新・Java入門編33: ストリームAPIについて学習しよう

インタフェースFunction
https://docs.oracle.com/javase/jp/17/docs/api/java.base/java/util/function/Function.html

String.valueOf
https://docs.oracle.com/javase/jp/17/docs/api/java.base/java/lang/String.html#valueOf(java.lang.Object)

import java.util.*;
import java.util.function.*;

public class Main {
    public static void main(String... args) {
        Function<Object, String> f1 = String::valueOf;
        Function<Object, String> f2 = t -> String.valueOf(t);

        Object o = new Object();
        System.out.println(f1.apply(o));
        System.out.println(f2.apply(o));
    }
}