03.Java Basics - Polymorphism

Basic concept

Polymorphism is the ability of the same behavior to have multiple different manifestations or forms.

Polymorphism is the embodiment of multiple expressions of objects.

Conditions for polymorphism: Inheritance, rewriting, upward transformation

Dynamic binding is the specific form of polymorphism implementation.

---

Example study

The following is an example to verify the above concept.

// 车class Car {    public void show(){
        System.out.println("This is a car");
    }
}//丰田class Toyota extends Car{    public void show(){
        System.out.println("This is a Japan car");
    }
}//别克class Buick extends Car{    public void show(){
        System.out.println("This is a USA car");
    }
}public class Test{  
    public static void main(String[] args) {
        Car c1 = new Toyota();
        Car c2 = new Buick();
        c1.show();
        c2.show();        // 打印内容：
        // This is a Japan car
        // This is a USA car
    }
}

Observing the code, different contents are printed when calling the show method of Car, which reflects the polymorphism of the class. At the same time, the example also meets the three basic conditions for polymorphism to occur:

• Inheritance: Toyota and Buick inherit from Car

• Rewrite: Toyota and Buick rewrite the show method of Car

• Upward transformation: Upward transformation occurs when creating Toyota and Buick objects Transformation

---

Let’s look at a typical question:

class A {    public String show(A obj) {        return ("A and A");
    }    public String show(D obj) {        return ("A and D");
    }
}

class B extends A {    public String show(A obj) {        return ("B and A");
    }    public String show(B obj) {        return ("B and B");
    }
}

class C extends B {
}

class D extends B {
}public class Test {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();        // 1.因此 B 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(b));        // 2.因为 C 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(c));        // 3.调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a1.show(d));        // 4.因为 B 向上转型成 A， 且 B 是 A 的子类，调用类 A 的 show(A obj) 方法
        // 但是由于 B 中重写了 show(A obj) 方法，导致运行期间发生的动态绑定，调用 类 B 的 show(A obj) 方法
        // ，输出：B and A
        System.out.println(a2.show(b));        // 5.同上
        System.out.println(a2.show(c));        // 6.B 向上转型成 A，默认调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a2.show(d));        // 7.调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(b));        // 8.C 是 B 的子类，调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(c));        // 9.调用继承自类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(b.show(d));

    }
}

---

Constructor and Polymorphism

Similarly, explore through an example

class Parent {    void print() {
        System.out.println("I am a Parent");
    }

    Parent() {
        print();
    }
}

class Son extends Parent {    int num = 1;    // 重写（override）了父类方法
    void print() {
        System.out.println("I am a Son " + num);
    }

    Son() {
        print();
    }
}public class Test {
    public static void main(String[] args) {
        Son son = new Son();        /**
         * 输出内容： 
         * I am a Son 0
         * I am a Son 1
         */
    }
}

We can see from the output content that the Parent's constructor called Son's print() due to dynamic binding. Since num has not been initialized, the value is 0.

Basic concept

Polymorphism is the ability of the same behavior to have multiple different manifestations or forms.

Polymorphism is the embodiment of multiple expressions of objects.

Conditions for polymorphism: Inheritance, rewriting, upward transformation

Dynamic binding is the specific form of polymorphism implementation.

---

Example study

The following is an example to verify the above concept.

// 车class Car {    public void show(){
        System.out.println("This is a car");
    }
}//丰田class Toyota extends Car{    public void show(){
        System.out.println("This is a Japan car");
    }
}//别克class Buick extends Car{    public void show(){
        System.out.println("This is a USA car");
    }
}public class Test{  
    public static void main(String[] args) {
        Car c1 = new Toyota();
        Car c2 = new Buick();
        c1.show();
        c2.show();        // 打印内容：
        // This is a Japan car
        // This is a USA car
    }
}

Observing the code, different contents are printed when calling the show method of Car, which reflects the polymorphism of the class. At the same time, the example also meets the three basic conditions for polymorphism to occur:

• Inheritance: Toyota and Buick inherit from Car

• Rewrite: Toyota and Buick rewrite the show method of Car

• Upward transformation: Upward transformation occurs when creating Toyota and Buick objects Transformation

---

Let’s look at a typical question:

class A {    public String show(A obj) {        return ("A and A");
    }    public String show(D obj) {        return ("A and D");
    }
}

class B extends A {    public String show(A obj) {        return ("B and A");
    }    public String show(B obj) {        return ("B and B");
    }
}

class C extends B {
}

class D extends B {
}public class Test {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();        // 1.因此 B 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(b));        // 2.因为 C 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(c));        // 3.调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a1.show(d));        // 4.因为 B 向上转型成 A，应该是调用类 A 的 show(A obj) 方法
        // 由于 B 中重写了 show(A obj) 方法，实际调用 类 B 的方法，，输出：B and A
        System.out.println(a2.show(b));        // 5.同上
        System.out.println(a2.show(c));        // 6.B 向上转型成 A，默认调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a2.show(d));        // 7.调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(b));        // 8.C 是 B 的子类，调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(c));        // 9.调用继承自类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(b.show(d));

    }
}

---

Constructor and Polymorphism

Similarly, we will explore the

class Parent {    void print() {
        System.out.println("I am a Parent");
    }

    Parent() {
        print();
    }
}

class Son extends Parent {    int num = 1;    // 关键 -> 重写了父类的方法
    void print() {
        System.out.println("I am a Son " + num);
    }

    Son() {
        print();
    }
}public class Test {
    public static void main(String[] args) {
        Son son = new Son();        // 输出内容： 
        // I am a Son 0
        // I am a Son 1
    }
}

analysis results through an example. In fact, it involves three contents:

• Son In the process of instantiation, the constructor of Parent, Parent's constructor is executed first.

• Son overrides the print method, so Parent calls the rewritten method of Son.

• In Parent, since num has not been initialized, the value is 0.

Basic concept

Polymorphism is the ability of the same behavior to have multiple different manifestations or forms.

Polymorphism is the embodiment of multiple expressions of objects.

Conditions for polymorphism: Inheritance, rewriting, upward transformation

Dynamic binding is the specific form of polymorphism implementation.

---

Example study

The following is an example to verify the above concept.

// 车class Car {    public void show(){
        System.out.println("This is a car");
    }
}//丰田class Toyota extends Car{    public void show(){
        System.out.println("This is a Japan car");
    }
}//别克class Buick extends Car{    public void show(){
        System.out.println("This is a USA car");
    }
}public class Test{  
    public static void main(String[] args) {
        Car c1 = new Toyota();
        Car c2 = new Buick();
        c1.show();
        c2.show();        // 打印内容：
        // This is a Japan car
        // This is a USA car
    }
}

Observing the code, different contents are printed when calling the show method of Car, which reflects the polymorphism of the class. At the same time, the example also meets the three basic conditions for polymorphism to occur:

• Inheritance: Toyota and Buick inherit from Car

• Rewrite: Toyota and Buick rewrite the show method of Car

• Upward transformation: Upward transformation occurs when creating Toyota and Buick objects Transformation

---

Let’s look at a typical question:

class A {    public String show(A obj) {        return ("A and A");
    }    public String show(D obj) {        return ("A and D");
    }
}

class B extends A {    public String show(A obj) {        return ("B and A");
    }    public String show(B obj) {        return ("B and B");
    }
}

class C extends B {
}

class D extends B {
}public class Test {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();        // 1.因此 B 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(b));        // 2.因为 C 是 A 的子类，调用类 A 的 show(A obj) 方法，输出：A and A
        System.out.println(a1.show(c));        // 3.调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a1.show(d));        // 4.因为 B 向上转型成 A， 且 B 是 A 的子类，调用类 A 的 show(A obj) 方法
        // 但是由于 B 中重写了 show(A obj) 方法，导致运行期间发生的动态绑定，调用 类 B 的 show(A obj) 方法
        // ，输出：B and A
        System.out.println(a2.show(b));        // 5.同上
        System.out.println(a2.show(c));        // 6.B 向上转型成 A，默认调用类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(a2.show(d));        // 7.调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(b));        // 8.C 是 B 的子类，调用类 B 的 show(B obj) 方法，输出：B and B
        System.out.println(b.show(c));        // 9.调用继承自类 A 的 show(D obj) 方法，输出：A and D
        System.out.println(b.show(d));

    }
}

---

Constructor and Polymorphism

Similarly, explore through an example

class Parent {    void print() {
        System.out.println("I am a Parent");
    }

    Parent() {
        print();
    }
}

class Son extends Parent {    int num = 1;    // 重写（override）了父类方法
    void print() {
        System.out.println("I am a Son " + num);
    }

    Son() {
        print();
    }
}public class Test {
    public static void main(String[] args) {
        Son son = new Son();        /**
         * 输出内容： 
         * I am a Son 0
         * I am a Son 1
         */
    }
}

We can see from the output content that the Parent's constructor called Son's print() due to dynamic binding. Since num has not been initialized, the value is 0.

Analysis results, in fact, it involves three contents:

• During the instantiation process of Son, the constructor of Parent and the constructor of Parent are executed first.

• Son overrides the print method, so Parent calls the rewritten method of Son.

• In Parent, since num has not been initialized, the value is 0.

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