Review Questions

Given the following program, which statement is true?

// Filename: MyClass.java
public class MyClass {
    public static void main(String[] args) {
        A[] arrA;
        B[] arrB;

        arrA = new A[10];
        arrB = new B[20];
        arrA = arrB;       // (1)
        arrB = (B[]) arrA; // (2)
        arrA = new A[10];
        arrB = (B[]) arrA; // (3)
    }
}

class A {}

class B extends A {}

Select the one correct answer.

Which is the first line that will cause compilation to fail in the following program?

// Filename: MyClass.java
class MyClass {
    public static void main(String[] args) {
        MyClass a;
        MySubclass b;

        a = new MyClass();             // (1)
        b = new MySubclass();          // (2)

        a = b;                         // (3)
        b = a;                         // (4)

        a = new MySubclass();          // (5)
        b = new MyClass();             // (6)
    }
}

class MySubclass extends MyClass {}

Select the one correct answer.

Given the following definitions and reference declarations, which one of the following assignments is legal?

// Definitions:
interface I1 {}
interface I2 {}
class C1 implements I1 {}
class C2 implements I2 {}
class C3 extends C1 implements I2 {}
// Reference declarations:
// ...
    C1 obj1;
    C2 obj2;
    C3 obj3;
// ...

Select the one correct answer.

Given the following class definitions and the reference declarations, what can be said about the statement y = (Sub) x?

// Class definitions:
class Super {}
class Sub extends Super {}

// Reference declarations
// ...
    Super x;
    Sub y;
// ...

Select the one correct answer.

Given the following class definitions and declaration statements, which one of these assignments is legal at compile time?

// Definitions:
interface A {}
class B {}
class C extends B implements A {}
class D implements A {}

// Declaration statements:
// [...]
    B b = new B();
    C c = new C();
    D d = new D();
// [...]

Select the one correct answer.

Which letters will be printed when the following program is run?

// Filename: MyClass.java
public class MyClass {
    public static void main(String[] args) {
        B b = new C();
        A a = b;
        if (a instanceof A) System.out.println("A");
        if (a instanceof B) System.out.println("B");
        if (a instanceof C) System.out.println("C");
        if (a instanceof D) System.out.println("D");
    }
}

class A {}
class B extends A {}
class C extends B {}
class D extends C {}

Select the three correct answers.

Given three classes A, B, and C, where B is a subclass of A and C is a subclass of B, which one of these boolean expressions is true when an object denoted by reference o has actually been instantiated from class B as opposed to from A or C?

Select the one correct answer.

When the following program is run, it will print all the letters I, J, C, and D. Is this statement true or false?

public class MyClass {
    public static void main(String[] args) {
        I x = new D();
        if (x instanceof I) System.out.println("I");
        if (x instanceof J) System.out.println("J");
        if (x instanceof C) System.out.println("C");
        if (x instanceof D) System.out.println("D");
    }
}

interface I{}
interface J{}
class C implements I {}
class D extends C implements J {}

Select the one correct answer.