Kotlin Val任务

Question

Ran into this and wanted to do this with the least amount of repetitive code. In Java I would do it this way (being an old Java programmer). For reference, data is a Map<String, String> .

Class action = SomeActivity.class;
if (data != null && data.get("abc") != null) {
    if (data.get("abc").contains("details")) {
        action = OtherActivity.class;
    } ... //may have more ifs for other contains()
}

Intent newIntent = new Intent(this, action);

I wanted to do the same in Kotlin but not repeat null conditions and use multiple assignment lines. This is what I have so far.

val action = data["abc"]?.let {
    if (it.contains("details")) {
        OtherActivity::class.java
    } else {
        SomeActivity::class.java
    }
} ?: kotlin.run { SomeActivity::class.java }

val intent = Intent(this, action)

To me the Java looks more efficient than Kotlin here, with Kotlin I have two assignment lines repeated the SomeActivity::class.java , whereas in Java I can set the variable to one class and if and only if all the null and if condition checks out will it assign the new class. So is there a way to do this in Kotlin without repeated assignment lines for SomeActivity::class.java ?

Answer 1

For the provided code, the following should work:

val intent = Intent(this, 
   data?.takeIf { it["abc"]?.contains("details") == true }
       ?.let { OtherActivity::class.java } 
       ?: SomeActivity::class.java
)

---

If you have additional branches, then of course you should use a when statement.

val abc = data?.get("abc")

val action = when {
    abc == null -> SomeActivity::class.java
    abc.contains("details") -> OtherActivity::class.java
    // other branches
    else -> SomeActivity::class.java
}
val intent = Intent(this, action)

Answer 2

Maybe something like the following will work for you:

val action = if (data?.get("abc")?.contains("details") == true) {
    OtherActivity::class.java
} else SomeActivity::class.java
val intent = Intent(this, action)

Note, you need the == true because the left part of the condition may still be null in which case the condition will result to: null == true , which then basically just returns the SomeActivity::class.java .

If you like ~one liners or when :

val intent = Intent(this, when {
  data?.get("abc")?.contains("details") == true -> OtherActivity::class.java
  /* Other conditions */ 
  else -> SomeActivity::class.java
})

Finally you could also just use the same construct as you had in Java by using var instead and specifying your general type, eg:

var action : Class<*> = SomeActivity::class.java
if (data?.get("abc")?.contains("details") == true) { 
  // Or add the conditions you actually had
  action = OtherActivity::class.java
}
Intent newIntent = new Intent(this, action);

Answer 3

You can use a when block to do it neatly:

val abc = data["abc"]
val action = when {
    abc == null -> OtherActivity::class.java
    abc.contains("details") -> DetailsActivity::class.java
    abc.contains("somedata") -> SomeActivity::class.java
    abc.contains("otherdata") -> UnknownActivity::class.java
    else -> DefaultActivity::class.java
}

Or without repeating variable name:

val action = data["abc"].run {
    when {
        this == null -> OtherActivity::class.java
        contains("details") -> DetailsActivity::class.java
        contains("somedata") -> SomeActivity::class.java
        contains("otherdata") -> UnknownActivity::class.java
        else -> DefaultActivity::class.java
    } 
}

Or if you can afford to use equals instead of contains:

val action = when(abc) {
    null -> OtherActivity::class.java
    "details" -> DetailsActivity::class.java
    "somedata" -> SomeActivity::class.java
    "otherdata" -> UnknownActivity::class.java
    else -> DefaultActivity::class.java
}

Note that catching null as the first case lets other branches smart-cast to non-nullable type.

Answer 4

You do this in this way

    val action = data["abc"]?.let {
        if (it.contains("details")) OtherActivity::class.java
            else SomeActivity::class.java
    }

    val intent = Intent(this,action)

if data["abc"] is null the let block is not called. so you can do this.

prevent null assignment (if deta["abc"

   val action= data["abc"]?.let {
        if (it.contains("details")) OtherActivity::class
            else SomeActivity::class
    } ?: DefaultActivity::class

    val intent = Intent(this,action.java)

Answer 5

You can use when , and an intermediate null to not repeat the else branch.

val action: KClass<out Activity> = data?.get("abc")?.let {
   when {
     it.contains("details") -> OtherActivity::class
     it.contains("bla")     -> BlaActivity::class
     else                   -> null
  }
} ?: SomeActivity::class

val intent = Intent(this, action.java)

Note that data["abc"] will not work if data can be null!