不可变 class：空白最终字段可能尚未初始化

Question

I have to do an immutable class but I'm given a mistake.

public final class ClassX{
   private final String a;
   private final String b;
   private final int c;


   public ClassX(String a, String b, int c){
       this.a=a;
       this.b=b;
       if(c>=1||c<=100)
       this.c=c;
   }

The blank final field c may not have been initialized

Am I doing something wrong?

without if I don't get any errors but I need variable c to be in range 1-100

Answer 1

Balance your if with an else

In your phrasing:

if( … ) { this.c = c ; }

… you have not addressed the negative case. If the test in the if test results in a false , your member field this.c will not have been assigned a value by you. That member field of int will be assigned a default value of zero.

(For object references in contrast to primitives, the default will be null . That means no value at all in that member field.)

We can see this defaulting behavior in the following code example.

package work.basil.example;

public class DefaultingInt
{
    private int count;

    public int getCount ( ) { return this.count; }  // Getter method (accessor). 

    @Override
    public String toString ( )
    {
        return "DefaultingInt{ " +
                "count=" + count +
                " }";
    }

    public static void main ( String[] args )
    {
        DefaultingInt di = new DefaultingInt();
        System.out.println( "di = " + di );
    }
}

When run:

di = DefaultingInt{ count=0 }

With the code above, the value of the member field could always be changed later is some other code.

final requires a value be assigned

Your code adds the complication of marking the member field final . Using final means that you intend to assign a value once, as a constant. The compiler ensures that no code is allowed to change that value.

Your IDE or compiler is flagging an error because in the case of your if test returning false , you will not have assigned an explicit value to the final field. After the constructor ends, you cannot later assign a value. Since you insisted you would immediately set a single final value, and your code at runtime may miss your two chances to have set that value (static assignment, or in the constructor), the compiler knows something is wrong.

Solution: Add else

To take care of the missing case when your if returns a false , change your code to add an else block.

if( … ) { this.c = c ; } else { this.c = your-default-value-here ; }

As others pointed out, the logic of your particular if test is flawed. Use logical AND rather than OR.

if( ( c >= 1 ) && ( c <= 100 ) )  // If input value is within the bounds allowed, assign to field.
{ 
    this.c = c ; 
} 
else // Else input value is outside the bounds allowed.
{ 
    this.c = your-default-value-here ; 
}

If you cannot specify a default value, throw an exception instead.

if( ( c >= 1 ) && ( c <= 100 ) )  // If input value is within the bounds allowed, assign to field.
{ 
    this.c = c ; 
} 
else // Else input value is outside the bounds allowed. Throw exception.
{ 
    throw new  IllegalArgumentException( "Requires an input value of 1 to 100 inclusive." ) ;
}

Full example code:

package work.basil.example;

public class DefaultingInt
{
    final private int count;

    public DefaultingInt ( int count )
    {
        if ( ( count >= 1 ) && ( count <= 100 ) )  // If input value is within the bounds allowed, assign to field.
        {
            this.count = count;
        } else // Else input value is outside the bounds allowed. Throw exception.
        {
            throw new IllegalArgumentException( "Requires an input value of 1 to 100 inclusive." );
        }
    }

    public int getCount ( ) { return this.count; }  // Getter method (accessor). 

    @Override
    public String toString ( )
    {
        return "DefaultingInt{ " +
                "count=" + count +
                " }";
    }

    public static void main ( String[] args )
    {
        DefaultingInt di = new DefaultingInt( 101 );
        System.out.println( "di = " + di );
    }
}

When run:

Exception in thread "main" java.lang.IllegalArgumentException: Requires an input value of 1 to 100 inclusive.

---

Java record

By the way, for a simple immutable object whose main job is carrying data, consider using the new records feature due to arrive in Java 16 and previewed in earlier versions.

You simply declare the member fields. Java implicitly creates constructor, getters, and overrides of toString , equals , and hashCode .

In a record, marking final on a member field is superfluous. Every member field on record is implicitly final , a read-only constant, with an implicit getter accessor method. A record is meant to be a non-modifiable nominal tuple (an immutable assortment of named fields). The content within an object referenced by a record's member field may be mutable as with any other class, but the assignment of that one particular object to that member field cannot be changed. Primitives such as your int field are not object references, but simply the immediate value, and are immutable when used as a field in a record.

record ClassX( String a , String b , int c ) {}

You can still provide your own constructor to run your if test for data validation.

package work.basil.example;

public record DefaultingInt(int count)
{
    public DefaultingInt ( int count )
    {
        if ( ( count >= 1 ) && ( count <= 100 ) )  // If input value is within the bounds allowed, assign to field.
        {
            this.count = count;
        } else // Else input value is outside the bounds allowed. Throw exception.
        {
            throw new IllegalArgumentException( "Requires an input value of 1 to 100 inclusive." );
        }
    }

    public static void main ( String[] args )
    {
        DefaultingInt di = new DefaultingInt( 42 );
        System.out.println( "di = " + di );
    }
}

When run:

di = DefaultingInt[count=42]

Answer 2

this.c = Math.min(100, Math.max(c,1));

Answer 3

Your if is useless.

If you want c to be in the range 1-100 you should change || to &&

If you want c to be initialised to max or min value every time you can use @A.Alexander solution