Khi ch&uacute;ng ta sử dụng <code>final</code> với phương thức, c&aacute;c lớp con kh&ocirc;ng thể ghi đ&egrave; l&ecirc;n phương thức đ&oacute;. H&atilde;y xem một v&iacute; dụ:
<pre class="language-java" data-content="class%20Engine%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%0A%C2%A0%C2%A0%C2%A0%C2%A0%2F%2F%20create%20a%20final%20method%0A%C2%A0%C2%A0%C2%A0%C2%A0final%20void%20setEngine()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Four-Stroke%20Engine%22)%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%7D%0A%7D%0A%C2%A0%0Aclass%20PetrolEngine%20extends%20Engine%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%0A%C2%A0%C2%A0%C2%A0%C2%A0%2F%2F%20try%20to%20override%20setEngine()%0A%C2%A0%C2%A0%C2%A0%C2%A0void%20setEngine()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Four-Stroke%20Petrol%20Engine%22)%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%7D%0A%7D%0A%C2%A0%0Aclass%20Main%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0public%20static%20void%20main(String%5B%5D%20args)%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%2F%2F%20object%20of%20subclass%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0PetrolEngine%20obj%20%3D%20new%20PetrolEngine()%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%2F%2F%20access%20the%20overridden%20method%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0obj.setEngine()%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%7D%0A%7D" data-language="java" data-ace-language="java" data-name-language="Java" data-main-file="Main.java"><code>class Engine {
 
 // create a final method
 final void setEngine() {
 System.out.println("Four-Stroke Engine");
 }
}
 
class PetrolEngine extends Engine {
 
 // try to override setEngine()
 void setEngine() {
 System.out.println("Four-Stroke Petrol Engine");
 }
}
 
class Main {
 public static void main(String[] args) {
 
 // object of subclass
 PetrolEngine obj = new PetrolEngine();
 
 // access the overridden method
 obj.setEngine();
 }
}</code></pre>
Trong v&iacute; dụ tr&ecirc;n, ch&uacute;ng ta đ&atilde; tạo phương thức final <code>setEngine()</code> b&ecirc;n trong lớp <code>Engine</code>.
<pre class="language-java" data-content="final%20void%20setEngine()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Four-Stroke%20Engine%22)%3B%0A%7D" data-language="java" data-ace-language="java" data-name-language="Java" data-main-file="Main.java"><code>final void setEngine() {
 System.out.println("Four-Stroke Engine");
}</code></pre>
Ở đ&acirc;y, lớp <code>PetrolEngine</code> kế thừa <code>Engine</code> v&agrave; ghi đ&egrave; phương thức <code>setEngine()</code> b&ecirc;n trong n&oacute;.
<pre class="language-java" data-content="void%20setEngine()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Four-Stroke%20Petrol%20Engine%22)%3B%0A%7D" data-language="java" data-ace-language="java" data-name-language="Java" data-main-file="Main.java"><code>void setEngine() {
 System.out.println("Four-Stroke Petrol Engine");
}</code></pre>
B&acirc;y giờ, nếu chạy chương tr&igrave;nh tr&ecirc;n, ch&uacute;ng ta sẽ gặp lỗi.
<pre class="language-java" data-content="error%3A%20setEngine()%20in%20PetrolEngine%20cannot%20override%20setEngine()%20in%20Engine%0A%C2%A0%C2%A0%C2%A0%C2%A0void%20setEngine()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%5E%0A%C2%A0%C2%A0overridden%20method%20is%20final" data-language="java" data-ace-language="java" data-name-language="Java" data-main-file="Main.java"><code>error: setEngine() in PetrolEngine cannot override setEngine() in Engine
 void setEngine() {
 ^
 overridden method is final</code></pre>
Đ&oacute; l&agrave; bởi v&igrave; khi một phương thức được khai b&aacute;o l&agrave; <code>final</code> th&igrave; lớp con kh&ocirc;ng thể ghi đ&egrave; l&ecirc;n n&oacute;.
<h2 style="text-align: justify;">Trắc nghiệm final với phương thức</h2>
Bạn c&oacute; thể nh&igrave;n ra vấn đề với chương tr&igrave;nh sau kh&ocirc;ng?
<pre class="language-java" data-content="class%20Animal%20%7B%0A%0A%C2%A0%C2%A0%C2%A0%C2%A0final%20String%20name%3B%0A%0A%C2%A0%C2%A0%C2%A0%C2%A0final%20void%20print()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Animal%22)%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%7D%0A%7D%0A%0Aclass%20Dog%20extends%20Animal%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0void%20print()%20%7B%0A%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0%C2%A0System.out.println(%22Dog%22)%3B%0A%C2%A0%C2%A0%C2%A0%C2%A0%7D%0A%7D" data-language="java" data-ace-language="java" data-name-language="Java" data-main-file="Main.java"><code>class Animal {

 final String name;

 final void print() {
 System.out.println("Animal");
 }
}

class Dog extends Animal {
 void print() {
 System.out.println("Dog");
 }
}</code></pre>

A. Ch&uacute;ng ta đ&atilde; tạo biến v&agrave; phương thức final c&ugrave;ng l&uacute;c.

B. Ch&uacute;ng ta sử dụng từ kh&oacute;a final với phương thức.

C. Ch&uacute;ng ta ghi đ&egrave; phương thức final b&ecirc;n trong lớp Dog.

D. Tất cả c&aacute;c đ&aacute;p &aacute;n tr&ecirc;n

Final với phương thức

Final với phương thức