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; }} |
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); } }} |
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 **;
}
