理解 Java 默认值, Getters/Setters, and Constructors：避免 Null Pointer Exceptions

目录
1. 介绍 …………………………………………………….. 第 1 页
2. 理解 Java 中的 Null Pointer Exceptions ……………………. 第 2 页
3. Java 实例变量的默认值 ……………………….. 第 3 页
4. 使用 Getters/Setters 初始化数值 ……………………….. 第 4 页
5. 探索 Java Constructors ……………………………………… 第 5 页
6. 实用示例与代码：运行 Java Class 方法 …………… 第 6 页
7. 图示：处理 Null 值和数值初始化 ……………… 第 7 页
8. 结论 …………………………………………………….. 第 8 页

1. 介绍

本电子书专为初学者及具备基本 Java 知识的开发者设计。在接下来的章节中，我们将讨论与默认值和 null pointer exceptions 相关的常见陷阱，解释如何使用 Getters/Setters 分配数值，并介绍 Constructors 作为初始化实例变量的机制。

在今天的教程中，我们将分析 Java 中常见的错误场景 —— null pointer exception，并提供关于如何通过适当的数值初始化来避免这些错误的见解。我们还将通过 Java Class 方法及代码示例进行实际演示。这些信息总结自讲座记录，并由提供的项目代码示例支持。

下表总结了主要讨论的主题：

主题	描述
Null Pointer Exception	由于比较 null 默认值而导致的错误
默认值	Java 实例变量通常默认为 null 或零
Getters and Setters	安全设置和检索变量数据的方法
Constructors	用于初始化实例变量的特殊方法

此外，请参阅下表，了解使用 getters, setters 和 constructors 时的典型数值范围和默认状态：

变量	默认值	初始化后数值
doors	“open”	“closed” (当通过 setter 设置时)
engine	“off”	“on” (当通过 setter 设置时)
driver	“away”	“seated” (当通过 setter 设置时)
speed	0	10 (当通过 setter 设置时)

2. 理解 Java 中的 Null Pointer Exceptions

当程序尝试使用未赋值（即指向 null）的对象引用时，就会发生 null pointer exception。在我们的讲座记录中，演讲者解释了字符串的默认实例值被初始化为 null，并且将 null 与实际数值进行比较会抛出异常。

关键点：

Null 表示 “指向无处”。
比较 null 值与非 null 值会导致错误。
使用 getters, setters 或 constructors 正确初始化变量可以避免这些问题。

3. Java 实例变量的默认值

在 Java 中，如果没有明确赋值，实例变量会采用默认值。对于字符串而言，其默认值为 null。在实际情况下，这种默认行为是许多错误的根源，例如在比较字符串数值时所遇到的 null pointer exception。

讲座记录强调，无论是通过 setters 还是 constructors，正确初始化都是至关重要的。例如，如果表示门状态的变量保持为 null，则尝试将其与 “closed” 进行比较将会导致错误。

4. 使用 Getters/Setters 初始化数值

讲座记录展示了如何使用 getters 和 setters 来解决 null pointer 问题。与其比较未初始化（null）的实例变量，不如通过 setters 分配特定的数值。

示例用例：

将 doors 设置为 “closed”
将 driver 状态设置为 “seated”
将 engine 设为 “on”
将 speed 设置为 10

通过 getters，可以检索这些数值以验证对象是否已被正确初始化。这确保比较操作不会抛出异常。

5. 探索 Java Constructors

Constructors 提供了另一种有效的方法，通过在对象创建时自动分配默认值来初始化实例变量。例如，你可能有如下默认值：

Doors: open
Engine: off
Driver: away
Speed: 0

讲座介绍了构造函数的概念，作为立即覆盖这些默认值的一种手段。这种方法可以防止 null pointer exception，因为对象字段在创建时已经预先分配了安全的默认值。

6. 实用示例与代码：运行 Java Class 方法

以下是一个示例 Java 代码片段，演示了如何设置 getters, setters 和 constructors。该示例源自 “S06L05 – Run Java Class methods” 项目的文件结构。

/* Car.java - Represents a Car class with default values and getter/setter methods */
package org.studyeasy;

public class Car {
    // Instance variables with default values
    private String doors = "open";    // Default value assigned using declaration
    private String engine = "off";    // Default value: engine initially off
    private String driver = "away";   // Default value for driver status
    private int speed = 0;            // Default speed is 0
    
    // Default Constructor: (optional, as defaults are set above)
    public Car() {
        // Constructor can be used to override default assignments if needed.
    }
    
    // Getters and Setters with explanatory comments:
    public String getDoors() {
        return doors;
    }
    
    public void setDoors(String doors) {
        // Set the doors to the specified state (e.g., "closed")
        this.doors = doors;
    }
    
    public String getEngine() {
        return engine;
    }
    
    public void setEngine(String engine) {
        // Set engine status (e.g., "on" or "off")
        this.engine = engine;
    }
    
    public String getDriver() {
        return driver;
    }
    
    public void setDriver(String driver) {
        // Assign the driver's status (e.g., "seated")
        this.driver = driver;
    }
    
    public int getSpeed() {
        return speed;
    }
    
    public void setSpeed(int speed) {
        // Set the vehicle's speed
        this.speed = speed;
    }
}

/* Main.java - Executes the Car methods */
package org.studyeasy;

public class Main {
    public static void main(String[] args) {
        // Create a new Car instance; default values are set to "open", "off", "away", and 0
        Car car = new Car();
        
        // Using setters to initialize values properly, avoiding null pointer exceptions.
        car.setDoors("closed");        // Now doors are closed
        car.setEngine("on");           // Engine turned on
        car.setDriver("seated");       // Driver is now seated
        car.setSpeed(10);              // Speed set to 10
        
        // Using getters to retrieve current car state and make decisions.
        if(car.getEngine().equals("on") && car.getSpeed() > 0) {
            System.out.println("running");
        } else {
            System.out.println("not running");
        }
    }
}

/* Car.java - Represents a Car class with default values and getter/setter methods */

package org.studyeasy;

public class Car {

// Instance variables with default values

private String doors = "open"; // Default value assigned using declaration

private String engine = "off"; // Default value: engine initially off

private String driver = "away"; // Default value for driver status

private int speed = 0; // Default speed is 0

// Default Constructor: (optional, as defaults are set above)

public Car() {

// Constructor can be used to override default assignments if needed.

}

// Getters and Setters with explanatory comments:

public String getDoors() {

return doors;

}

public void setDoors(String doors) {

// Set the doors to the specified state (e.g., "closed")

this.doors = doors;

}

public String getEngine() {

return engine;

}

public void setEngine(String engine) {

// Set engine status (e.g., "on" or "off")

this.engine = engine;

}

public String getDriver() {

return driver;

}

public void setDriver(String driver) {

// Assign the driver's status (e.g., "seated")

this.driver = driver;

}

public int getSpeed() {

return speed;

}

public void setSpeed(int speed) {

// Set the vehicle's speed

this.speed = speed;

}

/* Main.java - Executes the Car methods */

package org.studyeasy;

public class Main {

public static void main(String[] args) {

// Create a new Car instance; default values are set to "open", "off", "away", and 0

Car car = new Car();

// Using setters to initialize values properly, avoiding null pointer exceptions.

car.setDoors("closed"); // Now doors are closed

car.setEngine("on"); // Engine turned on

car.setDriver("seated"); // Driver is now seated

car.setSpeed(10); // Speed set to 10

// Using getters to retrieve current car state and make decisions.

if(car.getEngine().equals("on") && car.getSpeed() > 0) {

System.out.println("running");

} else {

System.out.println("not running");

}

逐步解释：

Car class 通过默认值初始化其字段。这些默认值确保即使未调用 setter，字段也具有可预测的状态。
Main class 实例化了一个 Car 对象。在执行依赖这些变量的操作之前，代码显式地通过 setter 方法设置数值。
main 方法中的条件检查确保 engine 状态正确且 speed 大于零。如果两者都满足，则输出 “running”；否则输出 “not running”。
这种逐步方法防止了将 null（默认值）与实际数值比较时出现的 null pointer exception。

程序输出：

running

running

该输出表明车的 engine 已经开启且 speed 大于零，展示了 getters, setters 的正确应用以及正确的初始化。

7. 图示：处理 Null 值和数值初始化

下图为说明此过程的简化图示：

                +------------------------+
                | Object Creation (Car)  |
                +------------------------+
                           |
                           v
                +------------------------+          (Default values)
                |  Instance Variables:   | ----------------->
                | doors="open", engine="off", ...        |
                +------------------------+
                           |
             +-------------+-------------+
             |                           |
             v                           v
    [Using Getters/Setters]      [Using a Constructor]
             |                           |
             v                           v
   +-----------------------+       +-----------------------+
   | Values Updated:       |       | Values Pre-Assigned:  |
   | doors="closed", etc.  |       | doors="open", etc. to  |
   |                       |       | safe defaults         |
   +-----------------------+       +-----------------------+
                           |
                           v
                +------------------------+
                | Conditional Check:     |
                | if (engine=="on" ...)   |
                +------------------------+
                           |
                           v
                 +----------------------+
                 |   Output "running"   |
                 +----------------------+

+------------------------+

| Object Creation (Car) |

+------------------------+

+------------------------+ (Default values)

| Instance Variables: | ----------------->

| doors="open", engine="off", ... |

+------------------------+

+-------------+-------------+

| |

v v

[Using Getters/Setters] [Using a Constructor]

| |

v v

+-----------------------+ +-----------------------+

| Values Updated: | | Values Pre-Assigned: |

| doors="closed", etc. | | doors="open", etc. to |

| | | safe defaults |

+-----------------------+ +-----------------------+

+------------------------+

| Conditional Check: |

| if (engine=="on" ...) |

+------------------------+

+----------------------+

| Output "running" |

+----------------------+

8. 结论

在本电子书中，我们探讨了有助于防止常见运行时错误（如 null pointer exception）的关键 Java 概念。通过理解默认值、getters 和 setters 的重要性，以及 constructors 的主要作用，开发者可以编写更健壮、抗错误的代码。

主要收获：

当比较未初始化（null）的数值时，会发生 null pointer exception。
Getters 和 setters 是初始化和访问变量数值的有效方案。
Constructors 提供了一种在对象创建时直接设置默认值的可靠方法。
实际代码示例证明，正确设置数值可以避免运行时错误，并确保程序按预期运行。

通过这些实际的见解和代码演示，你现在拥有了扎实的 Java 初始化处理基础。继续探索和实践这些概念，以构建更具韧性的应用程序。

SEO 优化关键词： Java programming, null pointer exception, getters and setters, default values, constructors, Java tutorial, programming basics, Java error handling, Java initialization, technical writing

附件

字幕记录：

1 Hey there, welcome back. Let's continue our journey and in this video we will see how to fix the error, the null pointer exception and we will discuss about exceptions, these errors in a greater detail in our upcoming videos. But for now, the reason why we are getting this error is because of the default values. As we have discussed earlier, the default value for string entity, string instance variable is null and in Java we cannot compare null with a value. Null is what? Null is pointing to nowhere. That is the reason why we cannot compare nothing to something. And as a result, we will get an error if we try to compare it. Now whenever we want to like set some values, we can make use of getters and setters. Isn't it? And if I do this, for example, set doors equal to closed, then driver seated, engine on, speed is 10, then definitely the error will go away and we will get the output as running. For example, the engine is off. In that case, we will say the car is not running. Not running. So this is cool. This is nice. This is how we can set values in order to get value. We can also do that. The current value of, for example, speed, we can do car.get getSpeed and this will return the current value of the speed of the car, which is 10. Here we go. So everything is good. We have getters and setters, but what if we want to initiate these values, these instance values by some particular values. For example, doors by default will be open. For example, engine will be off, driver will be away and speed will be zero. These can be the default values. Now how we can do that? What is a constructor? Why constructor can be used and where constructor can be used is something which we will discuss in a greater detail moving forward in this section. I hope you guys enjoyed this video. Thanks for watching. Have a nice day and take care.

1 Hey there, welcome back. Let's continue our journey and in this video we will see how to fix the error, the null pointer exception and we will discuss about exceptions, these errors in a greater detail in our upcoming videos. But for now, the reason why we are getting this error is because of the default values. As we have discussed earlier, the default value for string entity, string instance variable is null and in Java we cannot compare null with a value. Null is what? Null is pointing to nowhere. That is the reason why we cannot compare nothing to something. And as a result, we will get an error if we try to compare it. Now whenever we want to like set some values, we can make use of getters and setters. Isn't it? And if I do this, for example, set doors equal to closed, then driver seated, engine on, speed is 10, then definitely the error will go away and we will get the output as running. For example, the engine is off. In that case, we will say the car is not running. Not running. So this is cool. This is nice. This is how we can set values in order to get value. We can also do that. The current value of, for example, speed, we can do car.get getSpeed and this will return the current value of the speed of the car, which is 10. Here we go. So everything is good. We have getters and setters, but what if we want to initiate these values, these instance values by some particular values. For example, doors by default will be open. For example, engine will be off, driver will be away and speed will be zero. These can be the default values. Now how we can do that? What is a constructor? Why constructor can be used and where constructor can be used is something which we will discuss in a greater detail moving forward in this section. I hope you guys enjoyed this video. Thanks for watching. Have a nice day and take care.

项目文件详情（来自档案）：

File: S06L05 - Run Java Class methods/pom.xml 
File: S06L05 - Run Java Class methods/src/main/java/org/studyeasy/Car.java 
File: S06L05 - Run Java Class methods/src/main/java/org/studyeasy/Main.java 
File: S06L05 - Run Java Class methods/target/classes/org/studyeasy/Car.class 
File: S06L05 - Run Java Class methods/target/classes/org/studyeasy/Main.class

File: S06L05 - Run Java Class methods/pom.xml

File: S06L05 - Run Java Class methods/src/main/java/org/studyeasy/Car.java

File: S06L05 - Run Java Class methods/src/main/java/org/studyeasy/Main.java

File: S06L05 - Run Java Class methods/target/classes/org/studyeasy/Car.class

File: S06L05 - Run Java Class methods/target/classes/org/studyeasy/Main.class

这篇关于有效 Java 初始化实践的综合电子书文章到此结束。祝你学习愉快，编码顺利！

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#Java编程 #NullPointerException #GettersSetters #Constructors #Java教程