关于方法调用的 Java 多态性的混淆

Question

I am having trouble with this Java Question :

Consider the following classes:

​
public class Computer extends Mineral {
    public void b() {
        System.out.println("Computer b");
        super.b();
    }

    public void c() {
        System.out.println("Computer c");
    }
}
​
public class Mineral extends Vegetable {
    public void b() {
        System.out.println("Mineral b");
        a();
    }
}
​
public class Animal extends Mineral {
    public void a() {
        System.out.println("Animal a");
    }

    public void c() {
        b();
        System.out.println("Animal c");
    }
}
​
public class Vegetable {
    public void a() {
        System.out.println("Vegetable a");
    }
​
    public void b() {
        System.out.println("Vegetable b");
    }
}

Suppose the following variables are defined:

Vegetable var1 = new Computer();
Mineral   var2 = new Animal();
Vegetable var3 = new Mineral();
Object    var4 = new Mineral();

Indicate on each line below the output produced by each statement shown. If the statement produces more than one line of output indicate the line breaks with slashes as in a/b/c to indicate three lines of output with a followed by b followed by c. If the statement causes an error, write the word error to indicate this.

For the execution of

var1.b()

I was confused about the output

Through careful analysis, we must notice that when we call the method b() of mineral:

public void b() {
        System.out.println("Mineral b");
        a();
    }

We are also calling a method

a()

And therefore, using a class hierarachy diagram, can call the method

Vegetable.a()

source

Answer 1

In Vegetable var1 = new Computer(); , you have a reference variable of type Vegetable , pointing to an object of type Computer . The assignment is valid, if Vegetable is a super-type of Computer.

The expression var1.b() will be legal (compilation pass), if the type of the reference variable (Vegetable) has a method b() . If the Vegetable type doesn't have a method b() , then the expression will give a compilation error.

If the compilation passes: at runtime , calling var1.b() will invoke the b() method on the object the variable var1 points to (that is, an instance of type Computer). Computer.b() overrides Mineral.b() , so that method will be invoked.