The for-each Loop in Java – Java中的for-each循环

最后修改: 2022年 3月 27日

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1. Overview

1.概述

In this tutorial, we’ll discuss the for-each loop in Java along with its syntax, working, and code examples. Finally, we’ll understand its benefits and drawbacks.

在本教程中，我们将讨论Java中的for-each循环，以及它的语法、工作原理和代码示例。最后，我们将了解它的好处和坏处。

2. Simple for Loop

2.简单的for循环

The simple for loop in Java essentially has three parts – initialization, boolean condition & step:

Java中简单的for loop本质上有三个部分–初始化、boolean条件& 步骤：

for (initialization; boolean-condition; step) {
    statement;
}

It starts with the initialization of a loop variable, followed by a boolean expression. If the condition is true, it executes the statement(s) in the loop and increments/decrements the loop variable. Otherwise, it terminates the loop.

它以一个循环变量的初始化开始，然后是一个boolean表达式。如果条件是真，它将执行循环中的语句，并增加/减少循环变量。否则，它将终止循环。

This pattern makes it slightly complex and difficult to read. Moreover, if we do not write the condition properly, there’s always a chance to get into an infinite loop.

这种模式使得它略显复杂，难以阅读。此外，如果我们没有正确地写出条件，总是有机会进入无限循环。

3. for-each Loop

3.for-each 循环

The for-each loop was introduced in Java 5. We also call it an enhanced for loop.

for-each循环是在Java 5中引入的。我们也称它为增强型for循环。。

It’s an alternate traversing technique specifically introduced to traverse arrays or collections. Noticeably, it also uses the for a keyword. However, instead of using a loop counter variable, we assign a variable of the same type as that of an array or a collection.

这是一种替代性的遍历技术，专门用于遍历数组或集合。值得注意的是，它也使用了for a关键字。但是，我们没有使用循环计数器变量，而是分配了一个与数组或集合类型相同的变量。

The name for-each signifies that each element of an array or a collection is traversed, one after another.

for-each这个名字表示数组或集合中的每个元素都被逐一遍历。

3.1. Syntax

3.1 语法

The for-each loop consists of the declaration of a loop variable followed by a colon (:), which is followed by the name of an array or collection:

for-each循环由一个循环变量的声明组成，后面是冒号（:），冒号后面是一个数组或集合的名称。

for (data_type var_name : array | collection) {
    // code
}

3.2. Working

3.2.工作

For each iteration, the for-each loop takes each element of the collection and stores it in a loop variable. Thus, it executes the code written in the body of the loop for each element of the array or collection.

对于每一次迭代，for-each 循环会获取集合中的每个元素，并将其存储在一个循环变量中。因此，它为数组或集合的每个元素执行写在循环主体中的代码。

Most importantly, the traversal happens until the last element of the array or collection.

最重要的是，这种遍历一直发生到数组或集合的最后一个元素。

3.3. Examples

3.3.实例

Let’s see an example of traversing an array with the for-each loop:

让我们看一个用for-each循环遍历数组的例子。

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

for (int number : numbers) {
    System.out.print(number + " ");
}

Here, the for-each loop traverses over each element of the array numbers one by one until the end. Therefore, there’s no need to access the array elements using indexing.

在这里，for-each循环逐一遍历数组numbers中的每个元素，直到结束。因此，不需要使用索引来访问数组元素。

Now, let us see some examples of traversing various collections with the for-each loop.

现在，让我们看看用for-each循环遍历各种集合的一些例子。

Let’s start with a List:

让我们从一个列表开始。

String[] wordsArray = { "Java ", "is ", "great!" };
List<String> wordsList = Arrays.asList(wordsArray);

for (String word : wordsList) {
    System.out.print(word + " ");
}

Similarly, we can traverse through all the elements of a Set:

同样地，我们可以遍历Set的所有元素。

Set<String> wordsSet = new HashSet();
wordsSet.addAll(wordsList);

for (String word : wordsSet) {
    System.out.print(word + " ");
}

Additionally, we can also use the for-each loop to traverse through a Map<K, V> as well:

此外，我们还可以使用for-each循环来遍历Map<K, V> 。

Map<Integer, String> map = new HashMap<>();
map.put(1, "Java");
map.put(2, "is");
map.put(3, "great!");

for (Entry<Integer, String> entry : map.entrySet()) {
    System.out.println(
      "number: " + entry.getKey() +
      " - " +
      "Word: " + entry.getValue());
}

In the same way, we can use a for-each loop to iterate through various other data structures in Java.

同样，我们可以使用for-each循环来迭代Java中的其他各种数据结构。

However, if the array or collection is null, it throws a NullPointerException:

然而，如果数组或集合是null，它会抛出一个NullPointerException：。

int[] numbers = null;
for (int number : numbers) {
    System.out.print(number + " ");
}

The above code throws a NullPointerException:

上述代码抛出一个NullPointerException。

Exception in thread "main" java.lang.NullPointerException
    at com.baeldung.core.controlstructures.loops.ForEachLoop.traverseArray(ForEachLoop.java:63)
    ..

Hence, we must check if the array or collection is null before passing it to the for-each loop.

因此，我们必须在将数组或集合传递给for-each循环之前检查其是否为null。

The for-each loop doesn’t execute at all if the array or collection is empty.

如果数组或集合是空的，for-each循环就根本不执行。

3.4. Pros and Cons

3.4.优点和缺点

The for-each loop is one of the important features introduced in Java 5. However, it also has its own benefits and drawbacks.

for-each循环是Java 5中引入的重要功能之一。然而，它也有自己的好处和缺点。

The benefits of the for-each loop are:

for-each循环的好处是。

  • It helps us avoid programming errors.
  • It makes the code precise and readable.
  • It’s easier to implement.
  • It avoids the chance of an infinite loop.

Because of these benefits, we prefer the for-each loop over the for loop, especially while working with arrays or collections.

由于这些好处，我们更喜欢for-each循环而不是for循环，尤其是在处理数组或集合时。

The drawbacks of the for-each loop are:

for-each循环的缺点是。

  • We can’t jump over an element as it traverses through each element.
  • Traversing in reverse order is not possible.
  • We can’t modify the array if we’re using a for-each loop.
  • It’s not possible to keep track of the index.
  • It has some performance overhead over the for a loop.

4. Conclusion

4.总结

In this article, we explored the for-each loop in Java along with its syntax, working, and examples. Finally, we saw its benefits and drawbacks.

在这篇文章中，我们探讨了Java中的for-each循环，以及它的语法、工作和例子。最后，我们看到了它的好处和坏处。

As always, the code for these examples is available over on GitHub.

像往常一样，这些例子的代码可以在GitHub上找到over。