SE 350 - Object-Oriented Software Development

Exam Review

Instructor: Stefan Mitsch

Programming Paradigms

public int f(int x, int y) {
  if (x < y) return x;
  else return f(x-y, y);
}

What is the result of the functional program above?

• f(5,2) == 1?
• f(5,3) == 2?
• f(3,1) == 0?
• f(7,4) == 2?

Object-Oriented Programming

True or false?

• Classes are objects?
• Every class must have at least 1 field and 1 method?
• An object is an instance of a class?
• An object has no state?
• A class cannot have a name, only objects can have names?
• A class can extend itself?

Object-Oriented Programming

public class C {
  private int i = 1;
  public void f() { i = i+2; }
  public void g() { i = 1; }
}

How many times to call f() until i > 6?

• i never becomes larger than 6?
• 1 time?
• 2 times?
• 3 times?
• 4 times?

How many times to call the sequence f(); g(); until i > 6?

Method Overloading

public class A {
  private String f() { return "f()"; }
  private String f(String s) { return "f(String)"; }
  private String f(String s, String t) { return "f(String,String)"; }
  private String f(String s, int t) { return "f(String,int)"; }
}

True or false result?

• f() == "f()"?
• f("x") == "f(String)"?
• f("1") == "f(String,int)"?
• f("x",1) == "f(String,int)"?
• f("x","1") == "f(String,int)"?

Inheritance

public class A {
  public static String f() { return "A"; }
  public String g() { return A.f(); }
  public String h() { return f(); }
}

public class B extends A {
  public static String f() { return "B"; }
  public String g() { return "B"; }
  public String h() { return f(); }
}

What is the result when using the following reference A x = new B();:

Inheritance

public final class C {
  public final String s = "C";
  public static String t = "C";
  public static int f() { return 1; }
  public final int g() { return 2; }
}

• Class C can be extended?
• The value of s can be changed?
• The value of t can be changed?
• The method f can be overridden?
• The method g can be overridden?

Inheritance

public class A {}
public class B extends A {}

Which casts are safe, which ones always fail, and which ones must be checked?

• A x1 = new A()?
• B x2 = new B()?
• A x3 = new B()?
• B x4 = new A()?
• B x5 = x3?
• x1 = x2?
• x2 = x3?

Constructor Chaining

public class A {
  public A() { this(0, 0); }
  public A(int x) { this(x, 0); }
  public A(int x, int y) {}
}
public class B extends A {
  public B(int x) { super(x); }
}
public class C extends B {
  public C() { super(1); }
}

What is the constructor sequence when instantiating a new C()?

Java Classes

public abstract final class a {
  protected String empty() { return ""; }
}

True or false?

• The class name violates Java coding conventions?
• The class compiles?
• The method empty must be abstract?
• The class declares no fields?

Java Instantiation

public class A {
  public A(int a) {}
  public A(String a) {}
}

public class B {}

Which of the following instantiation statements compile?

• A a = new A()?
• A a = new A(1)?
• A a = new A("1")?
• A a = new A(1.0)?
• B b = new B()?
• B b = new B(1)?

Visibility

public class A {
  private   String a = "a";
  protected String b = "b";
  public    String c = "c";
}
public class B extends A {
  public  String getA() { return a; }
  public  String getB() { return b; }
  private String getC() { return c; }
}

True or false?

• Field a is only visible within class A?
• Method getA() compiles
• Method getC() must be public because c is public?
• Field b is not visible to subclasses of class A?

Test-Driven Development

• Which test passes?
• How to fix the code of the failing test?

public class A {
  public String a = "A";
  public A(String a) { this.a = a; }
}

public class ATests {
  @Test public void test1() { assertEquals("A", new A().a); }
  @Test public void test2() { assertEquals("A", new A("a").a); }
}

UML Class Diagrams

True or false?

• When an A is destroyed, the C remains alive by itself?
• A D is an A?
• An A can have any number of Bs?
• An A has a public field a?
• Bs cannot be added to or removed from their A?