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    Java Variable Declaration and Initialization: Best Practices

    Rumman Ansari September 28, 2024 48,262 views
    Java Variable Declaration and Initialization: Best Practices

    Variable Declaration and initialization

    Variables are used to represent values that may be changed in the program. In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here: 

    type identifier [ = value][, identifier [= value] ...] ;
    • The type is one of Java’s primitive types or the name of a class or interface.
    • The identifier is the name of the variable. You can initialize the variable by specifying an equal sign and a value. Keep in mind
    • You can initialize the variable by specifying an equal sign and a value. Keep in mind that the initialization expression must result in a value of the same (or compatible) type as that specified for the variable.
    • To declare more than one variable of the specified type, use a comma-separated list.

    int Variable Declaration and Variable Initialization in two steps:

    Save Source File Name as : IntExample1.java
    To compile : javac IntExample1.java
    To Run : java IntExample1

    Example :

    // A Java program to demonstrate int data type
class IntExample1 {
   public static void main(String args[])
     {
           int age;  // Declaration of variable age
           age = 20;  // initialization of the variable age
           System.out.println(age);
     }
  }
    Output : 
    20
 press any key to continue ...

    Types of variable with respect to data type:

    Variable Delcaration:

    Here are examples of how to declare variables of all the primitive data types in Java: 

    data_type variable_name ;
    byte    myByte;
short   myShort;
char    myChar;
int     myInt;
long    myLong;
float   myFloat;
double  myDouble;

    Here myByte, myShort, myChar, myInt, myLong, myFloat, myDouble all are variable name which have data type byte, short, char, int, long, float, double respectively


    Here are examples of how to declare variables of the object types in Java: 

    Byte       myObjectByte;
Short      myObjectShort;
Character  myObjectChar;
Integer    myObjectInt;
Long       myObjectLong;
Float      myObjectFloat;
Double     myObjectDouble;
String     myObjectString;

    Notice the uppercase first letter of the object types.

    Here myObjectByte, myObjectShort, myObjectChar, myObjectInt, myObjectLong, myObjectFloat, myObjectDouble all are variable name which have data type Byte, Short, Char, Int, Long, Float, Double respectively

    Variable initialization:

    Here are examples of how to variables are initialize to all the primitive data types in Java: 

    myByte = 21 ;
myShort = 22;
myChar = 'A';
myInt = 232 ;
myLong = 2121314312321;
myFloat = 23.54f ;
myDouble = 12441124124.4343;

    Here are examples of how to initialize variables of the object types in Java: 

    myObjectByte = 21 ;
myObjectShort = 22;
myObjectChar = 'A';
myObjectInt = 232 ;
myObjectLong = 2121314312321;
myObjectFloat = 23.54f ;
myObjectDouble = 12441124124.4343;
    Practice Quiz 25 MCQs Smart Learning

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    MCQ Section

    MCQ 1
    Choose all the lines which if inserted independently instead of "//insert code here" will allow the following code to compile: 
    public class Test{   
        public static void main(String args[]){
	        add(); 
		add(1);
		add(1, 2); 
	}

	// insert code here
}
    MCQ 2
    What is the result of compiling and running the following code? 
    class Base{
        private Base(){
                System.out.print("Base");
        }
}
public class test extends Base{
        public test(){
                System.out.print("Derived");
        }
        public static void main(String[] args){
                new test();
        }
}
    MCQ 3
    What is the result of compiling and running the following code? 
    public class Tester{
	static int x = 4;
	public Tester(){
		System.out.print(this.x); // line 1
		Tester();
	}
	public static void Tester(){ // line 2
		System.out.print(this.x); // line 3
	}
	public static void main(String... args){ // line 4
		new Tester();
	}
}
    MCQ 4
    What is the result of compiling and running the following code? 
    public class Tester{
	static int x = 4;
	int y = 9;  	
	public Tester(){
		System.out.print(this.x); // line 1
		printVariables();
	}
	public static void printVariables(){
		System.out.print(x); // line 2
		System.out.print(y); // line 3
	}
	public static void main(String... args) { // line 4
		new Tester();
	}
}
    MCQ 5
    Suppose a class has public visibility. In this class we define a protected method. Which of the following statements is correct?
    MCQ 6
    The object is created with new keyword
    MCQ 7
    Consider the following two classes declared and defined in two different packages, what can be added in class B to form what considered a correct access to class A from main() method of class B? 
    package subPackage;
public class A { }


package anotherPackage;
// line 1
	public class B{
		public static void main(String[] args){
		// line 2
	}
}
    1. At line1 add noting; At line2 add: new A();
    2. At line 1 add: import package.*; at line
    MCQ 8
    Determine output: 
    public class InitDemo{
	static int i = demo();
	static{
                System.out.print(i);
        }
	InitDemo(){
		System.out.print("hello1");
	}
	public static void main(String... args){
                System.out.print("Hello2");
	}
 	static int demo(){
        	System.out.print("InsideDemo");
		return 10;
	}
}
    MCQ 9
    Which statements are most accurate regarding the following classes? 
    class A{
        private int i;
        protected int j;
}

class B extends A{
        private int k;
        protected int m;
}
    MCQ 10
    A package is a collection of
    MCQ 11
    A method within a class is only accessible by classes that are defined within the same package as the class of the method. Which one of the following is used to enforce such restriction?
    MCQ 12
    Choose the correct statement 
    public class Circle{
      private double radius;  
      public Circle(double radius){
            radius = radius;
      }
}
    MCQ 13
    Choose the correct statement. Restriction on static methods are: I. They can only call other static methods.

    I.   They can only call other static methods.
    II.   They must only access static data.
    III. They cannot refer this or super in any way.
    MCQ 14
    You have the following code in a file called Test.java 
    class Base{
      public static void main(String[] args){
            System.out.println("Hello");
      }
}
public class Test extends Base{}
    What will happen if you try to compile and run this?
    MCQ 15
    What will be the output? 
    public class Test{
	static{
		int a = 5;
	}

	public static void main(String args[]){
		new Test().call();
	}

	void call(){
		this.a++;
		System.out.print(this.a);
	}
}
    MCQ 16
    What will be the output for the below code? 
    public class Test{
      static{
            int a = 5; 
      }

      public static void main(String[] args){
            System.out.println(a);
      }
}
    MCQ 17
    Name the keyword that makes a variable belong to a class, rather than being defined for each instance of the class.
    MCQ 18
    What will be the output? 
    public class Test{
      public static void main(String[] args){
            String value = "abc";
            changeValue(value);
            System.out.println(value);
      }

      public static void changeValue(String a){
            a = "xyz";
      }
}
    MCQ 19
    What is the output for the below code ? 
    public class A{
      static{
            System.out.println("static");
      }

      {
            System.out.println("block");
      }

      public A(){
            System.out.println("A");
      }

      public static void main(String[] args){
            A a = new A();
      }
}
    MCQ 20
    Determine Output: 
    class MyClass{
      static final int a = 20;

      static final void call(){
            System.out.println("two");
      }
	
      static{
            System.out.println("one");
      }
}

public class Test{
      public static void main(String args[]){
            System.out.println(MyClass.a);
      }
}
    MCQ 21
    Determine output: 
    class A{
	{
		System.out.print("b1 ");
	}
	public A(){
		System.out.print("b2 ");
	}
}

class B extends A{
	static{
		System.out.print("r1 ");
	}
	public B(){
		System.out.print("r2 ");
	}
	{
		System.out.print("r3 ");
	}
	static{
		System.out.print("r4 ");
	}
}

public class Test extends B{
	public static void main(String[] args){
		System.out.print("pre ");
		new Test();
		System.out.println("post ");
	}
}
    MCQ 22
    What will be the output for the below code? 
    static public class Test{
      public static void main(String[] args){
            char c = 'a';

            switch(c){
                  case 65 : System.out.println("one");break;
		  case 'a': System.out.println("two");break;
		  case 3  : System.out.println("three");
            }	
      }
}
    MCQ 23
    What will be the output after compiling and running following program code? 
    public class Test{
      static int a;
      public static void main(String[] args){
            System.out.println("one");
            call(1);
      }

      static void call(int a){
            this.a=10;
            System.out.println("two "+a);
      }
}
    MCQ 24
    What will be the output after the following program is compiled and executed? 
    public class Test{
      public static void main(String args[]){
            int x = 10;
	    x = myMethod(x--);
	    System.out.print(x);
      }

      static int myMethod(final int x){
            return x--;
      }
}
    MCQ 25
    What can directly access and change the value of the variable qusNo? 
    package com.mypackage;

public class Test{
    private int qusNo = 100;
}