For Loop in Java | Important points

Prerequisite: Loops in Java

The structure of basic for loop:

for(initialization; boolean expression; update statement)
{
    //Body
}

Let’s look at some basic examples of using for loop and the common pitfalls in using for loop

1. Providing expression in for loop is must : For loop must consist a valid expression in the loop statement failing which can lead to an infinite loop. The statement

   for ( ; ; ) 
   is similar to
   while(true)

   // Java program to illustrate // infinite loop public class Example1  {     public static void main(String[] args)      {         for( ; ; )         {             System.out.println("This is an infinite loop");         }     }    }

   Output: This code prints the statement “This is an infinite loop” repeatedly.

2. Initializing multiple variables : In Java, multiple variables can be initialized in initialization block of for loop regardless of whether you use it in the loop or not.

   // Java program to illustrate // Initializing multiple variables  // in initialization block public class Example2  {        public static void main(String[] args)      {         int x = 2;         for(long y = 0, z = 4; x < 10 && y < 10; x++, y++)          {             System.out.println(y + " ");         }                System.out.println(x);     } }

   In the above code, there is simple variation in the for loop. Two variables are declared and initialized in the initialization block. The variable ‘z’ is not being used. Also, the other two components contain extra variable. So, it can be seen that the blocks may include extra variables which may not be referenced by each other.

3. Redeclaration of a variable in initialization block : Suppose, an initialization vaiable is already declared as integer. Can we re-declare it in for loop with other data type? No, See the example:

   // Java program to illustrate // redeclaring a variable // in initialization block public class Example3  {     public static void main(String[] args)      {             // x is integer         int x = 0;                    // redeclaring x as long will not work         for(long y = 0, x = 1; x < 5; x++)          {              System.out.print(x + " ");         }                } }

   Example3.java:12: error: variable x is already defined in method main(String[])
           for(long y = 0, x = 1; x < 5; x++)

   Here, x was already initialized to zero as integer and is being re-declared in the loop with data type long.

   But this problem can be fixed by slightly modifying the code. Here, the variables x and y are declared in a
   different way.

   
   
   

   // Java program to illustrate // redeclaring a variable // in initialization block public class Example3  {     public static void main(String[] args)      {             // x is integer         int x = 0;         long y = 10;                    for (y = 0, x = 1; x < 5; x++)          {              System.out.print(x + " ");         }                } }

   Output:

   1 2 3 4

4. Variables declared in the initialization block must be of same type : It is just a common sense that when we declare a variable as

    int x, y;

   both variables are of same type. Its just the same in for loop initialization block too.

   // Java program to illustrate // declaring a variable // in initialization block public class Example4  {     public static void main(String[] args)      {             // This will cause error;          // int x;                    // redeclaring x as long will not work         for (long y = 0, x = 1; x < 5; x++)          {              System.out.print(x + " ");         }                } }

5. Variables in the loop are accessible only within: The variables that are declared in the initialization block can be accessed only within the loop. For more on scope of variables, Refer here

   // Java program to illustrate // scope of Initializing variables  // within the loop public class Example5  {     public static void main(String[] args)      {         // x and y scope is only          // within for loop         for(int x = 0, y = 0; x < 3 && y < 3; x++, y++)          {             System.out.println(y + " ");         }                System.out.println(x);     } }

   Error

   Example5.java:13: error: cannot find symbol
           System.out.println(x);

   Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.