Why do we use “companion object” as a kind of replacement for Java static fields in Kotlin?

What is the intended meaning of “companion object”? So far I have been using it just to replace Java’s static when I need it. I am confused with: Why is it called “companion”? Does it mean that to create multiple static properties, I have to group it together inside companion object block? To instantly create […]

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Kotlin – Property initialization using “by lazy” vs. “lateinit”

In Kotlin if you don’t want to initiate a class property inside the constructor or in the top of the class body, you have basically these two options ( from the language reference): Lazy Initialization lazy() is a function that takes a lambda and returns an instance of Lazy which can serve as a delegate […]

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How to convert a kotlin source file to a java source file

I have a Kotlin source file, but I want to translate it to Java. How can I convert Kotlin to Java source? Solution In IntelliJ or Android Studio, you just have to do the following to get java code from kotlin: Menu>Tools>Kotlin>Show Kotlin Bytecode Click on the Decompile button Copy the java code This question was reposted […]

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What Java 8 Stream.collect equivalents are available in the standard Kotlin library?

In Java 8, there is Stream.collect which allows aggregations on collections. In Kotlin, this does not exist in the same way, other than maybe as a collection of extension functions in the stdlib. But it isn’t clear what the equivalences are for different use cases. For example, at the top of the JavaDoc for Collectors […]

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In Kotlin, what is the idiomatic way to deal with nullable values, referencing or converting them

If I have a nullable type Xyz?, I want to reference it or convert it to a non-nullable type Xyz. What is the idiomatic way of doing so in Kotlin? For example, this code is in error: val something: Xyz? = createPossiblyNullXyz() something.foo() // Error: “Only safe (?.) or non-null asserted (!!.) calls are allowed […]

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Idiomatic way of logging in Kotlin

Kotlin doesn’t have the same notion of static fields as used in Java. In Java, the generally accepted way of doing logging is: public class Foo { private static final Logger LOG = LoggerFactory.getLogger(Foo.class); } Question is what is the idiomatic way of performing logging in Kotlin? Solution In majority of mature Kotlin code, you […]

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Kotlin and new ActivityTestRule : The @Rule must be public

I’m trying to make UI test for my android app in Kotlin. Since the new system using ActivityTestRule, I can’t make it work: it compiles correctly, and at runtime, I get: java.lang.Exception: The @Rule ‘mActivityRule’ must be public. at org.junit.internal.runners.rules.RuleFieldValidator.addError(RuleFieldValidator.java:90) at org.junit.internal.runners.rules.RuleFieldValidator.validatePublic(RuleFieldValidator.java:67) at org.junit.internal.runners.rules.RuleFieldValidator.validateField(RuleFieldValidator.java:55) at org.junit.internal.runners.rules.RuleFieldValidator.validate(RuleFieldValidator.java:50) at org.junit.runners.BlockJUnit4ClassRunner.validateFields(BlockJUnit4ClassRunner.java:170) at org.junit.runners.BlockJUnit4ClassRunner.collectInitializationErrors(BlockJUnit4ClassRunner.java:103) at org.junit.runners.ParentRunner.validate(ParentRunner.java:344) at org.junit.runners.ParentRunner.(ParentRunner.java:74) at […]

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Kotlin Ternary Conditional Operator

What is the equivalent of this expression in Kotlin? a ? b : c This is not a valid code in Kotlin. Solution In Kotlin, if statements are expressions. So the following code is equivalent: if (a) b else c This question was reposted from Stack Overflow

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