Generic For Loop in Java

When we know that we have to iterate over a whole set or list, then we can use Generic For Loop. Java’s Generic has a new loop called for-each loop. It is also called enhanced for loop. This for-each loop makes it easier to iterate over array or generic Collection classes.

In normal for loop, we write three statements :

for( statement1; statement 2; statement3 )
{
     //code to be executed 
}

Statement 1 is executed before the execution of code, Statement 2 states the condition to be satisfied to execute the code and Statement 3 gets executed after the execution of the code block.

But if we look into the Generic For loop or for-each loop,

The generic for loop consists of three parameters :

• Iterator function: It gets called when the next value is needed. It receives both the invariant state and control variable as parameters. Returns nil signals for termination.
• Invariant state: This doesn’t change during the iteration. It is basically the subject of the iteration such as String, table or user data.
• Control variable: It represents the initial value of the iteration.

Syntax 

for( ObjectType variable : iterable/array/collections ){

     // code using name variable
}

Equivalent to

for( int i=0 ; i< list.size() ; i++) {

   ObjectType variable = list.get(i);

   // statements using variable
}

Example: 

Java

// Java program to illustrate Generic for loop

import java.io.*;

class GenericsForLoopExample {

    public static void main(String[] args)
    {
        // create an array
        int arr[] = { 120, 100, 34, 50, 75 };

        // get the sum of array elements
        int s = sum(arr);

        // print the sum
        System.out.println(s);
    }

    // returns the sum of array elements
    public static int sum(int arr[])
    {

        // initialize the sum variable
        int sum = 0;

        // generic for loop where var stores the integer
        // value stored at every index of array
        for (int var : arr) {
            sum += var;
        }

        return sum;
    }
}

Output

379

Example: Showing that 

Java

// Java program to demonstrate that Generic
// for loop can be used in iterating
// Collections like HashMap

import java.io.*;
import java.util.*;

class EnhancedForLoopExample {

    public static void main(String[] args)
    {
        // create a empty hashmap
        HashMap<String, String> map = new HashMap<>();

        // enter name/url pair
        map.put("GFG", "geeksforgeeks.org");
        map.put("Github", "www.github.com");
        map.put("Practice", "practice.geeksforgeeks.org");
        map.put("Quiz", "www.geeksforgeeks.org");

        // using keySet() for storing
         // all the keys in a Set
        Set<String> key = map.keySet();

        // Generic for loop for iterating all over the
        // keySet
        for (String k : key) {
            System.out.println("key :" + k);
        }

        // Generic for loop for iterating all over the
        // values
        for (String url : map.values()) {
            System.out.println("value :" + url);
        }
    }
}

Output

key :Quiz
key :Github
key :Practice
key :GFG
value :www.geeksforgeeks.org
value :www.github.com
value :practice.geeksforgeeks.org
value :geeksforgeeks.org

Limitations of Generic For loop or for-each loop:

1. Not appropriate when we want to modify the list.

for( Integer var : arr)
{
     // only changes the var value and not the value of the data stored inside the arr
     var = var + 100; 
} 

2. We cannot keep track of the index.

for( Integer var : arr){
     
     if(var == target)
     {   
         // don't know the index of var to be compared with variable target         
         return **; 
     }           
} 

4. Iterates a single step in forward direction only.

// This cannot be converted to Generic for loop
for( int i = n-1 ; i >= 0 ; i-- ) {

     // code
                            
}                  

4. Cannot process two decision-making statements at once.

// This loop cannot be converted to Generic for loop
for( int i = 0; i < arr.length; i++ ) {

     if( arr[i]==num )
         return **;
}