AP CSA 1.14：调用实例方法

June 28, 2026

In the previous lessons, we created objects with constructors. In this lesson, we use instance methods to make a specific object perform an action or return information about itself.

前几课我们使用构造器创建了对象。这一课，我们学习调用实例方法，让某个具体对象执行动作，或者返回与自身有关的信息。

A class method is called on a class; an instance method is called on a specific object. 类方法通过类调用；实例方法通过具体对象调用。

Core Concepts:
核心知识点

1. Class Methods vs. Instance Methods
类方法与实例方法

A class method belongs to a class and includes static in its method header.

类方法（class method） 属于类，并且方法头中包含 static。

double root = Math.sqrt(25);

An instance method belongs to objects of a class and does not use static.

实例方法（instance method） 属于类创建出来的对象，并且不使用 static。

yertle.forward();

Method Type	Call Pattern	Example
Class method	ClassName.methodName(arguments)	Math.sqrt(25)
Instance method	objectName.methodName(arguments)	yertle.forward(50)

A quick AP clue: class names usually begin with an uppercase letter, while object variable names usually begin with a lowercase letter.

一个实用判断方法：类名通常以大写字母开头，对象变量名通常以小写字母开头。

2. Calling an Instance Method
调用实例方法

To call an instance method from main or another class, first create the object and then use the dot operator.

要在 main 或另一个类中调用实例方法，必须先创建对象，再使用点运算符。

World world = new World(300, 300);
Turtle yertle = new Turtle(world);

yertle.forward();
yertle.turnRight();

The general pattern is:

一般格式是：

objectName.methodName(arguments);

Read this call:

yertle.forward(50);

as:

Ask the object yertle to run forward using the argument 50.

可以理解为：

让对象 yertle 执行 forward 方法，并传入实参 50。

Inside an instance method of the same class, another instance method can usually be called using only its name:

在同一个类的实例方法内部，通常可以直接使用方法名调用另一个实例方法：

printSlices(slicesPerPerson);

3. Method Signatures and Arguments
方法签名与实参

A method signature consists of the method name and the ordered list of parameter types.

方法签名由方法名和按顺序排列的参数类型列表组成。

Example method header:

public void forward(int pixels)

Method signature:

forward(int)

Method call:

yertle.forward(200);

Here:

Item	Role
pixels	parameter
200	argument
forward(int)	method signature

The arguments must match the parameters in:

实参必须在以下方面匹配参数：

• number

• order

• compatible types

• 数量

• 顺序

• 兼容的数据类型

For example, if the signature is:

moveTo(int, int)

this is valid:

yertle.moveTo(50, 100);

but this is not:

yertle.moveTo(50);

4. Overloaded Instance Methods
重载的实例方法

A class may contain multiple methods with the same name but different signatures.

一个类可以包含多个同名但签名不同的方法，这叫方法重载。

For example, a Turtle class may provide:

forward()
forward(int)

These calls use different versions:

yertle.forward();
yertle.forward(75);

Java selects the matching method based on the number and types of arguments.

Java 会根据实参的数量和类型选择匹配的方法。

5. Void and Non-void Instance Methods
void 与非 void 实例方法

A void instance method performs an action but does not return a value.

void 实例方法会执行动作，但不返回值。

yertle.forward();
yertle.turnLeft();

These calls may change the object’s state, such as its position or direction.

这些调用可能会改变对象的状态，例如位置或方向。

A non-void instance method returns a value.

非 void 实例方法会返回一个值。

int width = yertle.getWidth();
System.out.println(yertle.getXPos());

The returned value should normally be:

返回值通常应该被：

• stored in a variable

• printed

• used in an expression

• 存入变量

• 打印

• 用在表达式中

Calling a getter without using its result is legal but usually useless:

调用访问器却不使用返回值，虽然可能合法，但通常没有意义：

yertle.getWidth();

6. Tracing Changes to Object State
追踪对象状态的变化

Instance methods may change the attributes of the object on which they are called.

实例方法可能会改变被调用对象的属性。

Consider this class:

public class Rectangle
{
    private int width;
    private int height;

    public Rectangle(int w, int h)
    {
        width = w;
        height = h;
    }

    public void resize(int amount)
    {
        width += amount;
    }

    public int getArea()
    {
        return width * height;
    }
}

Now trace this code:

Rectangle r = new Rectangle(10, 15);
r.resize(5);
System.out.println(r.getArea());

Step	width	height
after construction	10	15
after r.resize(5)	15	15
r.getArea()	returns 225

Output:

225

The important AP habit is to update the object’s attributes immediately after every method that changes its state.

AP 题中要养成一个习惯：每执行一个会改变对象状态的方法，就马上更新对象的属性值。

7. null and NullPointerException
null 与空指针异常

A reference variable can contain null, meaning that it does not currently refer to an object.

引用变量可以保存 null，表示它目前没有指向任何对象。

Turtle yertle = null;

This compiles, but the following call causes a runtime error:

下面的声明可以编译，但继续调用方法会产生运行时错误：

yertle.forward();

The error is a:

NullPointerException

Fix:

World world = new World(300, 300);
Turtle yertle = new Turtle(world);

yertle.forward();

Remember:

declaring a reference variable ≠ creating an object

也就是：

声明引用变量 ≠ 创建对象

8. Common Beginner Mistakes
常见初学者错误

Mistake	Wrong Code	Why Wrong	Correct Code	中文解释
Using a class name for an instance method	Turtle.forward();	forward must be called on a Turtle object	yertle.forward();	实例方法通过对象调用
Forgetting the object name	forward(); in main	main has no current Turtle object	yertle.forward();	在 main 中要写对象名
Calling a method on null	Turtle t = null; t.forward();	no object exists	create with new Turtle(world)	null 没有指向对象
Wrong argument count	yertle.forward(50, 20);	no matching signature	yertle.forward(50);	实参数量必须匹配
Ignoring a returned value	yertle.getWidth();	result is unused	int w = yertle.getWidth();	返回值应被使用
Printing a void method	System.out.println(yertle.forward());	forward returns nothing	call yertle.forward(); alone	void 方法不能作为值打印

9. Debugging Example
调试例子

Buggy code:

public class TurtleArea
{
    public static void main(String[] args)
    {
        World world = new World(300, 300);
        Turtle yertle = new Turtle(world);

        int area;
        yertle.getWidth() * getHeight;

        System.out.println("Yertle's area is: " + area);
    }
}

Problems:

• getHeight is missing the object name and parentheses.
• The multiplication result is not assigned to area.
• area is used before receiving a value.

问题：

• getHeight 缺少对象名和括号。
• 乘法结果没有赋给 area。
• area 在得到值之前就被使用。

Fixed code:

public class TurtleArea
{
    public static void main(String[] args)
    {
        World world = new World(300, 300);
        Turtle yertle = new Turtle(world);

        int area = yertle.getWidth() * yertle.getHeight();

        System.out.println("Yertle's area is: " + area);
    }
}

Bug	Type	Fix
getHeight without ()	compile-time error	use getHeight()
missing object name	compile-time error	use yertle.getHeight()
result not assigned	compile-time / logic issue	assign expression to area
calling on null	runtime error	create an object first

10. Mini Practice
小练习

Practice 1: Choose the Valid Call
练习 1：选择合法调用

Consider:

public class Party
{
    public void inviteFriend()
    {
    }

    public void cancelParty()
    {
    }
}

Assume myParty is a properly initialized Party object. Which call is valid?

A. myParty.cancelParty(); B. myParty.inviteFriend(2); C. Party.cancelParty(); D. System.out.println(myParty.cancelParty());

Answer:

A. myParty.cancelParty();

Explanation: cancelParty() is a no-argument void instance method.

解释：cancelParty() 是无参数的 void 实例方法，应该通过对象调用。

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

public class Liquid
{
    private double currentTemp;

    public Liquid()
    {
        currentTemp = 50;
    }

    public void lowerTemp()
    {
        currentTemp -= 10;
    }

    public double getTemp()
    {
        return currentTemp;
    }
}

What is printed?

Liquid water = new Liquid();
water.lowerTemp();
System.out.println(water.getTemp());

Answer:

40.0

Explanation: the constructor sets the temperature to 50, and lowerTemp() changes it to 40.

解释：构造器把温度设为 50，然后 lowerTemp() 将它改为 40。

Practice 3: Trace Returned Values
练习 3：追踪返回值

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

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

What is printed?

MethodTrace traceObj = new MethodTrace();
System.out.println(traceObj.square(2) + traceObj.divide(6, 2));

Answer:

7

Explanation: square(2) returns 4, and divide(6, 2) returns 3.

解释：square(2) 返回 4，divide(6, 2) 返回 3，所以结果是 7。

Practice 4: Fix the Code
练习 4：修复代码

World world = new World(300, 300);
Turtle yertle = null;

yertle.forward();

Fixed code:

World world = new World(300, 300);
Turtle yertle = new Turtle(world);

yertle.forward();

Explanation: an instance method cannot be called through a null reference.

解释：不能通过 null 引用调用实例方法。

Quick Checklist
快速检查清单

Before answering an instance-method question, check:

做实例方法题前，检查：

• Is this a class method or an instance method?
• Should the call use a class name or an object name?
• Has the object been properly created and initialized?
• Could the object reference be null?
• Does the method name exactly match the class definition?
• Do the argument number, order, and types match the signature?
• Is the method void or non-void?
• If it returns a value, is that value stored, printed, or used?
• Does the method change the object’s attributes?
• Did you update the object’s state after each method call?
• After a method returns, which statement runs next?
• Is the error a compile-time error, runtime error, or logic error?