这个 kotlin 代码与 java 的等价物是什么

Question

The following is some kotlin code that prints the execution time of any statement or block of statements:

fun exeTime(code: () -> Unit) {
    var start = System.currentTimeMillis()
    code()
    var end = System.currentTimeMillis() - start
    println("Execution time: " + end + "ms")
}

It can be used like this:

exeTime {
    // some code
    // ...
}

What is the equivalent construct in Java for the exeTime function?

Answer 1

I will be something like:

public void exeTime(Runnable code){
    long start = System.currentTimeMillis();
    code.run();
    long end = System.currentTimeMillis() - start;
    System.out.println("Execution time: " + end + "ms");
}

Read this doc about higher-order functions and lambdas

Answer 2

Well, Java has no data type as Unit, so basically you can't pass a method instead of it's value. When you pass a method as an argument of a method in Java, it executes. If you need to do the same in Java easiest way to use reflection. Something similar to this.

void <T> execTime (Class<T> type, String methodName, Object... methodParams) {

}

Answer 3

That's an easy example of a "higher order function", ie a function that takes another function as an argument. In Java you can also use lambdas as of 1.8 which could look as follows:

public static void main(String[] args) {
    exeTime(v -> System.out.println("from Lambda"));
}

static void exeTime(Consumer<Void> code) {
    long start = System.currentTimeMillis();
    code.accept(null);
    long end = System.currentTimeMillis() - start;
    System.out.println("Execution time: " + end + "ms");
}

Also Runnable could be used as the interface type.

Before 1.8 you would have used an anonymous function instead of the lambda passed to the exeTime function.