[英]How to iterate over MultipartFile array using Java lambda and streams
[英]How to iterate over lambda functions in Java
我能用Python完成它,我的Python代码是:
signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}
a = 5
b = 3
for i in signs.keys():
print(signs[i](a,b))
输出是:
8
2
如何通过HashMap在Java中执行相同的操作?
你可以在这种情况下使用BinaryOperator<Integer>
,如下所示:
BinaryOperator<Integer> add = (a, b) -> a + b;//lambda a, b : a + b
BinaryOperator<Integer> sub = (a, b) -> a - b;//lambda a, b : a - b
// Then create a new Map which take the sign and the corresponding BinaryOperator
// equivalent to signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}
Map<String, BinaryOperator<Integer>> signs = Map.of("+", add, "-", sub);
int a = 5; // a = 5
int b = 3; // b = 3
// Loop over the sings map and apply the operation
signs.values().forEach(v -> System.out.println(v.apply(a, b)));
输出
8
2
注意Map.of("+", add, "-", sub);
我正在使用Java 10,如果您不使用Java 9+,您可以像这样添加到地图中:
Map<String, BinaryOperator<Integer>> signs = new HashMap<>();
signs.put("+", add);
signs.put("-", sub);
正如@Boris中的Spider和@Holger在评论中已经说过的,最好使用IntBinaryOperator
来避免装箱,最后你的代码看起来像这样:
// signs = {"+" : lambda a, b : a + b, "-" : lambda a, b : a - b}
Map<String, IntBinaryOperator> signs = Map.of("+", (a, b) -> a + b, "-", (a, b) -> a - b);
int a = 5; // a = 5
int b = 3; // b = 3
// for i in signs.keys(): print(signs[i](a,b))
signs.values().forEach(v -> System.out.println(v.applyAsInt(a, b)));
创建一个漂亮的,类型安全的enum
:
enum Operator implements IntBinaryOperator {
PLUS("+", Integer::sum),
MINUS("-", (a, b) -> a - b);
private final String symbol;
private final IntBinaryOperator op;
Operator(final String symbol, final IntBinaryOperator op) {
this.symbol = symbol;
this.op = op;
}
public String symbol() {
return symbol;
}
@Override
public int applyAsInt(final int left, final int right) {
return op.applyAsInt(left, right);
}
}
您可能希望lambda为其他运算符返回double
而不是int
。
现在,只需将其转储到Map
:
final var operators = Arrays.stream(Operator.values())
.collect(toMap(Operator::symbol, identity()));
但是,对于您的示例,您根本不需要Map
:
Arrays.stream(Operator.values())
.mapToInt(op -> op.applyAsInt(a,b))
.forEach(System.out::println);
使用:
import static java.util.function.Function.identity;
import static java.util.stream.Collectors.toMap;
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.