CODEWITHSUNDEEP
javasubclassingjls
Kiril Kirilov
Kiril Kirilov

Reputation: 11257

If you overwrite a field in a subclass of a class, the subclass has two fields with the same name(and different type)?

I have 3 classes:

public class Alpha {
    public Number number;
}

public class Beta extends Alpha {
    public String number;
}

public class Gama extends Beta {
    public int number;
}

Why does the following code compile? And, why does the test pass without any runtime errors?

@Test
public void test() {
    final Beta a = new Gama();
    a.number = "its a string";
    ((Alpha) a).number = 13;
    ((Gama) a).number = 42;

    assertEquals("its a string", a.number);
    assertEquals(13, ((Alpha) a).number);
    assertEquals(42, ((Gama) a).number);
}

Upvotes: 81

Views: 81106

Answers (4)

yoAlex5
yoAlex5

Reputation: 34215

Java Hiding a field

When successor has a field with the same name as a superclass's field it is called - Hiding a field

Java's field does not support polymorphism and does not take a field's type into account

class A {
    String field = "A: field";

    String foo() {
        return "A: foo()";
    }
}

class B extends A {
    //B's field hides A's field
    String field = "B: field";

    String foo() {
        return "B: foo()";
    }
}

@Test
public void testPoly() {
    A a = new A();
    assertEquals("A: field", a.field);
    assertEquals("A: foo()", a.foo());

    B b = new B();
    assertEquals("B: field", b.field);
    assertEquals("B: foo()", b.foo());

    //B cast to A
    assertEquals("A: field", ((A)b).field);  //<--
    assertEquals("B: foo()", ((A)b).foo());
}

[Swift override property]

Upvotes: 5

fm-sys
fm-sys

Reputation: 199

As a workaround, you can use getter methods:

class A {
    private String field = "A: field";

    String getField() {
        return field;
    }
}

class B extends A {
    private String field = "B: field";

    @Override        
    String getField() {
        return field;
    }
}

Upvotes: 3

Jesper
Jesper

Reputation: 206796

Member variables cannot be overridden like methods. The number variables in your classes Beta and Gama are hiding (not overriding) the member variable number of the superclass.

By casting you can access the hidden member in the superclass.

Upvotes: 98

Jon Skeet
Jon Skeet

Reputation: 1500405

Fields can't be overridden; they're not accessed polymorphically in the first place - you're just declaring a new field in each case.

It compiles because in each case the compile-time type of the expression is enough to determine which field called number you mean.

In real-world programming, you would avoid this by two means:

  • Common-sense: shadowing fields makes your code harder to read, so just don't do it
  • Visibility: if you make all your fields private, subclasses won't know about them anyway

Upvotes: 61

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