Java For loop

Understanding for loop in Java

Key Take aways :

  • For loop consist of three parts initialization, condition check and expression evaluation section.
  • During the first run of for loop initialization and condition check happens and then for loop block gets executed.
  • After the first run for all subsequent run the expression Evaluation(increment, decrement ) gets evaluated and then condition check happens and then code block gets executed.
  • The condition check only accepts boolean Type as argument. No other type is acceptable (No int , Integer, long value).
  • The condition check looks for boolean Type of a expression which results into Boolean.

 

Lets go through examples.

 

  • For Loop with int i declared in initialization section and visible only in for block

 

 public void loop1() {

        // For loop consists of three part
        // Initialization , conditional check ,  expression evaluation (increment decrement)

        for (int i = 0; i <= 3; i++) {
            System.out.println("value of i inside LOOP1 method is : " + i);
        }

        // System.out.println( i);  error.  i can be accessed only within the for loop

    }

value of i inside LOOP1 method is : 1
value of i inside LOOP1 method is : 2
value of i inside LOOP1 method is : 3


[addToAppearHere]
  • For Loop with int i declared outside of initialization section and visible only in for block
 public void loop2() {

        // For loop consists of three part
        // Initialization , conditional check (boolean) ,  expression evaluation (increment decrement)

        int i = 0;

        for (; i <= 3; i++) {
            System.out.println("value of i inside LOOP2 method is : " + i);
        }

        System.out.println("value of i in Looop2 outside for loop  is " + i);

    }


value of i inside LOOP2 method is : 0
value of i inside LOOP2 method is : 1
value of i inside LOOP2 method is : 2
value of i inside LOOP2 method is : 3
value of i in Looop2 outside for loop is 4
  • Infinite Loop in For loop
  public void loop3() {

        for (; true; ) {
            // infinite for loop in Java
        }

        // for(;1;)  1 is not taken as true.  For accepts only boolean condition
    }
  • condition check parameter supplied from a method 
 private int getConditionSize() {
        System.out.println("getConditionSize is being called");
        return 3;
    }


 public void loop4() {

        for (int i = 0; i <= getConditionSize(); i++) {
            System.out.println("value of i inside LOOP4 method is : " + i);
        }

    }

getConditionSize is being called
value of i inside LOOP4 method is : 0
getConditionSize is being called
value of i inside LOOP4 method is : 1
getConditionSize is being called
value of i inside LOOP4 method is : 2
getConditionSize is being called
value of i inside LOOP4 method is : 3
getConditionSize is being called

[addToAppearHere]

  • Iterating over a iterator using for loop
 public void loop5() {

        List names = Arrays.asList("Maverick", "Rock");

        for (String name : names) {
            System.out.println("In LOOP5 name is " + name);
        }

        for (int i = 0; i < names.size(); i++) {
            System.out.println("In LOOP5 iterating through traditional way is " + names.get(i));
        }

    }


In LOOP5 name is Maverick
In LOOP5 name is Rock
In LOOP5 iterating through traditional way is Maverick
In LOOP5 iterating through traditional way is Rock
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