Java for-each循环(遍历循环)详解

// 标准语法 (Java 5+)
for (ElementType element : Iterable/Array) {
    // 操作element
}

// 等效传统实现
Iterator<ElementType> it = collection.iterator();
while (it.hasNext()) {
    ElementType element = it.next();
    // 操作element
}

List<String> list = Arrays.asList("A", "B");
for (Integer num : list) {} // 编译错误：类型不匹配

for (final String s : list) { 
    s = "new"; // 编译错误：final修饰符
}

for (Object obj : null) { ... } // 运行时抛出NullPointerException

List<String> list = new ArrayList<>(Arrays.asList("A", "B"));
for (String s : list) {
    list.remove(s); // 抛出ConcurrentModificationException
}

List<StringBuilder> list = new ArrayList<>();
list.add(new StringBuilder("Hello"));
for (StringBuilder sb : list) {
    sb.append(" World"); // 修改对象内部状态 ✔️
    sb = new StringBuilder(); // 仅改变局部引用 ✖️
}

int[][] matrix = {{1,2}, {3,4}};
for (int[] row : matrix) {
    for (int num : row) {
        System.out.print(num + " ");
    }
}

class MyIterable implements Iterable<String> {
    public Iterator<String> iterator() {
        return Arrays.asList("A","B").iterator();
    }
}

list.parallelStream().forEach(element -> {
    // 多线程处理（注意线程安全）
});

// 防御性复制（针对可能被修改的集合）
for (String s : new ArrayList<>(originalList)) {
    originalList.remove(s); // 安全操作
}

var entries = Map.of(1,"A",2,"B").entrySet();
for (var entry : entries) {
    System.out.println(entry.getKey() + ":" + entry.getValue());
}

record Point(int x, int y) {}
List<Point> points = List.of(new Point(1,2), new Point(3,4));
for (Point(var x, var y) : points) { // 模式匹配解构（Java 17预览特性）
    System.out.println(x + "," + y);
}