1. Overview
1.概述
In this article, we’ll look at a core aspect of the Java language – executing a statement or a group of statements repeatedly using a for loop.
在这篇文章中，我们将研究Java语言的一个核心方面–使用for循环重复执行一条或一组语句。
2. Simple for Loop
2.简单的for循环
A for loop is a control structure that allows us to repeat certain operations by incrementing and evaluating a loop counter.
一个for 循环是一个控制结构，它允许我们通过增加和评估一个循环计数器来重复某些操作。
Before the first iteration, the loop counter gets initialized, then the condition evaluation is performed followed by the step definition (usually a simple incrementation).
在第一次迭代之前，循环计数器被初始化，然后进行条件评估，接着是步骤定义（通常是简单的递增）。
The syntax of the for loop is:
for 循环的语法是。
for (initialization; Boolean-expression; step)
statement;Let’s see it in a simple example:
让我们在一个简单的例子中看到它。
for (int i = 0; i < 5; i++) {
System.out.println("Simple for loop: i = " + i);
}The initialization, Boolean-expression, and step used in for statements are optional. Here’s an example of an infinite for loop:
在for语句中使用的初始化、布尔表达式和步骤是可选的。下面是一个无限for循环的例子。
for ( ; ; ) {
// Infinite for loop
}2.1. Labeled for Loops
2.1.标记的for循环
We can also have labeled for loops. It’s useful if we’ve got nested for loops so that we can break/continue from aspecific for loop:
我们也可以有标记的for循环。如果我们有嵌套的for循环，这很有用，这样我们就可以从特定的for循环中break/continue。
aa: for (int i = 1; i <= 3; i++) {
if (i == 1)
continue;
bb: for (int j = 1; j <= 3; j++) {
if (i == 2 && j == 2) {
break aa;
}
System.out.println(i + " " + j);
}
}3. Enhanced for Loop
3.增强的for循环
Since Java 5, we have a second kind of for loop called the enhanced for which makes it easier to iterate over all elements in an array or a collection.
从Java 5开始，我们有了第二种for循环，叫做enhanced for，它使我们更容易遍历数组或集合中的所有元素。
The syntax of the enhanced for loop is:
增强的for循环的语法是。
for(Type item : items)
statement;Since this loop is simplified in comparison to the standard for loop, we need to declare only two things when initializing a loop:
由于这个循环与标准的for循环相比是简化的，所以在初始化循环时我们只需要声明两件事。
- The handle for an element we’re currently iterating over
- The source array/collection we’re iterating
Therefore, we can say that: For each element in items, assign the element to the item variable and run the body of the loop.
因此，我们可以这样说。对于items中的每个元素，将该元素分配给item变量并运行循环的主体。
Let’s have a look at the simple example:
让我们看一下这个简单的例子。
int[] intArr = { 0,1,2,3,4 };
for (int num : intArr) {
System.out.println("Enhanced for-each loop: i = " + num);
}We can use it to iterate over various Java data structures:
我们可以用它来迭代各种Java数据结构。
Given a List<String> list object – we can iterate it:
给定一个List<String> list对象–我们可以对其进行迭代。
for (String item : list) {
System.out.println(item);
}We can similarly iterate over a Set<String> set:
我们同样可以在一个Set<String>集合上进行迭代。
for (String item : set) {
System.out.println(item);
}And, given a Map<String,Integer> map we can iterate over it as well:
而且，给定一个Map<String,Integer> map，我们也可以对它进行迭代。
for (Entry<String, Integer> entry : map.entrySet()) {
System.out.println(
"Key: " + entry.getKey() +
" - " +
"Value: " + entry.getValue());
}3.1. Iterable.forEach()
3.1.Iterable.forEach()
Since Java 8, we can leverage for-each loops in a slightly different way. We now have a dedicated forEach() method in the Iterable interface that accepts a lambda expression representing an action we want to perform.
从Java 8开始，我们可以用一种稍微不同的方式来利用for-each循环。我们现在在Iterable接口中有一个专门的forEach()方法，它接受一个代表我们要执行的动作的lambda表达式。
Internally, it simply delegates the job to the standard loop:
在内部，它只是将这项工作委托给标准循环。
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}Let’s have a look at the example:
让我们看一下这个例子。
List<String> names = new ArrayList<>();
names.add("Larry");
names.add("Steve");
names.add("James");
names.add("Conan");
names.add("Ellen");
names.forEach(name -> System.out.println(name));4. Conclusion
4.结论
In this quick tutorial, we explored Java’s for loop.
在这个快速教程中，我们探讨了Java的for循环。
As always, examples can be found over on GitHub.
一如既往，可以在GitHub上找到实例。