You can also define your own getter and setter methods class Person(age: Int, name: String) copy code Copy the code Safe conversion of types (as?) val kotliner: Kotliner =. If a variable can be null, it's not safe to call a method on the variable because this can cause a NullPointerException. It is a way to indicate which variables can possibly hold a null value.
Var: the default value is getterand setter The most important difference between Kotlin's and Java's type systems is Kotlin's explicit support for nullable types.Need to call the inherited constructor (default is no parameter constructor).extendsThe keyword is changed :, which is consistent with the interface implementation.notes are changed to overrideThe keywordīecause Kotlin's class is final by default, you need to add the open keyword to make it inheritable.Kotlin Val simpleClass = simpleClass (9) println(simpleclass.x) simpleclass.y () copies the code Copy the code Interface Definition Java SimpleClass simpleClass = new SimpleClass(9) (simpleClass.x) simpleClass.y() Copy the code Copy the code Intuitively, by omitting the new keyword, you no longer need new to get the object You can also define it directly on a class Class instantiation Use this if you need to define other constructors constructorKeyword creation The Kotlin class takes a no-parameter constructor by default. Kotlin class member variable, the method is similar to Java This section focuses on the type definition and simple use of Kotlin The definition of a class The Kotlin class defaults to public and has nothing to omit In Kotlin, the type system distinguishes between references that can hold null (nullable references) and those that cannot (non-null references).Posted on March 17, 2023, 4:24 a.m. Other issues caused by external Java code. For example, a piece of Java code might add null into a Kotlin MutableList, therefore requiring a MutableList for working with it. Nullability issues with generic types being used for Java interoperation.
Usage of the !! operator that is described below.ĭata inconsistency with regard to initialization, such as when:Īn uninitialized this available in a constructor is passed and used somewhere (a "leaking this").Ī superclass constructor calls an open member whose implementation in the derived class uses an uninitialized state.Īttempts to access a member of a null reference of a platform type The only possible causes of an NPE in Kotlin are:Īn explicit call to throw NullPointerException(). Nullability is a concept commonly found in many programming languages. In Java this would be the equivalent of a NullPointerException, or an NPE for short. This codelab teaches you about nullability and the importance of null safety. One of the most common pitfalls in many programming languages, including Java, is that accessing a member of a null reference will result in a null reference exception.
Kotlin's type system is aimed at eliminating the danger of null references, also known as The Billion Dollar Mistake. Null safety Nullable types and non-null types
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