Java Enhanced for Loop (for-each): Complete Guide with Examples, Best Practices, and Common Pitfalls

1. Introduction

2. Overview of the Enhanced for Loop (for-each Loop)

• Available in Java 5 and later
• Provides easy access to all elements of arrays and collections
• Shortens code and improves readability
• Helps prevent boundary-related errors and index mistakes

3. Basic Syntax and Usage of the Enhanced for Loop

for (DataType variable : arrayOrCollection) {
    // Processing for each element
}

Example: Enhanced for Loop with Arrays

int[] numbers = {1, 2, 3, 4, 5};

for (int num : numbers) {
    System.out.println(num);
}

Example: Lists and Other Collections

List<String> names = Arrays.asList("田中", "佐藤", "鈴木");

for (String name : names) {
    System.out.println(name);
}

Sample Output

1
2
3
4
5

田中
佐藤
鈴木

4. Differences Compared to the Traditional for Loop

Differences in Syntax

for (int i = 0; i < array.length; i++) {
    System.out.println(array[i]);
}

for (DataType element : arrayOrCollection) {
    System.out.println(element);
}

Comparison Table: Enhanced for Loop vs. Traditional for Loop

Which Should You Use? Key Decision Points

• You want to process all elements of an array or collection
• You want concise, readable code
• You do not need index values or reverse processing

• You need index values (e.g., access specific positions, reverse-order loops, or skip certain elements)
• You need to add or remove elements, or perform more complex operations using iterators

5. Practical Use Cases of the Enhanced for Loop

Looping Through a Map

Map<String, Integer> scores = new HashMap<>();
scores.put("田中", 80);
scores.put("佐藤", 90);
scores.put("鈴木", 75);

for (Map.Entry<String, Integer> entry : scores.entrySet()) {
    System.out.println(entry.getKey() + "：" + entry.getValue());
}

Looping Through a Two-Dimensional Array

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

for (int[] row : matrix) {
    for (int num : row) {
        System.out.print(num + " ");
    }
    System.out.println();
}

Looping Through Arrays or Lists of Objects

class Person {
    String name;
    Person(String name) {
        this.name = name;
    }
}

Person[] people = {
    new Person("田中"),
    new Person("佐藤"),
    new Person("鈴木")
};

for (Person person : people) {
    System.out.println(person.name);
}

Using Enhanced for Loop with Sets and Other Collections

Set<String> fruits = new HashSet<>(Arrays.asList("リンゴ", "バナナ", "オレンジ"));

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

6. Caveats and Cases Where the Enhanced for Loop Should Not Be Used

When You Need an Index

int[] numbers = {1, 2, 3, 4, 5};
for (int i = 0; i < numbers.length; i++) {
    if (i % 2 == 0) {
        System.out.println(numbers[i]);
    }
}

When Adding or Removing Elements

List<String> names = new ArrayList<>(Arrays.asList("田中", "佐藤", "鈴木"));
Iterator<String> iterator = names.iterator();
while (iterator.hasNext()) {
    String name = iterator.next();
    if (name.equals("佐藤")) {
        iterator.remove();
    }
}

Handling null or Empty Arrays/Collections

int[] numbers = null;
if (numbers != null) {
    for (int num : numbers) {
        System.out.println(num);
    }
}

Reverse Order Processing or Conditional Skipping

7. Common Errors and Troubleshooting

NullPointerException

List<String> names = null;
for (String name : names) { // ← NullPointerException
    System.out.println(name);
}

List<String> names = null;
if (names != null) {
    for (String name : names) {
        System.out.println(name);
    }
}

ConcurrentModificationException When Removing Elements

List<String> names = new ArrayList<>(Arrays.asList("田中", "佐藤", "鈴木"));
for (String name : names) {
    if (name.equals("佐藤")) {
        names.remove(name); // ← ConcurrentModificationException
    }
}

Iterator<String> iterator = names.iterator();
while (iterator.hasNext()) {
    String name = iterator.next();
    if (name.equals("佐藤")) {
        iterator.remove(); // Safe removal
    }
}

Changing the Size of Arrays or Collections

Type Mismatch Errors

List<Integer> numbers = Arrays.asList(1, 2, 3);
// for (String num : numbers) { ... } // ← Compile error
for (int num : numbers) { // or Integer num : numbers
    System.out.println(num);
}

8. Summary

9. Frequently Asked Questions (FAQ)

for (Map.Entry<K, V> entry : map.entrySet()) {
    ...
}

10. Reference Links and Related Articles

Official Documentation and Useful External Resources

• Java™ Tutorials (Oracle Official): Enhanced for Statement An official explanation of the enhanced for loop from Oracle’s Java documentation, including syntax and examples.
• Java Platform SE 8 API Specification – java.lang.Iterable The official documentation for the Iterable interface, which can be used with the enhanced for loop.

Recommended Books for Further Learning

• “Sukkiri Wakaru Java Nyumon (3rd Edition)” by Kiyotaka Nakayama / Impress
• “Java Language Programming Lessons” by Hiroshi Yuki / SB Creative