第三知识单元

面向对象程序设计

Java程序设计 第6讲，主讲人：李欣

Created: 2023-09-12 Tue 00:26

0.1. 互动课堂

Click to host the seminar.

0.2. 签到

https://xin.blue/tool/attendance/

0.3. 本次课的目标

  • 理解多态的含义
  • 理解方法的重载
  • 理解构造函数的重载
  • 包结构与使用
  • 接口与接口实现

1. 方法覆盖实现的多态

public class Main {
    public static void main(String[] args) {
        ChinesePeople c = new ChinesePeople();
        c.setName("张三");
        c.sayHello();
        JapanesePeople j = new JapanesePeople();
        j.setName("田中一郎");
        j.sayHello();
        People p = new People();
        p.setName("Lucy");
        p.sayHello();
    }
}
class People {
    private String name;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你！");
    }
}
class JapanesePeople extends People {
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

你好，我是张三。很高兴认识你！
こんにちは、田中一郎です。よろしくお願いします。
Hi, I'm Lucy. Nice to meet you!

2. 方法重载实现的多态

public class Main {
    public static void main(String[] args) {
        ChinesePeople c = new ChinesePeople();
        c.setName("张三");
        c.sayHello();
        System.out.println("。");
        c.sayHello("李四");
        System.out.println();
        c.sayHello("李四", "王五");
        System.out.println();
    }
}
class People {
    private String name;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    public void sayHello(){
        System.out.print("我是" + this.getName() + "。很高兴认识你");
    }
    public void sayHello(String guestName) {
        System.out.print("你好，请问你是" + guestName + "吗？");
        sayHello();
        System.out.print("。");
    }
    public void sayHello(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？");
        sayHello();
        System.out.print("们。");
    }
}

我是张三。很高兴认识你。
你好，请问你是李四吗？我是张三。很高兴认识你。
你好，请问你们是李四和王五吗？我是张三。很高兴认识你们。

3. 对象引用的多态

对象的 引用型 变量 具有 多态性 ，因为一个 引用型 变量 可以指向不同形式的对象， 这主要是基于一个事实：

子类 对象可以作为 父类 对象来使用。 

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            p[i].sayHello();
        }
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

你好，我是张三。很高兴认识你。
こんにちは、田中一郎です。よろしくお願いします。
你好，我是李四。很高兴认识你。
こんにちは、山下次郎です。よろしくお願いします。

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            p[i].sayHello();
        }
        ChinesePeople c = new People("张三");
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

javac Main.java

javac Main.java
# Main.java:11: error: incompatible types: People cannot be converted to ChinesePeople
#         ChinesePeople c = new People("张三");
#                           ^
# 1 error

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            p[i].sayHello();
        }
        p[0].sayHelloToSomebody("李四");
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

javac Main.java

javac Main.java
# Main.java:11: error: cannot find symbol
#         p[0].sayHelloToSomebody("李四");
#             ^
#   symbol:   method sayHelloToSomebody(String)
#   location: class People
# 1 error

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            p[i].sayHello();
        }
        ((ChinesePeople)p[0]).sayHelloToSomebody("李四");
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

你好，我是张三。很高兴认识你。
こんにちは、田中一郎です。よろしくお願いします。
你好，我是李四。很高兴认识你。
こんにちは、山下次郎です。よろしくお願いします。
你好，请问你是李四吗？我是张三。很高兴认识你。

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            p[i].sayHello();
        }
        People person = new People("张三");
        ChinesePeople c = (ChinesePeople) person;
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

javac Main.java

javac Main.java
java Main

javac Main.java
java Main
# 你好，我是张三。很高兴认识你。
# こんにちは、田中一郎です。よろしくお願いします。
# 你好，我是李四。很高兴认识你。
# こんにちは、山下次郎です。よろしくお願いします。
# Exception in thread "main" java.lang.ClassCastException: class People cannot be cast to class ChinesePeople (People and ChinesePeople are in unnamed module of loader 'app')
#         at Main.main(Main.java:12)

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三");
        p[1] = new JapanesePeople("田中一郎");
        p[2] = new ChinesePeople("李四");
        p[3] = new JapanesePeople("山下次郎");
        for (int i = 0; i < 4; i++) {
            System.out.print((p[i] instanceof People) + " " +
                             (p[i] instanceof ChinesePeople) + " " +
                             (p[i] instanceof JapanesePeople) + " ");
            p[i].sayHello();
        }
        System.out.print((((ChinesePeople)p[0]) instanceof People) + " " +
                         (((ChinesePeople)p[0]) instanceof ChinesePeople) + " ");
        ((ChinesePeople)p[0]).sayHelloToSomebody("李四");
    }
}
class People {
    private String name;
    People(String name) {
        this.name = name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}
class JapanesePeople extends People {
    JapanesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("こんにちは、" + this.getName() + "です。よろしくお願いします。");
    }
}

true true false 你好，我是张三。很高兴认识你。
true false true こんにちは、田中一郎です。よろしくお願いします。
true true false 你好，我是李四。很高兴认识你。
true false true こんにちは、山下次郎です。よろしくお願いします。
true true 你好，请问你是李四吗？我是张三。很高兴认识你。

4. 构造函数的重载

4.1. 构造函数的重载

构造函数 的 重载 是指 同一个类 中存在着 若干个 具有 不同参数列表 的 构造函数 。

当一个类定义了 若干个构造函数 时，创建该类对象的语句会 自动 根据给出的 实际参数 的 数目 、 类型 和 顺序 来确定调用 哪个构造函数 来完成对新对象的 初始化工作 。 

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new People();
        p[1] = new People("张三");
        p[2] = new People("张三", "男");
        p[3] = new People("张三", "男", 28);
        for (int i = 0; i < 4; i++) {
            System.out.print(p[i].getName() + " " + p[i].getGender() + " " + p[i].getAge() + " : ");
            p[i].sayHello();
        }
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() { }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this.name = name;
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this.name = name;
        this.gender = gender;
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}

null null 0 : Hi, I'm null. Nice to meet you!
张三 null 0 : Hi, I'm 张三. Nice to meet you!
张三 男 0 : Hi, I'm 张三. Nice to meet you!
张三 男 28 : Hi, I'm 张三. Nice to meet you!

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new People();
        p[1] = new People("张三");
        p[2] = new People("张三", "男");
        p[3] = new People("张三", "男", 28);
        for (int i = 0; i < 4; i++) {
            System.out.print(p[i].getName() + " " + p[i].getGender() + " " + p[i].getAge() + " : ");
            p[i].sayHello();
        }
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() { }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}

null null 0 : Hi, I'm null. Nice to meet you!
张三 null 0 : Hi, I'm 张三. Nice to meet you!
张三 男 0 : Hi, I'm 张三. Nice to meet you!
张三 男 28 : Hi, I'm 张三. Nice to meet you!

4.2. 调用父类的构造函数

4.2.1. 子类没有定义构造函数的情况

public class Main {
    public static void main(String[] args) {
        ChinesePeople c = new ChinesePeople();
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() {
        System.out.println("Constructor People() from People class.");
    }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}

Constructor People() from People class.

4.2.2. 子类定义了构造函数的情况

public class Main {
    public static void main(String[] args) {
        People p = new People("Lucy");
        System.out.println(p.getName());
        ChinesePeople c = new ChinesePeople("张三");
        System.out.println(c.getName());
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() {
        System.out.println("Constructor People() from People class.");
    }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        setName(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}

Lucy
Constructor People() from People class.
张三

4.2.3. 子类定义的构造函数中使用 super 调用父类构造函数的情况

public class Main {
    public static void main(String[] args) {
        People p = new People("Lucy");
        System.out.println(p.getName());
        ChinesePeople c = new ChinesePeople("张三");
        System.out.println(c.getName());
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() {
        System.out.println("Constructor People() from People class.");
    }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    ChinesePeople(String name) {
        super(name);
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String guestName) {
        System.out.println("你好，请问你是" + guestName + "吗？我是" +
                           this.getName() + "。很高兴认识你。");
    }
    public void sayHelloToSomebody(String firstGuestName, String secondGuestName) {
        System.out.print("你好，请问你们是" + firstGuestName + "和" + secondGuestName + "吗？我是" +
                         this.getName() + "。很高兴认识你们。");
    }
}

Lucy
张三

4.2.4. 构造函数重载及调用父类构造函数综合应用

public class Main {
    public static void main(String[] args) {
        People[] p = new People[4];
        p[0] = new ChinesePeople("张三", "北京市石景山区");
        p[1] = new ChinesePeople("李四", "男", "北京市海淀区");
        p[2] = new ChinesePeople("王五", "男", 28, "北京市朝阳区");
        p[3] = new People("Lucy", "Female", 23);
        for (int i = 0; i < 4; i++) {
            System.out.print(p[i].getName() + " " + p[i].getGender() + " " + p[i].getAge() + " " +
                             ((p[i] instanceof ChinesePeople) ?
                              ((ChinesePeople)p[i]).getAncestralHome() : "") +
                             " : ");
            p[i].sayHello();
        }
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() {
        System.out.println("Constructor People() from People class.");
    }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getGender() {
        return gender;
    }
    public void setGender(String gender) {
        this.gender = gender;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public void sayHello() {
        System.out.println("Hi, I'm " + this.name + ". Nice to meet you!");
    }
}
class ChinesePeople extends People {
    private String ancestralHome;
    ChinesePeople(String name, String ancestralHome) {
        super(name);
        this.ancestralHome = ancestralHome;
    }
    ChinesePeople(String name, String gender, String ancestralHome) {
        super(name, gender);
        setAncestralHome(ancestralHome);
    }
    ChinesePeople(String name, String gender, int age, String ancestralHome) {
        super(name, gender, age);
        setAncestralHome(ancestralHome);
    }
    public String getAncestralHome() {
        return ancestralHome;
    }
    public void setAncestralHome(String ancestralHome) {
        this.ancestralHome = ancestralHome;
    }
    public void sayHello(){
        System.out.println("你好，我是" + this.getName() + "。很高兴认识你。");
    }
}

张三 null 0 北京市石景山区 : 你好，我是张三。很高兴认识你。
李四 男 0 北京市海淀区 : 你好，我是李四。很高兴认识你。
王五 男 28 北京市朝阳区 : 你好，我是王五。很高兴认识你。
Lucy Female 23  : Hi, I'm Lucy. Nice to meet you!

4.3. 对象初始化的过程

参考教材

  • 5.5.3 pp.131-133

4.3.1. 为对象分配内存空间后的属性单元初始化

class Main {
    public static void main(String[] args) {
        int i;
        boolean b;
        String s;
        System.out.println("i: " + i + ", b: " + b + ", s: " + s);
    }
}

javac Main.java
# Main.java:6: error: variable i might not have been initialized
#         System.out.println("i: " + i + ", b: " + b + ", s: " + s);
#                                    ^
# Main.java:6: error: variable b might not have been initialized
#         System.out.println("i: " + i + ", b: " + b + ", s: " + s);
#                                                  ^
# Main.java:6: error: variable s might not have been initialized
#         System.out.println("i: " + i + ", b: " + b + ", s: " + s);
#                                                                ^
# 3 errors

class Main {
    public static void main(String[] args) {
        Test test = new Test();
        System.out.println("i: " + test.i + ", b: " + test.b + ", s: " + test.s);
    }
}
class Test {
    int i;
    boolean b;
    String s;
}

i: 0, b: false, s: null

class Main {
    public static void main(String[] args) {
        Test test;
        System.out.println("test: " + test);
    }
}
class Test {
    int i;
    boolean b;
    String s;
}

javac Main.java
# Main.java:4: error: variable test might not have been initialized
#         System.out.println("test: " + test);
#                                       ^
# 1 error

class Main {
    public static void main(String[] args) {
        Test t = new Test();
        System.out.println("t.test: " + t.test);
    }
}
class Test {
    int i;
    boolean b;
    String s;
    Test test;
}

t.test: null

class Main {
    static int i;
    static boolean b;
    static String s;
    public static void main(String[] args) {
        System.out.println("i: " + i + ", b: " + b + ", s: " + s);
    }
}

i: 0, b: false, s: null

4.3.2. 执行显式初始化

class Main {
    public static void main(String[] args) {
        Test t = new Test();
        System.out.println("i: " + t.i + ", b: " + t.b + ", s: " + t.s);
    }
}
class Test {
    int i;
    boolean b = true;
    String s;
}

i: 0, b: true, s: null

4.3.3. 执行构造方法中的初始化

class Main {
    public static void main(String[] args) {
        Test t = new Test();
        System.out.println("i: " + t.i + ", b: " + t.b + ", s: " + t.s);
    }
}
class Test {
    int i;
    boolean b;
    String s;
    Test() {
        i = 100;
        b = true;
        s = "test";
    }
}

i: 100, b: true, s: test

public class Main {
    public static void main(String[] args) {
        People p = new ChinesePeople("张三");
        System.out.println(p.getName() + ", " +
                           p.getGender() + ", " +
                           p.getAge() + ", " +
                           ((ChinesePeople)p).getAncestralHome());
    }
}
class People {
    private String name;
    private String gender;
    private int age;
    People() {
        name = "Lucy";
        gender = "?";
    }
    People(String name) {
        this.name = name;
    }
    People(String name, String gender) {
        this(name);
        this.gender = gender;
    }
    People(String name, String gender, int age) {
        this(name, gender);
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public String getGender() {
        return gender;
    }
    public int getAge() {
        return age;
    }
}
class ChinesePeople extends People {
    private String ancestralHome;
    ChinesePeople(String name) {
        ancestralHome = "籍贯未填写";
    }
    ChinesePeople(String name, String ancestralHome) {
        super(name);
        this.ancestralHome = ancestralHome;
    }
    ChinesePeople(String name, String gender, String ancestralHome) {
        super(name, gender);
        this.ancestralHome = ancestralHome;
    }
    ChinesePeople(String name, String gender, int age, String ancestralHome) {
        super(name, gender, age);
        this.ancestralHome = ancestralHome;
    }
    public String getAncestralHome() {
        return ancestralHome;
    }
}

Lucy, ?, 0, 籍贯未填写

5. 包及其使用

参考教材

  • 5.6 pp.133-138

6. 接口

参考教材

  • 5.7 pp.138-142