JAVA8新特性Optional

老的代碼

public static String getName(User u) {
    if (u == null || u.name == null)
        return "Unknown";
    return u.name;
}

新的寫法

public static String getName(User u) {
    return Optional.ofNullable(u)
                    .map(user->user.name)
                    .orElse("Unknown");
}

老的代碼

public static String getChampionName(Competition comp) throws IllegalArgumentException {
    if (comp != null) {
        CompResult result = comp.getResult();
        if (result != null) {
            User champion = result.getChampion();
            if (champion != null) {
                return champion.getName();
            }
        }
    }
    throw new IllegalArgumentException("The value of param comp isn't available.");
}

新的寫法

public static String getChampionName(Competition comp) throws IllegalArgumentException {
    return Optional.ofNullable(comp)
            .map(Competition::getResult)  // 相当于c -> c.getResult()，下同
            .map(CompResult::getChampion)
            .map(User::getName)
            .orElseThrow(()->new IllegalArgumentException("The value of param comp isn't available."));
}

//of()：为非null的值创建一个Optional
Optional optional = Optional.of("bam");
// isPresent()：如果值存在返回true，否则返回false
optional.isPresent();           // true
//get()：如果Optional有值则将其返回，否则抛出NoSuchElementException
optional.get();                 // "bam"
//orElse()：如果有值则将其返回，否则返回指定的其它值
optional.orElse("fallback");    // "bam"
//ifPresent()：如果Optional实例有值则为其调用consumer，否则不做处理
optional.ifPresent((s) -> System.out.println(s.charAt(0)));     // "b"