Question

1. What is an abstract method and why are they useful? Illustrate your answer using an example of where you might use an abstract method.

2. What is the difference between a static variable and a non-static variable? Given the example of a class representing Dogs, give an example of a variable that may be static and another that may non-static.

3. In Java, when you modify a String as shown in the code below, Java makes a new String to contain the result, rather than just adding the item onto the end of the original String. Why does it do this?

String s="Hello";

s = s + ", Pravesh";

4. Explain why the following code returns John, despite Java using pass by reference.

Describe what happens in memory, describing each object created, whether the name points directly to the value or to a pointer to the value, what item is sent to the method, etc.

public static void main(String args[]){

String k = "John";

change(k);

System.out.println(k);

}

public static void change(String k){

k = "Mike";

}

5.An evil jailer has presented two prisoners with a problem. He will put a hat on each of their heads, with each hat being either black or white. There is no other restriction on the hat choice, so the hats could be the same colour or different colours. The jailer will free both prisoners if either prisoner correctly guesses his own hat colour. Note that this means the other prisoner can get his hat colour wrong and still be freed. Each prisoner can see the other ones hat colour but not his own. Before the game starts, the prisoners are allowed to discuss a strategy for guessing.

Find a strategy they can agree on that always gets the prisoners freed in each of the following two scenarios:

(a) Theyareallowedtoguessinturnandthesecondprisonercanhearthefirstprisonersguess. [2 marks]

(b) They must guess simultaneously and so have no information about each others guess. [3 marks]

Answer 1

1. What is an abstract method and why are they useful? Illustrate your answer using an example of where you might use an abstract method.

Answer: A method without a body or with no implementation is known as an abstract method. An abstract method must always be declared in an abstract class or interface in JAVA, or in other words, you can say that if a class has an abstract method, it should be declared abstract as well. This is how an abstract method looks in java:

example of an abstract method:

public abstract int sum(int n1, int n2);

Sometimes you will want to create a superclass that only defines a generalized form that will be shared by all of its subclasses, leaving it to each subclass to fill in the details. Such a class determines the nature of the methods that the subclasses must implement.

Below example illustrate where we might use an abstract method.

Suppose we were modeling the behavior of animals, by creating a class hierarchy that started with a base class called Animal. Animals are capable of doing different things like flying, digging and walking, but there are some common operations as well as eating and sleeping and making noise. Some common operations are performed by all animals, but in a different way as well. When an operation is performed in a different way, it is a good candidate for an abstract method (forcing subclasses to provide a custom implementation). Let's look at a very primitive Animal base class, which defines an abstract method makenoise() for making a sound (such as a dog barking, a cow mooing, or a pig oinking).

public abstract Animal {

public void sleep{

// sleeping time

}

public void eat(food)

{

//eat something

}

public abstract void makeNoise();

}

public Dog extends Animal {

public void makeNoise() {

System.out.println("Bark! Bark!");

}

}

public Cow extends Animal {

public void makeNoise() {

System.out.println("Moo! Moo!");

}

}

Q2. What is the difference between a static variable and a non-static variable? Given the example of a class representing Dogs, give an example of a variable that may be static and another that may non-static.

Answer:

Non-static variable Static variable These variables are preceded by static keyword These variable should not be preceded by any static keyword Example Example class A class A int a, static int b; Memory is allocated for these variable whenever an object is created Memory is allocated for these variable at the time 2 of loading of the class. Memory is allocated multiple time whenever a new object is created Memory is allocated for these variable only once in the program. Memory is allocated at the time of loading of variable Static variable are common for every object that Non-static variable also known as instance variable while because memoryclass so that these are also known as class is allocated whenever instance is created. 4 Non-static variable are specific to an object means t here memory location can be sharable by every object reference or same class Non-static variable can access with object reference Static variable can access with class reference Syntax Syntax 6 class name.variable name obj_ref.variable_name

class Dogs
{
String name;
int age;
static String Breed_name="BullDog";
}
class StaticDemo
{
public static void main(String args[])
{
Dogs d1=new Dogs();
d1.age=100;
d1.name="Michal";
System.out.println(d1.age);
System.out.println(d1.name);
System.out.println(Dogs.Breed_name); 
//or System.out.println(d1.Breed_name); but first is use in real time.
Dogs  d2=new  Dogs();
d2.age=200;
d2.name="zyx";
System.out.println(d2.age);
System.out.println(s2.name);
System.out.println(Dogs.Breed_name); 
}
}

Q3. In Java, when you modify a String as shown in the code below, Java makes a new String to contain the result, rather than just adding the item onto the end of the original String. Why does it do this?

String s="Hello";

s = s + ", Pravesh";

Answer: In Java string is an object. if the object already exists in the memory it does not create a new Object rather it assigns the same old object to the new instance, that means even though we have two string instances.