英文:
guard/if let in Java - declare a property or an object if it can be unwrapped
问题
在Swift中，有guard let / if let模式，允许我们仅在可以解包时声明对象或属性。
它的工作方式如下：
func getMeaningOfLife() -> Int? {
42
}
func printMeaningOfLife() {
if let name = getMeaningOfLife() {
print(name)
}
}
func printMeaningOfLife() {
guard let name = getMeaningOfLife() else {
return
}
print(name)
}
我的问题是：是否有Java版本的类似功能？
英文:
In swift there is the guard let / if let pattern allowing us to declare an object or a property only if it can be unwrapped.
it works a follow:
func getMeaningOfLife() -> Int? {
42
}
func printMeaningOfLife() {
if let name = getMeaningOfLife() {
print(name)
}
}
func printMeaningOfLife() {
guard let name = getMeaningOfLife() else {
return
}
print(name)
}
My question here is: Is there a Java version of it ?
答案1
得分: 2
Java没有像这样的guard/let子句。您示例的最接近等价物是使用OptionalInt（或对象的Optional）及其ifPresent方法：
OptionalInt getMeaningOfLife() {
return OptionalInt.of(42);
}
void printMeaningOfLife() {
getMeaningOfLife().ifPresent(name ->
System.out.println(name));
}
请注意，Java确实有一种“较小”的形式的这些类型的子句，以instanceof <typeName> <variableName>的形式进行流类型化，这是通过JEP 394: instanceof的模式匹配引入的。
英文:
Java doesn't have guard/let clauses like this. The closest equivalent of your example is using OptionalInt (or Optional for objects) and its ifPresent method:
OptionalInt getMeaningOfLife() {
return OptionalInt.of(42);
}
void printMeaningOfLife() {
getMeaningOfLife().ifPresent(name ->
System.out.println(name));
}
Note that Java does have a "lesser" form of these types of clauses in the form of flow-typing using instanceof <typeName> <variableName> introduced with JEP 394: Pattern Matching for instanceof.
答案2
得分: 1
答案是不。
显然，这种语法在Clojure中也存在，根据这个Stack Overflow回答，如果可以在Java中取消包装，就没有办法声明一个属性。
英文:
The answer is No.
Apparently this syntax also exists in Clojure and according to this Stack Overflow answer there is no way to declare a property if it can be unwrapped in Java.
答案3
得分: 0
这个技巧就是，没有所谓的条件声明。实际上发生的是，条件引入了一个新的作用域，在这个作用域中变量总是被声明，或者引入了一个防卫条件以进行短路操作。
Java 也可以做到类似的事情，尽管从词法上来看可能没有那么漂亮。
Integer getMeaningOfLife() {
return 42;
}
void printMeaningOfLife() {
{ //这个大括号引入了一个新的作用域
Integer name = getMeaningOfLife();
if (name != null) {
print(name);
}
} //关闭作用域
//name 现在超出了作用域，无法使用
}
void printMeaningOfLife() {
Integer name = getMeaningOfLife();
if (name == null) {
return;
}
print(name);
}
这基本上就是你已经自己想明白的，除了第一个 printMeaningOfLife() 中引入作用域的大括号可能除外。
英文:
The trick is that there is no such thing as a conditional declaration. What's really happening is that the conditional is introducing a new scope in which the variable is always declared, or a guard condition which short-circuits.
Java can do the same, though lexically it is not as pretty.
Integer getMeaningOfLife() {
return 42;
}
void printMeaningOfLife() {
{ //this brace bracket introduces a new scope
Integer name = getMeaningOfLife();
if (name != null) {
print(name);
}
} //closes scope
//name is now out of scope and cannot be used
}
void printMeaningOfLife() {
Integer name = getMeaningOfLife();
if (name == null) {
return;
}
print(name)
}
This is basically what you have already figured out for yourself, except maybe for the brace brackets which introduce scope in the first printMeaningOfLife().
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