Object-Oriented Java

Question 1

Java objects’ state and behavior

Answer 1

In Java, instances of a class are known as objects. Every object and behavior in the form of the instance fields and methods respectively.

EX.

public class Person {
// state of an object
int age;
String name;

// behavior of an object
public void set_value() {
    age = 20;
    name = “Robin”;
}
public void get_value() {
     System.out.println(“Age is “ + age);
     System.out.println(“Name is “ + name);    }

//main method
public static void main(String [] args) {
// creates a new Person object
Person p = new Person();

    // changes state through behavior
    p.set_value();
 } }

Question 2

Java instance

Answer 2

Java instances are objects that are based on classes. For example, Bob may be an instance of the class Person.

Every instance has access to its own set of variables which are known as instance shields, which are variables declared within the scope of the instances. Values for instance fields are assigned within the constructor method.

EX.

public class Person {
int age;
String name;

// Constructor method
public Person(int age, String name) {
     this.age = age;
     this.name = name;
}

public static void main(String[] args) {
Person Bob = new Person(31, “Bob”);
Person Alice = new Person(27, “Alice”);
}
}

Question 3

Java dot notation

Answer 3

In Java programming language, we use . to access variables and methods of an object or a Class.

This is known as dot notation and the structure looks like this-

instanceOrClassName.fieldOrMethodName

EX.

public class Person {
int age;

public static void main(String [] args){
Persons p = new Person();

   //here we use dot notation to set age
   p.age = 20;

   // here we use dot notation to access age and print
  System.out.println(“Age is “ + p.age);
  // Output: Age is 20
 } }

Question 4

Constructor Method in Java

Answer 4

Java classes contain a constructor method which is used to create instances of the class.

The constructor is named after the class. If no constructor is defined, a default empty constructor is used.

EX.

public class Maths {
public Maths() {
System.out.println(“I am constructor”);
}
public static void main(String [] args) {
System.out.println(“I am main”);
Maths obj1 = new Maths();
}
}

Question 5

Creating a new Class instance in Java

Answer 5

In Java, we use the new keyword followed by a call to the class constructor in order to create a new instance of a class.

The constructor can be used to provide initial values to instance fields.

Ex.

Public class Person {
int age;
// Constructor:
public Person(int a) {
age = a;
}

public static void main(String [] args) {
//Here, we create a new instance of the Person class:
Person p = new Person(20);
System.out.println(“Age is “ + p.age); //Prints: Age is 20
}
}

Question 6

Reference Data Types

Answer 6

A variable with a reference data type has a value that references the memory address of an instance. During variable declaration, the class name is used as the variable’s type

EX.

public class Cat {
public Cat() {
//instructions for creating a Cat instance
}

public static void main(String[] args) {
    //garfield is declared with reference data type
    Cat garfield = new Car();
    System.out.println(garfield); //Prints: Cat@76ed5528
  } }

Question 7

Constructor Signatures

Answer 7

A class can contain multiple constructors as long as they have different parameter values. A signature helps the compiler differentiate between the different constructors.

A signature is made up of the constructor’s name and a list of its parameters.

EX.

//The signature is Cat(String furLength, boolean hasClaws).
public class Cat {
String furType;
boolean containsClaws;

public Cat(String furLength, boolean hasClaws){
    furtype = furLength;
    containsClaws = hasClaws; } public static void main(String[] args) {
Cat garfield = new Cat(“Long-hair”, true);   } }

Question 8

null Values

Answer 8

null is a special value that denotes that an object has a void reference.

EX.

public class Bear {
String species;
public Bear(String speciesOfBear;) {
species = species ofBear;
}

public static void main(String[] args){
Bear ball = new Bear(“Sloth bear”);
System.out.println(ball); //prints: Bear@4517d9a3
// set object to null
ball = null;
System.out.println(ball); //Prints: null
}
}

Question 9

The body of a Java method

Answer 9

In Java, we use curly brackets {} to enclose the boys of a method.

The statements written inside the {} are executed when a method is called.

EX.

public class Maths {
public static void sum(int a, int b) { // Start of sum
int result = a + b;
System.out.println(“Sum is “ + result);
} // End of sum

public static void main(String [] args) {
// Here, we call the sum, method
sum(10, 20);
// Output: Sum is 30
}
}

Question 10

Method parameters in Java

Answer 10

in Java, parameters are declared in a method definition. The parameters act as variables inside the method and hold the value that was passed in. They can be used inside a method for printing or calculation purposes.

In the example, a and b are two parameters which, when the method is called, hold the value 10 and 20 respectively.

EX.

public class Maths {
public int sum(int a, int b) {
int k = a + b;
return k;
}

public static void main(String [] args) {
Maths m = new Maths();
int result = m.sum(10, 20);
System.out.println(“Sum is “ + result);
//prints - sum is 30
}
}

Question 11

Java Variables Inside a Method

Answer 11

Java variables defined inside a method cannot be used outside the scope of that method

EX.

//For example, ‘i’ and ‘j’ variables are available in the ‘main’ method only:

public class Maths {
public static void main(String [] args) {
int i, j;
System.out.println(“These two variables are available in the main method only”);
}
}

Question 12

Returning info from a Java method

Answer 12

A Java method can return any value that can be saved in a variable. The value returned must match with the return type specified in the method signature.

The value is returned using the return keyword.

EX.

public class Maths {

//return type is int
public int sum(int a, int b) {
    int k;
    k = a + b;

    //sum is returned using the return keyword
    return k;
  }

public static void main(String [] args) {
Maths m = new Maths();
int result;
result = m.sum(10, 20);
System.out.println(“Sum is “ + result);
// Output: Sum is 30
}
}

Question 13

Declaring a Method

Answer 13

Method declarations should define the following method information: scope (private or public), return type, method name, and any parameters it receives.

EX.

//Here is a public method named sum whose return type is int and has two int parameters a and b
public int sum(int a, int b) {
return(a + b);
}