Enhanced for loops are suitable for scenarios where no indexing and read-only operations are required. 1. Access elements one by one when iterating through an array or collection; 2. Check whether the object meets the conditions; 3. Accumulate the sum of numerical values; its syntax is for (type variable: array or collection), which can be applied to data structures such as array, ArrayList, HashSet and HashMap; but the content of the element cannot be modified, the index cannot be obtained, and it is not suitable for multi-dimensional array operations. Common errors include trying to delete elements or modify array values. At this time, traditional for loops should be used.
Enhanced for loops (also called for-each loops) are a way in Java to simplify traversing arrays or collections. It is simpler than traditional for loops and is suitable for scenarios where no indexing is required and read-only operation is required.
When to use enhanced for loops?
When you only need to access elements one by one , without caring about index positions or modifying the elements themselves, you can consider using an enhanced for loop. for example:
- Iterate through an array of strings to output everything
- Check whether each object in the collection meets a certain condition
- Accumulate the sum of values ??in the array
This writing method can reduce the chance of errors and the code looks refreshing.
Basic syntax structure
The writing of an enhanced for loop is very simple, and the format is as follows:
for (element type element variable: array or collection) {
// Perform an action}For example, suppose you have an array of integers:
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
System.out.println(num);
} The above code will print each number in the array in turn. Here num represents the array element currently looped to.
Note: Enhanced for loops can only be used to read elements , and cannot directly modify the content of an array or collection (such as assigning new values ??to array elements). If you want to change the content, you have to use the traditional for loop to cooperate with the index to operate.
What data structures can be used for?
Enhanced for loops are not only used on arrays, it also supports any collection class that implements Iterable interface. for example:
- ArrayList
- HashSet
- HashMap (slightly different)
Let’s take a look at an example used on List:
List<String> names = Arrays.asList("Tom", "Jerry", "Lucy");
for (String name : names) {
System.out.println(name);
}If you are traversing a Map, you will usually traverse its entrySet:
Map<String, Integer> scores = new HashMap<>();
scores.put("Tom", 80);
scores.put("Jerry", 90);
for (Map.Entry<String, Integer> entry : scores.entrySet()) {
System.out.println(entry.getKey() ": " entry.getValue());
}Things to note when using
- Element content cannot be modified : operations such as element assignment of arrays, deletion/addition in collections, etc. are not allowed in enhanced for.
- Unable to get index : If you need to know which element is currently, you have to switch back to a normal for loop.
- Not suitable for multi-dimensional array operations : Although it can be used, it is not intuitive enough to handle.
Some common mistakes include:
- Want to delete elements during traversal →
ConcurrentModificationExceptionwill be thrown - Want to modify the array value through loop → It will not actually take effect (just change local variables)
So when encountering these situations, it is better to be honest and practical for loop.
Basically that's it. Master the applicable scenarios and restrictions, the enhanced for loop is a very practical gadget.
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