Can You Remove Items Inside a Java foreach Loop? Why It Fails and How to Fix It

Iterating Elements

First, example code for iterating an array and an ArrayList using enhanced for loops.

String[] array = {"1","2","3"};
for (String i : array) {
    System.out.println(i);
}

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
for (String i : list) {
    System.out.println(i);
}

The output is:

1
2
3
111
222
333

Decompiled bytecode shows that array iteration uses a traditional for loop, while collection iteration uses an Iterator.

Removing Elements

Using a classic for loop

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (int i = 0; i < list.size(); i++) {
    list.remove("222");
}
log.info(list.toString());

Result: the element is removed successfully.

Using foreach

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (String i : list) {
    list.remove("222");
}
log.info(list.toString());

This throws java.util.ConcurrentModificationException because the iterator's expected modCount does not match the list's modCount after removal.

Reason

The iterator records the collection's modCount at the start of iteration; calling list.remove increments the list's modCount, causing a mismatch and the exception.

Correct removal with iterator

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
Iterator<String> it = list.iterator();
while (it.hasNext()) {
    String next = it.next();
    if (next.equals("222")) {
        it.remove();
    }
}
log.info(list.toString());

Modifying Elements

Using classic for

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (int i = 0; i < list.size(); i++) {
    list.set(i, "444");
}
log.info(list.toString());

All elements become "444".

Using foreach

ArrayList<String> list = new ArrayList<>();
list.add("111");
list.add("222");
list.add("333");
log.info(list.toString());
for (String i : list) {
    i = "444";
}
log.info(list.toString());

The list remains unchanged because reassigning the loop variable does not affect the collection.

Modifying Object Properties in foreach

Define a Student class with age and name fields and corresponding getters/setters.

public class Student {
    private int age;
    private String name;
    public int getAge() { return age; }
    public void setAge(int age) { this.age = age; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public Student() {}
    public Student(int age, String name) {
        this.age = age;
        this.name = name;
    }
}

Student s1 = new Student(1, "huge");
Student s2 = new Student(1, "xiaoyao");
List<Student> studentList = new ArrayList<>();
studentList.add(s1);
studentList.add(s2);
for (Student stu : studentList) {
    stu.setName("jingtian");
}
System.out.println(s1.getName());
System.out.println(s2.getName());

The output shows both names changed to "jingtian", demonstrating that while you cannot replace objects in a foreach loop, you can modify their internal state.

Summary

Both for and foreach can iterate arrays and collections; for may be more efficient in complex loops.

foreach cannot delete or replace collection elements directly; classic for can.

Both loops can modify the properties of objects contained in the collection.

Therefore, the classic for loop offers greater flexibility.