How to use final in java

Final is used in Java to declare immutable variables, non-overridable methods, and non-inheritable classes. It also helps in declaring constants and capturing external variables. The main usage is summarized as follows: final variable: unchangeable, read-only. final method: cannot be overridden, but can be implemented. Final class: cannot be inherited, but its methods can be called. final constant: cannot be changed, usually represented by uppercase letters. final anonymous inner class: can capture external variables.

Usage of final in Java

final is a keyword in Java, used to declare variables and methods and classes.

Variables

final variables are read-only and cannot be changed once assigned. This helps prevent accidental changes to sensitive data. For example:

final String NAME = "John Doe";
NAME = "Jane Doe"; // 编译错误

Method

final method cannot be overridden by subclasses. This helps prevent unexpected behavior by overriding critical methods. For example:

final void printName() {
    System.out.println("John Doe");
}

Class

final class cannot be inherited. This helps ensure that the class is unmodifiable and prevents the creation of its subclasses. For example:

final class Person {
    // ...
}

Other uses

final can also be used to declare constants and anonymous inner classes.

Constant

final constant is unchangeable and can be declared in classes, methods and interfaces. For example:

public static final int MAX_AGE = 100;

Anonymous inner class

Using final in an anonymous inner class can capture external variables. For example:

JButton button = new JButton("Click me");
button.addActionListener(new ActionListener() {
    final String name = "John";
    
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("Hello, " + name);
    }
});

Points to note

• Once a final variable is declared, its value cannot be changed.
• Final methods cannot be overridden, but can be implemented.
• The final class cannot be inherited, but its methods can be called in subclasses.
• Final constants are usually represented by uppercase letters.

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