如何在方法引用中使用 Java 11 repeat() 方法？

Question

public class X{
    public static void print(Integer n, Function<Integer, String> fn) {
        System.out.println(fn.apply(n));
    }

     public static void main(String []args){
        print(3, Integer::repeat()); //
     }
}

Example :

Since 3 is a given number, the value returned by the function and printed on the console should be "aaa" .

Answer 1

Your method reference is wrong in a couple ways. First, a method reference doesn't end in parentheses () . Second, the Integer class doesn't have any repeat method.

The String class defines the repeat method , available for the first time in Java 11. But you want to execute on a specific instance of a string, namely the constant "a" .

Try "a"::repeat .

Answer 2

Method references as well as lambda expressions should conform to a function interface , by providing an implementation of the abstract method defined by this interface.

And interface Function declares a method apply()

R apply(T t);

You can implement it like that with lambda expression:

Function<Integer, String> fun = num -> String.valueOf(num);

Method repeat() is an instance method of the String class, ie you have to invoke it on the instance of the String . So, need to define this string either by outside the lambda or hard-code it inside the body of lambda.

public static final String str = "a";

Function<Integer, String> fun = num -> str.repeat(num);

Now, in order to transform this lambda expression in a method reference , let's first pose the question of what the method reference is.

Method reference - is the way to utilize an existing method to provide an implantation of the behavior defined by a function interface .

There are four kinds of method references (a quote from the Oracles's tutorial ):

• Reference to a static method ContainingClass::staticMethodName
• Reference to an instance method of a particular object containingObject::instanceMethodName
• Reference to an instance method of an arbitrary object of a particular type ContainingType::methodName
• Reference to a constructor ClassName::new

Note that the syntax of method references doesn't require parentheses () because that how the language was designed, and I guess because method references are intended to be concise and expressive.

The type of method reference that we need to implement this function using repeat() method is a reference to an instance method of a particular object (because repeat() is an instance method). And that's how it might look like:

public static final String str = "a";

Function<Integer, String> fun = str::repeat;

print(3, fun); // or print(3, str::repeat);

Note that variables used inside both method references and lambda expressions must be final or effectively final, ie should be assigned only once.