Fail-fast

实际上，java.util.Iterator 的大多数实现都提供了故障快速修复（Fail-fast）的机制 ⎯⎯在利用迭代器遍历某一容器的过程中，一旦发现该容器的内容有所改变，迭代器就会抛出 ConcurrentModificationException 意外错并立刻退出。

public class FailFast {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        Iterator iterator = list.iterator();
        while (iterator.hasNext()) {
            iterator.next();
            list.add(4);
        }
    }
}

输出

Exception in thread "main" java.util.ConcurrentModificationException
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1009)
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:963)
    at sort.FailFast.main(FailFast.java:16)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:567)
    at com.intellij.rt.execution.application.AppMainV2.main(AppMainV2.java:131)

Wiki

Fail-fast systems or modules are desirable in several circumstances:

• When building a fault-tolerant system by means of redundant components, the individual components should be fail-fast to give the system enough information to successfully tolerate a failure.
• Fail-fast components are often used in situations where failure in one component might not be visible until it leads to failure in another component.
• Finding the cause of a failure is easier in a fail-fast system, because the system reports the failure with as much information as possible as close to the time of failure as possible. In a fault-tolerant system, the failure might go undetected, whereas in a system that is neither fault-tolerant nor fail-fast the failure might be temporarily hidden until it causes some seemingly unrelated problem later.
• A fail-fast system that is designed to halt as well as report the error on failure is less likely to erroneously perform an irreversible or costly operation.

Developers also refer to fail-fast code to a code that tries to fail as soon as possible at variable or object initialization. In OOP, a fail-fast designed object initializes the internal state of the object in the constructor, launching an exception if something is wrong (vs allowing non-initialized or partially initialized objects that will fail later due to a wrong "setter"). The object can then be made immutable if no more changes to the internal state are expected. In functions, fail-fast code will check input parameters in the precondition. In client-server architectures, fail-fast will check the client request just upon arrival, before processing or redirecting it to other internal components, returning an error if the request fails (incorrect parameters, ...). Fail-fast designed code decreases the internal software entropy, and reduces debugging effort.

实际上immutable和variable不是一个意思，但是中文翻译总是不可变。可见，要学英语放置被误导。这点在我学java的函数式编程时的copy by value深有体会