Java Variables

Tutorial Outline

•     Java Variables
•          Naming Conventions: Java Variables
•          Types of Java Variables
•          Type casting in Java
•     Variables Vs Default Values

Java Variables

• Variable is the name of the reserved area allocated in the memory.
• In other words, it is the name of memory location.
• Variable’ is a combination of ‘very’ + ‘able’, that means its values can be changed.

• Variable is like a container, which holds the values, while the Java program is executed.
• Variable is assigned with a data type.
• Variable can take different values, but one value at a time.

• Values can be changed during the execution of the program.
• For different types of values, we have different types of variables.
• Variables names can be anything, but it can be logical.

For every variable, we need three things:

                          i.            Type of variable

                        ii.            Name of the variable

         iii.            Data/Information

          

         Example:           int num = 10;

 Note: Where ‘num’ is a variable of the Integer data type.

         

         

         Naming Conventions: Java Variables

  ü It should start with a lowercase letter, such as: id, name, num, etc.

  ü It should not start with the special characters, like & (ampersand), $ (dollar), _ (underscore), etc.

  ü If the name contains multiple words, start it with the lowercase letter, followed by an uppercase letter, such as: firstName, lastName, etc.

  ü Avoid using one-character variables, like a, b, c, x, y, z, etc.

  Example: int a = 5; // valid; not recommanded

            int num = 5; // valid

            int num; // valid

            int 123abc; // not valid

            int abc123; // valid

            int abc$; // valid

            int ch; // valid

            int $_num; // not valid

       ·       int num = 8_00_00_00 // valid (ver 1.7)

       ·       double abc = 5; // implicit conversion

  Example:

  public class Test1{

    public static void main(String[] args){

      int num = 5;

      System.out.println(num);

    }

  }

  ð Save this file as ‘Test1.java’

  ð To Compile: $javac Test1.java

  ð To Execute: $java Test1

  ð Output: 5

  Example:

  public class Test2{

    public static void main(String[] args){

      int num = 5;

      // System.out.printf(“%d”, num);

      System.out.println(num);

      num = 8; // changing the value of variable ‘num’.

      System.out.println(num);

    }

  }

  ð Save this file as ‘Test2.java’

  ð To Compile: $javac Test2.java

  ð To Execute; $java Test2

  ð Output:

 5

 8

Types of Java Variables

•        Local variables
•        Instance variables
•      Static variables

Local variables

The variables which are declared inside the method, those variables are called local variables.

Example:

public class LocalVarExample{

    public static void main(String[] args){

        int num = 10; // local variable

        System.out.println(num);

  }

}

ð Save this file as ‘LocalVarExample.java’

ð To Compile: $javac LocalVarExample.java

ð To run/execute: $ java LocalVarExample

ð Output: 10

Instance variables

    ·       The variables which are declared inside the class, but outside of the method, those variables are called instance variables.

    ·       We can access instance variables in the method by creating an object.

Example:

public class InstanceVarExample{

    int num1 = 100;

    int num2 = 200; // instance variables

    public static void main(String[] args){

        // creating new object

  InstanceVarExample obj = new InstanceVarExample();

       // accessing instance variables

       System.ou.println(obj.num1);

       System.out.println(obj.num2);

    }

  }

  ð Save this file as ‘InstanceVarExample.java’

  ð To Compile: $javac InstanceVarExample.java

  ð To execute: $java InstanceVarExample

  ð Output:

  100

  200

Static variables

Static variables are declared inside the class and outside of the method with a ‘static’ modifier.

Methods to access static variables

   1.   Using class name // recommanded

Example: System.out.println(Test.varName);

   2.   Direct access // not recommanded

Example: System.out.println(varName);

    3.   By creating object // not recommanded

Example:

Test obj = new Test();

System.out.println(obj.varName);

Example:

public class StaticVarExample{

    static int num1 = 100;

    static int num2 = 200; // static variables

    public static void main(String[] args){

      // direct access

      System.out.println(num1); // not recommanded

      System.out.println(num2);

      // creating new object

      StaticVarExample obj = new StaticVarExample();

      System.out.println(obj.num1); // not recommanded

      System.out.println(obj.num2);

      // access using class name

      System.out.println(StaticVarExample.num1);

      // recommanded

      System.out.println(StaticVarExample.num2);

    }

  }

  ð Save this file as ‘StaticVarExample.java’

  ð To Compile: $javac StaticVarExample.java

  ð To Execute: $java StaticVarExample

  ð Output:

  100

  200

  100

  200

  100

  200

Type casting in Java

  double num = 5;

  int num1 = (int)num;

  System.out.println(num1);

Example:

  public class Test3{

    public static void main(String[] args){

      int num1 = 10; // local variable

      int num2 = 20;

      System.out.println(num1);

      System.out.println(num2);

      num2 = 30; // changing the value of variable

      System.out.println(num2);

      double num3 = 15.06;

      System.out.println(num3);

      // type casting

      int num4 = (int)num3;

      System.out.println(num4);

    }

  }

  ð Save this file as ‘Test3.java’

  ð To Compile: $javac Test3.java

  ð To Execute: $java Test3

  ð Output:

  10

  20

  30

  15.06

  15

Variables Vs Default Values

èDefault value of class à null

S. No.     Type      JVM        Default Value

1.         Premitive   byte             0
                      short             0
                      int               0
                      long              0

2.      Floating-point  float             0.0
                      double           0.0

3.      Character     char           single space

4.      Boolean    boolean (true/false)    false

Note:

    ·       Instance variable à JVM will assign the default value.

     ·       Static variable à JVM will assign the default value.

    ·       Local variable à JVM will not assign the default value, we must initialized local variables before using.

Example:

// instance variables; default value

public class Test4{

  int num;

  boolean b; // instance variables

  public static void main(String[] args){

    // creating new object

    Test4 t = new Test4();

    // accessing the instance variables

    System.out.println(t.num); // 0

    System.out.println(t.b); // false

  }

}

  

  ð Save this file as ‘Test4.java’

  ð To Compile: $javac Test4.java

  ð To Execute: $java Test4

  ð Output:

  0

  false

Example:

// static variable; default value

public class Test5{

  static int a;

  static double b;

  static boolean c; // static variables

  public static void main(String[] args){

    // accessing static variables using class name

    System.out.println(Test5.a); // 0

    System.out.println(Test5.b); // 0.0

    System.out.println(Test5.c); // false

  }

}

  ð Save this file as ‘Test5.java’

  ð To Compile: $javac Test5.java

  ð To Execute: $java Test5

  ð Output:

  0

  0.0

  false

Example:

// local variable; default value

public class Test6{

  public static void main(String[] args){

    int a;

    double b;

    boolean c; // local variables

    System.out.println(a);

    System.out.println(b);

    System.out.println(c);

  }

}

  ð Save this file as ‘Test6.java’

  ð To compile: $javac Test6.java

  ð To Execute: $java Test6

  ð Output: Error!! message appears

   

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