Why Is Underscore a Reserved Keyword in Java?

Underscore(_) can be used as a variable name in Java till Java 8 but from Java 9 release, underscore can't be used as an identifier or variable name.

Java makes underscore(_) as a reserved word since Java 9.

In Java 9 and later versions, if we use the underscore character ("_") as an identifier, the source code can no longer be compiled and the compiler will report a compile-time error.

Let's see some examples to understand, how the use of underscore as a variable is changed version after version.

In this example, we are using underscore(_) as a variable name and compile it using Java 8 version. See it executes fine and produces output.

public class Main { 
    public static void main(String[] args){
        
        int _ = 20;
        System.out.println(_);
        
    }
}

Output:

20

If we execute this example using Java 9 then compile throws a compile-time error that indicates that underscore is a reserved word and can not be used as a variable name.

public class Main { 
    public static void main(String[] args){
        
        int _ = 20;
        System.out.println(_);
        
    }
}

Output:

'_' should not be used as an identifier, since it is a reserved keyword from source level 1.8

If we use underscore with a variable name then compiler works fine. Using underscore in a variable like first_name is still valid. But using _ alone as the variable name is no more valid. See the example below.

public class Main { 
    public static void main(String[] args){
        
        int _a = 20;
        System.out.println(_a);
        
    }
}

Output:

20

In this example, we are using underscore with parameter names and executes. It works fine, see the example below.

public class Main {  
    public static void main(String[] args){
        
        int sum = add(10,20);
        System.out.println(sum);
    }   
    
    static int add(int _a, int _b) {
        return _a+_b;
    }
}

Output:

30