Introduction to Iterators

Input/Output and Streams in Java

Alex Liu

Software Development Engineer

Working with collections

Example HashSet for the rest of the video

import java.util.HashSet;

public class SampleHashSetData {
    public static void main(String[] args) {
        HashSet<String> fruits = new HashSet<>();

        fruits.add("Apple");
        fruits.add("Banana");

    }
}

{Apple, Banana}

Use For-Each loop

For-Each loop
- Format: for (type x: collection){}
- Automatically processes each element
- Works for Multiple Collections: List, Set, Map and more
Example: use sample set fruits: [Apple, Banana].

for (String x : fruits) {
    System.out.println(x);
}

Apple
Banana

Traversing a collection with Iterator

Use Iterator<>: Supports List, Set, Queue and more
Import Iterator class

import java.util.Iterator;


// Create `Iterator` object named `it` for sample set `fruits`
Iterator<String> it = fruits.iterator();
// Use `.hasNext()` method checks if more elements exist
while (it.hasNext()) {
    // User `.next()` retrieves the element
    System.out.print(it.next());}

Apple Banana

Removing elements with Iterator

Use .remove() to remove element with Iterator

Iterator<String> it = fruits.iterator();
while (it.hasNext()) {
    String fruit = it.next();
    if (fruit.startsWith("A")) {
        // Remove element if element startsWith `A`
        it.remove();
        }
}
// fruits now contains only "Banana"

Removing elements with Iterator(Continued)

Before Removal:

System.out.println(fruits);

[Apple, Banana]

After Removal:
```
System.out.println(fruits);
```
```
[Banana]
```
Ensures safe deletion and prevent ConcurrentModificationException

Icon representing safe deletion

Summary

For-Each loop
- Support
  - List(ArrayList,LinkedList)
  - Set(HashSet,TreeSet,LinkedHashSet)
  - Queue(PriorityQueue)
  - Stack(Stack)
- Not Support
  - Map
  - Element Removal

Iterator
- Support
  - List(ArrayList,LinkedList)
  - Set(HashSet,TreeSet,LinkedHashSet)
  - Queue(PriorityQueue)
  - Stack(Stack)
  - Map (Must use keySet().iterator, values().iterator or entrySet().iterator)
  - Element Removal

Let's practice!

Input/Output and Streams in Java