user.go

package main

import "fmt"

type User struct {
	Username string
}

func (u User) String() string {
	return u.Username
}

func main() {
	var win User = User{Username: "Thanabodee"}
	fmt.Println("Hello, playground", win)
}

User.java

public class User implements Stringer {
    private String username;
    
    User(String username) {
        this.username = username;
    }
    
    @Override 
    public String String() {
        return username;
    }
}