如何将此 Android Kotlin 代码转换为 Java 代码

Question

class MainAdapter : RecyclerView.Adapter<MainAdapter.ViewHolder>() {

    var list: List<Int> = arrayListOf()
    var tracker: SelectionTracker<Long>? = null

    init {
        setHasStableIds(true)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val number = list[position]
        tracker?.let {
            holder.bind(number, it.isSelected(position.toLong()))
        }
    }

I would like to understand this example. What does tracker?.let translate to? And also what's the equivalent of init in Java?

Answer 1

I would recommend another way to read kotlin code assuming you are well familiar with java. You can use below option in android studio.

After this just click decompile and Voila!, you will get java code.

So now you can read any kotlin code.

Answer 2

init

As per documentation - init is equivalent to a constructor in java. here we can initialize a variable.

let

As per documentation- Kotlin let is a scoping function wherein the variables declared inside the expression cannot be used outside.

tracker?.let {
    holder.bind(number, it.isSelected(position.toLong()))
}

here it is a copy of tracker.changing it will not affect on tracker variable.

Answer 3

MainAdapter equivalent in Java code is something like that:

class MainAdapter extends RecyclerView.Adapter<MainAdapter.ViewHolder> {

    List<Integer> list = new ArrayList<>();
    SelectionTracker<Long> tracker;

    public MainAdapter() {
        setHasStableIds(true);
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        int number = list.get(position);

        if (tracker != null) {
            holder.bind(number, tracker.isSelected(position.toLong()));
        }
    }
}

Answer 4

class M {
    var age: Int = 0
}

fun main(){
    val m = M()
    m.let {
        it.age = 11
     }
    println(m.age)
}

above output "11", so "let" doesn't copy the receiver, "it" is just point to the receiver, it has side-effect