countryList是一個List接口，擴展自Collection:
後者又擴展自Iteratable, forEach就是Iterable提供的方法。
forEach接受的方法參數爲Consumer<? super T> action):
Consumer暗示了該操作會造成side effect：
Consumer是一個FunctionalInterface：
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package java.lang;
import java.lang.annotation.*;
/**
* An informative annotation type used to indicate that an interface
* type declaration is intended to be a <i>functional interface</i> as
* defined by the Java Language Specification.
*
* Conceptually, a functional interface has exactly one abstract
* method. Since {@linkplain java.lang.reflect.Method#isDefault()
* default methods} have an implementation, they are not abstract. If
* an interface declares an abstract method overriding one of the
* public methods of {@code java.lang.Object}, that also does
* <em>not</em> count toward the interface's abstract method count
* since any implementation of the interface will have an
* implementation from {@code java.lang.Object} or elsewhere.
*
* <p>Note that instances of functional interfaces can be created with
* lambda expressions, method references, or constructor references.
*
* <p>If a type is annotated with this annotation type, compilers are
* required to generate an error message unless:
*
* <ul>
* <li> The type is an interface type and not an annotation type, enum, or class.
* <li> The annotated type satisfies the requirements of a functional interface.
* </ul>
*
* <p>However, the compiler will treat any interface meeting the
* definition of a functional interface as a functional interface
* regardless of whether or not a {@code FunctionalInterface}
* annotation is present on the interface declaration.
*
* @jls 4.3.2. The Class Object
* @jls 9.8 Functional Interfaces
* @jls 9.4.3 Interface Method Body
* @jls 9.6.4.9 @FunctionalInterface
* @since 1.8
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface FunctionalInterface {}
country爲臨時變量，類型爲countryList裏的元素，forEach裏的Consumer實際是一個Lambda Function，並且可以訪問函數體外定義的變量。
運行時能看出，consumer是一個Lambda Function：
forEach執行時方法內的調試：
實際上內部還是存在一個for循環，如1539行所示，遍歷集合每個元素，依次調用action的accept方法：
@Override
public void forEach(Consumer<? super E> action) {
Objects.requireNonNull(action);
final int expectedModCount = modCount;
final Object[] es = elementData;
final int size = this.size;
for (int i = 0; modCount == expectedModCount && i < size; i++)
action.accept(elementAt(es, i));
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
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