Inheritance

Definitions

A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).

• Superclass$\equiv$Base class$\equiv$Parent class
• Subclass$\equiv$Derived class$\equiv$Child class

Excepting Object, which has no superclass, every class has one and only one direct superclass (single inheritance).

A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

Subclass

What You Can Do in a Subclass?

A subclass inherits all of the public and protected members of its parent, no matter what package the subclass is in. If the subclass is in the same package as its parent, it also inherits the package-private members of the parent. You can use the inherited members as is, replace them, hide them, or supplement them with new members:

• You can declare a field in the subclass with the same name as the one in the superclass, thus hiding it (not recommended).
• You can declare new fields in the subclass that are not in the superclass.
• The inherited methods can be used directly as they are.
• You can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it.
• You can write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it.
• You can declare new methods in the subclass that are not in the superclass.
• You can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super.

A nested class has access to all the private members of its enclosing class—both fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.

Polymorphism

Method Override

Source Code

Test.java

public class Test {

private static class Parent {

public void print() {

System.out.println("Parent");

}

}

private static class Child extends Parent{

public void print() {

System.out.println("Child");

}

}

public static void main(String[] args) {

Parent o = new Child();

o.print();

}

}

Output

Run the following commands.

javac Test.java

# You should see the following four files now.

# 'Test$Child.class' 'Test$Parent.class' Test.class Test.java

# disassembles Test class file

javap -verbose Test

The complete output ofjavap -verbose Test is in the last section. Here we only focus on two parts.

Constant Pool

Constant pool:

#1 = Methodref #6.#20 // java/lang/Object."<init>":()V

#2 = Class #21 // Test$Child

#3 = Methodref #2.#20 // Test$Child."<init>":()V

#4 = Methodref #9.#22 // Test$Parent.print:()V

#5 = Class #23 // Test

#6 = Class #24 // java/lang/Object

#7 = Utf8 Child

#8 = Utf8 InnerClasses

#9 = Class #25 // Test$Parent

#10 = Utf8 Parent

#11 = Utf8 <init>

#12 = Utf8 ()V

#13 = Utf8 Code

#14 = Utf8 LineNumberTable

#15 = Utf8 main

#16 = Utf8 ([Ljava/lang/String;)V

#17 = Utf8 SourceFile

#18 = Utf8 Test.java

#19 = Utf8 NestMembers

#20 = NameAndType #11:#12 // "<init>":()V

#21 = Utf8 Test$Child

#22 = NameAndType #26:#12 // print:()V

#23 = Utf8 Test

#24 = Utf8 java/lang/Object

#25 = Utf8 Test$Parent

#26 = Utf8 print

Bytecode

public Test();

descriptor: ()V

flags: (0x0001) ACC_PUBLIC

Code:

stack=1, locals=1, args_size=1

0: aload_0

1: invokespecial #1 // Method java/lang/Object."<init>":()V

4: return

LineNumberTable:

line 1: 0

public static void main(java.lang.String[]);

descriptor: ([Ljava/lang/String;)V

flags: (0x0009) ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=2, args_size=1

0: new #2 // class Test$Child

3: dup

4: invokespecial #3 // Method Test$Child."<init>":()V

7: astore_1

8: aload_1

9: invokevirtual #4 // Method Test$Parent.print:()V

12: return

LineNumberTable:

line 15: 0

line 16: 8

line 17: 12

Bytecode Tear Down

Bytecode contains two parts: the constructor and the method.

The Constructor

public Test();

# it takes no argrments and it returns nothing.

descriptor: ()V

# it's public

flags: (0x0001) ACC_PUBLIC

Code:

stack=1, locals=1, args_size=1

0: aload_0

# implicitly invoke super(), which would call new Object() because every class inherits Object

1: invokespecial #1 // Method java/lang/Object."<init>":()V

4: return

LineNumberTable:

line 1: 0

The Method

public static void main(java.lang.String[]);

# it takes a String as an arguments and it returns nothing.

descriptor: ([Ljava/lang/String;)V

# it's public and static

flags: (0x0009) ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=2, args_size=1

# this is a Child object

0: new #2 // class Test$Child

3: dup

# call Child constructor

4: invokespecial #3 // Method Test$Child."<init>":()V

7: astore_1

8: aload_1

# call Parent print method

9: invokevirtual #4 // Method Test$Parent.print:()V

12: return

LineNumberTable:

line 15: 0

line 16: 8

line 17: 12

caution

Why Parent print method is called instead of Child's?

9: invokevirtual #4 // Method Test$Parent.print:()V

Because that is all the compiler knows.

• The compiler knows o is Parent type, so it calls Parent's method.

Constant Pool Tear Down

Let's read a few lines of Constant Pool.

Because the first line of the code is Parent o = new Child();, it will call super(); then it initialized itself.

#1 = Methodref #6.#20 // java/lang/Object."<init>":()V

#2 = Class #21 // Test$Child

#3 = Methodref #2.#20 // Test$Child."<init>":()V

More information are available at:

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.4

Polymorphism

From Bytecode, we see that

9: invokevirtual #4 // Method Test$Parent.print:()V

is going to be called, which is Parent.print .

But polymorphism gives us a different result. Why?

VTable

If take C++ approach as JVM's implementation, we can safely assume for each object, there is a VTable corresponding to it.

When we declare Parent o, a VTable is created

Name	Reference
Object.toString()	Object.toString()
Object.something else	Object.something else
Parent.print()	Parent.print()

✍ When o was initialized to Child, Parent.print() would be updated to Child.print(), resulting the following VTable :

Name	Reference
Object.toString()	Object.toString()
Object.something else	Object.something else
Parent.print()	Child.print()

JVM will look up this VTable at running time to find #4 Test$Parent.print and would find that it's actually pointing to Child.print(). That's why it's called running time binding or dynamic binding, or late binding.

Complete Output

Classfile $FULL_PATH_OF_THE_PROJECT/src/Test.class

Last modified $DATE; size 411 bytes

MD5 checksum dc8d7bc4039d222f5ff3d99c6bf3897d

Compiled from "Test.java"

public class Test

minor version: 0

major version: 55

flags: (0x0021) ACC_PUBLIC, ACC_SUPER

this_class: #5 // Test

super_class: #6 // java/lang/Object

interfaces: 0, fields: 0, methods: 2, attributes: 3

Constant pool:

#1 = Methodref #6.#20 // java/lang/Object."<init>":()V

#2 = Class #21 // Test$Child

#3 = Methodref #2.#20 // Test$Child."<init>":()V

#4 = Methodref #9.#22 // Test$Parent.print:()V

#5 = Class #23 // Test

#6 = Class #24 // java/lang/Object

#7 = Utf8 Child

#8 = Utf8 InnerClasses

#9 = Class #25 // Test$Parent

#10 = Utf8 Parent

#11 = Utf8 <init>

#12 = Utf8 ()V

#13 = Utf8 Code

#14 = Utf8 LineNumberTable

#15 = Utf8 main

#16 = Utf8 ([Ljava/lang/String;)V

#17 = Utf8 SourceFile

#18 = Utf8 Test.java

#19 = Utf8 NestMembers

#20 = NameAndType #11:#12 // "<init>":()V

#21 = Utf8 Test$Child

#22 = NameAndType #26:#12 // print:()V

#23 = Utf8 Test

#24 = Utf8 java/lang/Object

#25 = Utf8 Test$Parent

#26 = Utf8 print

{

public Test();

descriptor: ()V

flags: (0x0001) ACC_PUBLIC

Code:

stack=1, locals=1, args_size=1

0: aload_0

1: invokespecial #1 // Method java/lang/Object."<init>":()V

4: return

LineNumberTable:

line 1: 0

public static void main(java.lang.String[]);

descriptor: ([Ljava/lang/String;)V

flags: (0x0009) ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=2, args_size=1

0: new #2 // class Test$Child

3: dup

4: invokespecial #3 // Method Test$Child."<init>":()V

7: astore_1

8: aload_1

9: invokevirtual #4 // Method Test$Parent.print:()V

12: return

LineNumberTable:

line 15: 0

line 16: 8

line 17: 12

}

SourceFile: "Test.java"

NestMembers:

Test$Child

Test$Parent

info

You may notice that there isn't that much information about the inner classes Parent and Child. This is because they are compiled to the other two files:Test$Parent.class and Test$Child.class.