countryList是一個List接口，擴展自Collection:

后者又擴展自Iteratable, forEach就是Iterable提供的方法。

forEach接受的方法參數為Consumer<? super T> action):

Consumer暗示了該操作會造成side effect：

Consumer是一個FunctionalInterface：

/** Copyright (c) 2012, 2015, Oracle and/or its affiliates. All rights reserved.* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.** This code is free software; you can redistribute it and/or modify it* under the terms of the GNU General Public License version 2 only, as* published by the Free Software Foundation. Oracle designates this* particular file as subject to the "Classpath" exception as provided* by Oracle in the LICENSE file that accompanied this code.** This code is distributed in the hope that it will be useful, but WITHOUT* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License* version 2 for more details (a copy is included in the LICENSE file that* accompanied this code).** You should have received a copy of the GNU General Public License version* 2 along with this work; if not, write to the Free Software Foundation,* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.** Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA* or visit www.oracle.com if you need additional information or have any* questions.*/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為臨時變量，類型為countryList里的元素，forEach里的Consumer實際是一個Lambda Function，并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：

@Overridepublic 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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