4.1.1.1. How to iterate through Java List? Five (5) ways to Iterate Through Loop in Java

There are 5 ways you can iterate through List.

1. For Loop
2. Advanced For Loop
3. Iterator
4. While Loop
5. Collections’s stream() util (Java8)

Java Example:

You need JDK 8 to run below program as point-5 above uses stream() util.
void java.util.stream.Stream.forEach(Consumer<? super String> action) performs an action for each element of this stream.

4.1.1.1.1. Code

package crunchify.com.tutorial;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
 * @author Crunchify.com
 */
public class CrunchifyIterateThroughList {

    public static void main(String[] argv) {
        // create list
        List<String> crunchifyList = new ArrayList<String>();

        // add 4 different values to list
        crunchifyList.add("eBay");
        crunchifyList.add("Paypal");
        crunchifyList.add("Google");
        crunchifyList.add("Yahoo");

        // iterate via "for loop"
        System.out.println("==> For Loop Example.");
        for (int i = 0; i < crunchifyList.size(); i++) {
            System.out.println(crunchifyList.get(i));
        }

        // iterate via "New way to loop"
        System.out.println("\n==> Advance For Loop Example..");
        for (String temp : crunchifyList) {
            System.out.println(temp);
        }

        // iterate via "iterator loop"
        System.out.println("\n==> Iterator Example...");
        Iterator<String> crunchifyIterator = crunchifyList.iterator();
        while (crunchifyIterator.hasNext()) {
            System.out.println(crunchifyIterator.next());
        }

        // iterate via "while loop"
        System.out.println("\n==> While Loop Example....");
        int i = 0;
        while (i < crunchifyList.size()) {
            System.out.println(crunchifyList.get(i));
            i++;
        }

        // collection stream() util: Returns a sequential Stream with this collection as its source
        System.out.println("\n==> collection stream() util....");
        crunchifyList.forEach((temp) -> { System.out.println(temp);});
    }
}

4.1.1.1.2. Output

==> For Loop Example.
eBay
Paypal
Google
Yahoo

==> Advance For Loop Example..
eBay
Paypal
Google
Yahoo

==> Iterator Example...
eBay
Paypal
Google
Yahoo

==> While Loop Example....
eBay
Paypal
Google
Yahoo

==> collection stream() util....
eBay
Paypal
Google
Yahoo

4.1.1.2. Iterate through a HashMap [duplicate]

Iterate through the entrySet like so:

public static void printMap(Map mp) {
    Iterator it = mp.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry pair = (Map.Entry)it.next();
        System.out.println(pair.getKey() + " = " + pair.getValue());
        it.remove(); // avoids a ConcurrentModificationException
    }
}

Read more on Map

Map<String, Object> map = ...;
for (String key : map.keySet()) {
    // ...
}

If you only need the values, use values():

for (Object value : map.values()) {
    // ...
}

Finally, if you want both the key and value, use entrySet():

for (Map.Entry<String, Object> entry : map.entrySet()) {
    String key = entry.getKey();
    Object value = entry.getValue();
    // ...
}

One caveat: if you want to remove items mid-iteration, you'll need to do so via an Iterator (see karim79's answer). However, changing item values is OK (see Map.Entry).

4.1.1.3. How to convert HashMap to ArrayList in Java?

package com.crunchify;

/**
 * @author Crunchify.com
 */

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;    
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class CrunchifyHashmapToArrayList {

    public static void main(String... args) {
        HashMap<String, Integer> companyDetails = new HashMap<String, Integer>();

        // create hashmap with keys and values (CompanyName, #Employees)
        companyDetails.put("eBay", 4444);
        companyDetails.put("Paypal", 5555);
        companyDetails.put("IBM", 6666);
        companyDetails.put("Google", 7777);
        companyDetails.put("Yahoo", 8888);

        System.out.println("==> Size of companyDetails Map: " + companyDetails.size());
        Iterator it = companyDetails.entrySet().iterator();
        while (it.hasNext()) {
            Map.Entry pairs = (Map.Entry) it.next();
            System.out.println(pairs.getKey() + " = " + pairs.getValue());
        }

        // Converting HashMap keys into ArrayList
        List<String> keyList = new ArrayList<String>(companyDetails.keySet());
        System.out.println("\n==> Size of Key list: " + keyList.size());

        for (String temp : keyList) {
            System.out.println(temp);
        }

        // Converting HashMap Values into ArrayList
        List<Integer> valueList = new ArrayList<Integer>(companyDetails.values());
        System.out.println("\n==> Size of Value list: " + valueList.size());
        for (Integer temp : valueList) {
            System.out.println(temp);
        }

        List<Entry> entryList = new ArrayList<Entry>(companyDetails.entrySet());
        System.out.println("\n==> Size of Entry list: " + entryList.size());
        for (Entry temp : entryList) {
            System.out.println(temp);
        }
    }
}