lab5

lab/lab5/Question4/Animal.java

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+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022/03/19
+ * Description: This is the class of Animal.
+ */
+
+public class Animal {
+	/**
+	 * Animal's name
+	 */
+	private String name;
+
+	/**
+	 * Animal's weight
+	 */
+	private double weight;
+
+	/**
+	 * Constructor. Initialize the name and weight of the animal.
+	 */
+	public Animal(String name, double weight) {
+		this.name = name;
+		this.weight = weight;
+	}
+
+	/**
+	 * Get the name of the animal.
+	 * 
+	 * @return the name of the animal
+	 */
+	public String getName() {
+		return name;
+	}
+
+	/**
+	 * Get the weight of the animal.
+	 * 
+	 * @return the weight of the animal
+	 */
+	public double getWeight() {
+		return weight;
+	}
+
+	/**
+	 * Set the weight of the animal.
+	 * 
+	 * @param weight
+	 *            the weight of the animal
+	 */
+	public void setWeight(double weight) {
+		this.weight = weight;
+	}
+
+	/**
+	 * Test
+	 */
+	public static void testAnimal() {
+		Animal animal = new Animal("cat", 1.0);
+		System.out.println(animal.getName() == "cat");
+		System.out.println(animal.getWeight() == 1.0);
+		animal.setWeight(2.0);
+		System.out.println(animal.getName() == "cat");
+		System.out.println(animal.getWeight() == 2.0);
+	}
+}
+

lab/lab5/Question4/Cat.java

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+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-03-19
+ * Description: Cat class
+ */
+
+public class Cat extends Animal {
+	/**
+	 * Constructor. Initialize the name and weight of the cat
+	 */
+	public Cat(String name, double weight) {
+		super(name, weight);
+	}
+
+	/**
+	 * Feeding a cat adds 1.0 to its weight
+	 */
+	public void feed() {
+		setWeight(getWeight() + 1.0);
+	}
+
+	/**
+	 * Test
+	 */
+	public static void testCat() {
+		Cat c = new Cat("Meow", 2.0);
+
+		System.out.println(c.getName() == "Meow");
+		System.out.println(c.getWeight() == 2.0);
+		c.feed();
+		// The name is still the same but the weight increased by 1.0:
+		System.out.println(c.getName() == "Meow");
+		System.out.println(c.getWeight() == 3.0);
+	}
+}

lab/lab5/Question4/Dog.java

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+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022-03-19
+ * Description: Dog class
+ */
+
+public class Dog extends Animal {
+	/**
+	 * Constructor. Initialize the name and weight of the dog
+	 */
+	public Dog(String name, double weight) {
+		super(name, weight);
+	}
+
+	/**
+	 * Feeding a dog adds 2.0 to its weight
+	 */
+	public void feed() {
+		setWeight(getWeight() + 2.0);
+	}
+
+	/**
+	 * Test
+	 */
+	public static void testDog() {
+		Dog d = new Dog("Woof", 2.0);
+		System.out.println(d.getName() == "Woof");
+		System.out.println(d.getWeight() == 2.0);
+		d.feed();
+		// The name is still the same but the weight increased by 2.0:
+		System.out.println(d.getName() == "Woof");
+		System.out.println(d.getWeight() == 4.0);
+	}
+}

lab/lab5/Question4/Start.java

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+/**
+ * Answer to question.
+ *
+ * # Question 4
+ *
+ * Q: Now that the Student class uses an animal as pet, can you still use a cat
+ * object as the pet of a student? Why or why not?
+ *
+ * A: Yes, you can. The Student class has a method getPet() that returns the pet
+ * which is Animal type.
+ *
+ * Q: Can you now use a dog object as the pet of a student? Why or why not?
+ *
+ * A: Yes, you can. But only attributes and methods definded in the Animal class
+ * can be accessed. When you do so, the Dog object will be converted to an
+ * Animal object. Assign a child object to a parent object is called upcasting
+ * in Java. Upcasting is implicit.
+ *
+ */
+public class Start {
+	public static void main(String[] args) {
+		Cat.testCat();
+		Dog.testDog();
+		Student.testStudent();
+		Animal.testAnimal();
+	}
+}

lab/lab5/Question4/Student.java

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+/*
+ * Author: CHEN Yongyuan (Walter) 1930006025 from OOP(1007)
+ * Date: 2022/03/19
+ * Description: Student class
+ */
+
+public class Student {
+	/**
+	 * Student's name
+	 */
+	private String name;
+
+	/**
+	 * Student's pet
+	 */
+	private Animal pet;
+
+	/**
+	 * Constructor
+	 */
+	public Student(String name, Animal pet) {
+		this.name = name;
+		this.pet = pet;
+	}
+
+	/**
+	 * Getter for name
+	 *
+	 * @return String
+	 */
+	public String getName() {
+		return name;
+	}
+
+	/**
+	 * Getter for pet
+	 *
+	 * @return Animal
+	 */
+	public Animal getPet() {
+		return pet;
+	}
+
+	/**
+	 * Test
+	 */
+	public static void testStudent() {
+		Cat pet = new Cat("Tom", 1.0);
+		Student student = new Student("Walter", pet);
+		System.out.println(student.getName() == "Walter");
+		System.out.println(student.getPet() == pet);
+		System.out.println(student.getPet().getName() == "Tom");
+		System.out.println(student.getPet().getWeight() == 1.0);
+
+		Dog pet2 = new Dog("Jerry", 1.0);
+		Student student2 = new Student("Walter", pet2);
+		System.out.println(student2.getName() == "Walter");
+		System.out.println(student2.getPet() == pet2);
+		System.out.println(student2.getPet().getName() == "Jerry");
+		System.out.println(student2.getPet().getWeight() == 1.0);
+
+		Animal pet3 = new Animal("Unknown animal", 4.0);
+		Student student3 = new Student("Walter", pet3);
+		System.out.println(student3.getName() == "Walter");
+		System.out.println(student3.getPet() == pet3);
+		System.out.println(student3.getPet().getName() == "Unknown animal");
+		System.out.println(student3.getPet().getWeight() == 4.0);
+	}
+}