The Type! notation is called a Platform Type and is critical for Interoperability with the weaker-typed Java; it means "the null-ability is unknown" and it allows for relaxed null-ability enforcement. I found the examples in the linked question underwhelming.. although all the information is there.
下面的程序是accepted as type-safe，因为Kotlin允许x/Int的类型！要用作"Int或Int"的表达式.
var x = JavaIntegerList.get(0); // Int! type inferred by Kotlin
var y: Int? = x;
var z: Int = x;
However, the assignment to z (Int) will fail at run-time if x evaluates to null. (For class implementation types, the NPE may delayed to usage: basically, Kotlin chooses to 'get out of the way'.)
It's thus designed for cases when it is known that the value "cannot" be null in a valid program, although the value comes from Java outside of the Kotlin type system. Highly applicable to int-box-Integer in Java, although it is valid for any class type / value coming in from Java. If the Java code can return null, it is safest to put the value into a Type? immediately and then correctly deal with the null-ability.
将其与具有类似运行时失败语义的Java代码进行比较:
// Java
Integer x = JavaIntegerList.get(0);
Integer y = x; // "No OP" - all class types in Java are null-able
int z = (int)x; // kaboom *at run-time* if x is null!
While simple to show with ints, as in this example, this issue is not strictly related to Java's int/Integer types. The document linked above shows a case of String! being implicitly used, giving the example that item.substring(1), where item is a String! is allowed/type-safe but will fail at run-time if item evaluates to null.