	returns an `int`
`public static double divide(double x, double y)`	`double`	returns a `double`

AP trap:

AP 常见陷阱：

int x = printHello();   // wrong if printHello is void

A void method cannot be used as a value.

void 方法不能当作一个值来使用。

4. The `return` Statement
`return` 语句

A non-void method must return a value of the correct type.

非 void 方法必须返回正确类型的值。

public static int square(int number)
{
    int result = number * number;
    return result;
}

The return value goes back to the place where the method was called.

返回值会回到方法被调用的位置。

int y = square(5);

This means:

square(5) returns 25
y gets 25

也就是：

square(5) 返回 25
y 得到 25

The return statement also ends the method.

return 也会结束方法的执行。

5. Using Return Values
使用返回值

When you call a non-void method, you should usually do something with the value it returns.

调用非 void 方法时，通常应该使用它返回的值。

Option 1: Store it in a variable
方式 1：存入变量

int y = square(5);
System.out.println(y);

Output:

Option 2: Print it directly
方式 2：直接打印

System.out.println(square(4));

Output:

Option 3: Use it in an expression
方式 3：放进表达式

int total = square(3) + square(4);
System.out.println(total);

Output:

Because:

因为：

square(3) + square(4)
= 9 + 16
= 25

A call like this is legal but usually not useful:

下面这种写法通常没有意义：

square(5);

The method returns 25, but the program does not store, print, or use it.

这个方法返回了 25，但程序没有保存、打印或使用它。

6. Calling Methods from Another Class
调用其他类中的方法

When a static method is in another class, use:

如果 static 方法在另一个类中，使用：

ClassName.methodName(arguments);

Example:

double x = Math.pow(3, 2);
double y = Math.sqrt(9);

System.out.println(x);
System.out.println(y);

Output:

9.0
3.0

Important AP point:

AP 重点：

Math.pow and Math.sqrt return double values.

Math.pow 和 Math.sqrt 返回的是 double 值。

So this is correct:

double x = Math.pow(3, 2);

This is not correct:

int x = Math.pow(3, 2);

The method returns a double, and Java will not automatically store that into an int.

该方法返回 double，Java 不会自动把它存进 int。

7. Matching Arguments and Return Types
匹配实参与返回类型

When calling a method, check three things:

调用方法时，检查三件事：

number of arguments
order of arguments
compatible argument types
实参数量
实参顺序
实参类型是否兼容

Example:

public static double divide(double x, double y)
{
    return x / y;
}

Correct:

double result = divide(5, 2);

Output:

2.5

5 and 2 are int literals, but they can be widened to double.

5 和 2 是 int 字面量，但可以自动拓宽为 double。

Wrong return variable type:

int result = divide(5, 2);

The method returns a double, so the result should be stored in a double.

这个方法返回 double，所以结果应该存进 double 变量。

Wrong logical order:

double result = divide(2, 5);

This compiles, but it calculates 2 / 5, not 5 / 2.

这段代码可以编译，但它算的是 2 / 5，不是 5 / 2。

8. Method Calls in Expressions
表达式中的方法调用

A non-void method call can appear anywhere a value of that type can appear.

非 void 方法调用可以出现在任何需要该类型值的位置。

Example:

public class MethodTrace
{
    public static int square(int x)
    {
        return x * x;
    }

    public static int divide(int x, int y)
    {
        return x / y;
    }

    public static void main(String[] args)
    {
        System.out.println(square(3) + divide(5, 2));
    }
}

Output:

Trace:

Expression	Result
`square(3)`	`9`
`divide(5, 2)`	`2`
`9 + 2`	`11`

Notice that divide(5, 2) returns 2, not 2.5, because this version of divide uses int parameters and returns an int.

注意：这里 divide(5, 2) 返回 2，不是 2.5，因为这个版本的 divide 使用 int 参数并返回 int。

9. Common Beginner Mistakes
常见初学者错误

Mistake	Wrong Code	Why Wrong	Correct Code	中文解释
Ignoring a returned value	`square(5);`	Return value is not used	`int y = square(5);`	返回值没有被使用
Assigning `double` to `int`	`int x = Math.sqrt(9);`	`Math.sqrt` returns `double`	`double x = Math.sqrt(9);`	`double` 不能直接存进 `int`
Wrong number of arguments	`square(4, 2);`	`square` expects one argument	`square(4);`	实参数量不匹配
Wrong argument order	`divide(2, 5);`	Calculates the wrong direction	`divide(5, 2);`	顺序错可能导致逻辑错误
Using a `void` method as a value	`int x = printHello();`	`void` returns nothing	call `printHello();` alone	`void` 方法没有返回值
Forgetting class name for external class method	`sqrt(9);`	`sqrt` is in the `Math` class	`Math.sqrt(9);`	调用其他类的方法要加类名

10. Debugging Example
调试例子

Buggy code:

public class MathMethods
{
    public static int square(int number)
    {
        return number * number;
    }

    public static double divide(double x, double y)
    {
        return x / y;
    }

    public static void main(String[] args)
    {
        int result1 = square(4.0, 2);
        int result2 = divide(2, 5);

        System.out.println("4 squared is " + result1);
        System.out.println("5 divided by 2 is " + result2);
    }
}

Problems:

square expects one int argument, but the call gives two arguments.
divide returns a double, but result2 is declared as int.
divide(2, 5) calculates 2 / 5, not 5 / 2.

问题：

square 需要一个 int 实参，但调用时给了两个实参。
divide 返回 double，但 result2 被声明成了 int。
divide(2, 5) 算的是 2 / 5，不是 5 / 2。

Fixed code:

public class MathMethods
{
    public static int square(int number)
    {
        return number * number;
    }

    public static double divide(double x, double y)
    {
        return x / y;
    }

    public static void main(String[] args)
    {
        int result1 = square(4);
        double result2 = divide(5, 2);

        System.out.println("4 squared is " + result1);
        System.out.println("5 divided by 2 is " + result2);
    }
}

Output:

4 squared is 16
5 divided by 2 is 2.5

Bug	Type	Fix
`square(4.0, 2)`	compile-time error	`square(4)`
`int result2 = divide(5, 2)`	compile-time error	`double result2 = divide(5, 2)`
`divide(2, 5)`	logic error	`divide(5, 2)`

11. Mini Practice
小练习

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

What is printed?

public class Test
{
    public static int doubleIt(int x)
    {
        return x * 2;
    }

    public static void main(String[] args)
    {
        System.out.println(doubleIt(4) + 1);
    }
}

Answer:

Explanation: doubleIt(4) returns 8, and 8 + 1 is 9.

解释：doubleIt(4) 返回 8，8 + 1 是 9。

Practice 2: Predict the Result
练习 2：预测结果

What is printed?

public static int square(int x)
{
    return x * x;
}

public static int divide(int x, int y)
{
    return x / y;
}

public static void main(String[] args)
{
    System.out.println(square(3) + divide(5, 2));
}

Answer:

Explanation: square(3) returns 9; divide(5, 2) returns 2 because it uses integer division.

解释：square(3) 返回 9；divide(5, 2) 返回 2，因为这里是整数除法。

Practice 3: Match the Method Call
练习 3：匹配方法调用

Given this method:

public static double calculatePizzaBoxes(int numOfPeople, double slicesPerBox)
{
    return numOfPeople / slicesPerBox;
}

Which line compiles correctly?

A. int result = calculatePizzaBoxes(45, 9.0); B. double result = calculatePizzaBoxes(45.0, 9.0); C. int result = calculatePizzaBoxes(45.0, 9); D. double result = calculatePizzaBoxes(45, 9.0);

Answer:

D. double result = calculatePizzaBoxes(45, 9.0);

Explanation: the method needs an int first and a double second, and it returns a double.

解释：这个方法需要第一个实参是 int，第二个实参是 double，并且返回 double。

Practice 4: Fix the Code
练习 4：修复代码

Fix the code.

int x = Math.sqrt(25);
System.out.println(x);

Fixed code:

double x = Math.sqrt(25);
System.out.println(x);

Output:

5.0

Explanation: Math.sqrt returns a double.

解释：Math.sqrt 返回的是 double。

Quick Checklist
快速检查清单

Before answering a class-method question, check:

做类方法题前，检查：

Is the method static?
Is the method in the same class or another class?
If it is in another class, did you use ClassName.methodName(arguments)?
Is the method void or non-void?
If it is non-void, what type does it return?
Is the return value stored, printed, or used in an expression?
Are you assigning the return value to a compatible variable type?
Do the number of arguments match the parameter list?
Do the argument types match or widen safely?
Is the argument order correct?
If a method call is inside an expression, what value does it produce?
If Math.sqrt or Math.pow is used, are you expecting a double result?

Core Concepts:核心知识点

1. Main Idea核心理解

2. Static Class Methodsstatic 类方法

3. Void vs. Non-void Methodsvoid 与非 void 方法

4. The return Statementreturn 语句

5. Using Return Values使用返回值

Option 1: Store it in a variable方式 1：存入变量

Option 2: Print it directly方式 2：直接打印

Option 3: Use it in an expression方式 3：放进表达式

6. Calling Methods from Another Class调用其他类中的方法

7. Matching Arguments and Return Types匹配实参与返回类型

8. Method Calls in Expressions表达式中的方法调用

9. Common Beginner Mistakes常见初学者错误

10. Debugging Example调试例子

11. 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：预测结果

Practice 3: Match the Method Call练习 3：匹配方法调用

Practice 4: Fix the Code练习 4：修复代码

Quick Checklist快速检查清单

Core Concepts:
核心知识点

1. Main Idea
核心理解

2. Static Class Methods
static 类方法

3. Void vs. Non-void Methods
void 与非 void 方法

4. The `return` Statement
`return` 语句

5. Using Return Values
使用返回值

Option 1: Store it in a variable
方式 1：存入变量

Option 2: Print it directly
方式 2：直接打印

Option 3: Use it in an expression
方式 3：放进表达式

6. Calling Methods from Another Class
调用其他类中的方法

7. Matching Arguments and Return Types
匹配实参与返回类型

8. Method Calls in Expressions
表达式中的方法调用

9. Common Beginner Mistakes
常见初学者错误

10. Debugging Example
调试例子

11. 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：预测结果

Practice 3: Match the Method Call
练习 3：匹配方法调用

Practice 4: Fix the Code
练习 4：修复代码

Quick Checklist
快速检查清单