Lab 05: Mini Problems

Learning Objectives

Students should

• be able to organize program into packages
• be able to create and use nested classes

Initializing

To get the files, run the following command from your PE node. We recommend creating a new directory called mini to store all your lab mini problems.

cp -r ~cs2030s/mini/lab5/ .

The files will only be available on Wednesday, 9 October 2024.

Lab 04 Extension

Parts of this problem is an extension to mini problems of Lab 04. The solution to the mini problems of Lab 4 can be found in the accompanying directory.

In Ex 5, you will need to copy your solution instead of having the accompanying solution.

Mini Problem 1

We have created an immutable point in Lab 04, the partial code is shown below. While it may not fully solve Lab 04, it is sufficient for our purpose.

public final class Point {
  private final int x;
  private final int y;
  private static Point ORIGIN = new Point(0, 0);

  private Point(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public static Point of(int x, int y) {
    if (x == 0 && y == 0) {
      return Point.ORIGIN;
    }
    return new Point(x, y);
  }

  public Point moveTo(int x, int y) {
    return new Point(x, y);
  }

  @Override
  public String toString() {
    return "(" + this.x + "," + this.y + ")";
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj instanceof Point) {
      Point pt = (Point) obj;
      return this.x == pt.x && this.y == pt.y;
    }
    return false;
  }
}

Task 1: Packaging

We will now create a package called shapes. To do this, follow the steps below. Please revise the bash commands to do this.

1. Create a directory called shapes.
2. Copy Point.java into shapes.
3. Add the line package shapes; at the top of Point.java.
   • All classes in the package shapes must begin with package shapes;.
4. Add the line import shapes.Point; at the top of Task1.java.
   • Note that you should NOT use import shapes.*;. Instead, you should import the needed classes individually to avoid namespace polution.

At the end, you should see something like the following directory structure. The base directory is called mini1. All directory ends with / as part of their names.

mini1/
├── shapes/
│   └── Point.java
├── Task1.java
└── Task2.java

Now we can check if this works. We will test with Task1.java only.

1. Compile Task1.java with javac Task1.java.
   • Notice that Point.java inside the directory shapes will also be compiled.
2. Run Task1.java with java Task1.
   • You should get all ok.

Task 2: Adding Shapes

Given points, we can now create shapes. We have created Circle before, so let us recreate it inside the shapes package. Remember that all classes in the package shapes must begin with package shapes;.

Before we specify the requirement for Circle, we first remove the keyword public from the constructor of class Point. This will prevent us from creating a point directly from outside of the package. That is because beside the public and private modifier, we have the following modifiers. Without any modifier, the class Point can only be used by code in the same class or in the same package.

Modifier	Class	Package	Subclass	Others
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

Circle.java      Now we can create Circle inside the package shapes. Design your class in the following way.

• It is a public class with two private fields. The first field is a Point indicating the center point of the circle. The second field is the radius of the circle as int.
• It has a public constructor that accepts two parameters. The first parameter is the center point and the second parameter is the radius of the circle.
• The public method toString prints the circle as Circle @ <center> with radius <radius>. <center> is the string representation of its center point and <radius> is the string representation of its radius.
• The protected method getRadius returns the radius of the circle.
• Add the method public void moveTo(int x, int y) in the class Circle to move the center point of the circle. However, instead of invoking Point::moveTo, try creating new Point(x, y) directly in the Circle class.

You can check your implementation with Task2.java.

MyCircle.java      Now we can create ColoredCircle outside the package shapes. Design your class in the following way.

• It is a public class extending Circle with no fields.
• It has a public constructor that accepts one parameters: the radius of the circle. The center point is always at (0, 0).
• It has a single public method isSmallerThan(int radius) to compare the radius with the input parameter radius. The method returns true if the radius is strictly smaller than radius.

You can check your implementation with Task3.java.

Mini Problem 2

We have created a generic pair in Lab 04, the partial code is shown below. While it may not fully solve Lab 04, it is sufficient for our purpose. Although we are not extending pair, you may use it as an inspiration.

public class Pair<S, T> {
  private S first;
  private T second;

  public Pair(S first, T second) {
    this.first = first;
    this.second = second;
  }

  public S getFirst() {
    return this.first;
  }

  public T getSecond() {
    return this.second;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true; // optional, for efficiency
    }
    if (obj instanceof Pair<?, ?>) {
      Pair<?, ?> pair = (Pair<?, ?>) obj;

      if (this.first == null && this.second == null) {
        return pair.first == null && pair.second == null;
      }
      if (this.first == null) {
        return pair.first == null && this.second.equals(pair.second);
      }
      if (this.second == null) {
        return this.first.equals(pair.first); && pair.second == null;
      }

      return this.first.equals(pair.first)
          && this.second.equals(pair.second);
    }
    return false;
  }

  @Override
  public String toString() {
    return "(" + this.first + "," + this.second + ")";
  }
}

Task 1: Abstract Boolean Condition

We will now create a new class that is an abstraction of an boolean condition. We call this class Bool<T>. Your current task is to implement the abstract class Bool<T> with the following specification.

• It is a generic abstract class with one type parameter T.
• It has a single private field of type T.
• It has a private constructor that accepts a value of type T.
• It has a single concrete method T getVal() that returns the field of type T.
• It has two public abstract methods.
  • Bool<T> doThen(T val).
  • Bool<T> doElse(T val).

This class will have a factory method based on the second task below.

Task 2: Nested Class

We will now create two static nested classes.

True      The class True<T> extends Bool<T>. It overrides the two abstract methods as follows.

• Bool<T> doThen(T val): returns a new instance of True<T> such that the protected field of type T inherited from Bool<T> is set to the given value val.
• Bool<T> doElse(T val): returns itself without change.

False      The class False<T> extends Bool<T>. It overrides the two abstract methods as follows.

• Bool<T> doThen(T val): returns itself without change.
• Bool<T> doElse(T val): returns a new instance of False<T> such that the protected field of type T inherited from Bool<T> is set to the given value val.

Factory Method      Implement the factory method test(boolean cond) that takes in a boolean condition and returns Bool<T> such that

• if cond is true, it returns a new True<T> with initial value of null.
• if cond is false, it returns a new False<T> with initial value of null.

See the sample usage below.

jshell> Bool.<Integer>test(true).doThen(1).doElse(2).getVal()
$.. ==> 1
jshell> Bool.<Integer>test(false).doThen(1).doElse(2).getVal()
$.. ==> 2

jshell> Bool.<Integer>test(true).getVal()
$.. ==> null
jshell> Bool.<Integer>test(false).getVal()
$.. ==> null

jshell> Bool.<Integer>test(true).doThen(1).doThen(2).getVal()
$.. ==> 2
jshell> Bool.<Integer>test(false).doElse(2).doElse(1).getVal()
$.. ==> 1

jshell> Bool.<Integer>test(true).doElse(1).doElse(2).getVal()
$.. ==> null
jshell> Bool.<Integer>test(false).doThen(2).doThen(1).getVal()
$.. ==> null

Look Ma, no if-then-else statement!

You can check your implementation with Task1.java.