The const modifier marks a property as a compile-time constant. Use it with a top-level or object property initialized by a primitive value or String when the value must be known during compilation. section 3.2.3 section 12.1.1
Declaring a Java-friendly constant
const val UNIX_LINE_SEPARATOR = "\n" This exposes the Kotlin property to Java as a public static final field, rather than through an accessor method. The equivalent Java declaration is `public static final String UNIX_LINE_SEPARATOR = "\n";`.
Without const, a top-level val is normally exposed to Java through a getter. Adding const makes direct field access natural for Java callers and communicates that the value is fixed at compile time. section 3.2.3
Use const when these conditions apply:
- Declare the property at the top level or inside an object. section 12.1.1
- Initialize it with a primitive type or String value. section 3.2.3
- Supply it where a compile-time value is required, such as an annotation argument. section 12.1.1
Using a constant in an annotation
const val TEST_TIMEOUT = 10L class MyTest { @Test @Timeout(TEST_TIMEOUT) fun testMethod() { /* ... */ } } A regular property cannot be used in this position; the compiler requires `const val` for constant expressions. section 12.1.1
