java for 下一个,使用流中的Java 8 foreach循环移至下一个项目
I have a problem with the stream of Java 8 foreach attempting to move on next item in loop. I cannot set the command like continue;, only return; works but you will exit from the loop in this case. I need to move on next item in loop. How can I do that?
Example(not working):
try(Stream lines = Files.lines(path, StandardCharsets.ISO_8859_1)){
filteredLines = lines.filter(...).foreach(line -> {
...
if(...)
continue; // this command doesn't working here
});
}
Example(working):
try(Stream lines = Files.lines(path, StandardCharsets.ISO_8859_1)){
filteredLines = lines.filter(...).collect(Collectors.toList());
}
for(String filteredLine : filteredLines){
...
if(...)
continue; // it's working!
}
解决方案
Using return; will work just fine. It will not prevent the full loop from completing. It will only stop executing the current iteration of the forEach loop.
Try the following little program:
public static void main(String[] args) {
ArrayList stringList = new ArrayList<>();
stringList.add("a");
stringList.add("b");
stringList.add("c");
stringList.stream().forEach(str -> {
if (str.equals("b")) return; // only skips this iteration.
System.out.println(str);
});
}
Output:
a
c
Notice how the return; is executed for the b iteration, but c prints on the following iteration just fine.
Why does this work?
The reason the behavior seems unintuitive at first is because we are used to the return statement interrupting the execution of the whole method. So in this case, we expect the main method execution as a whole to be halted.
However, what needs to be understood is that a lambda expression, such as:
str -> {
if (str.equals("b")) return;
System.out.println(str);
}
... really needs to be considered as its own distinct "method", completely separate from the main method, despite it being conveniently located within it. So really, the return statement only halts the execution of the lambda expression.
The second thing that needs to be understood is that:
stringList.stream().forEach()
... is really just a normal loop under the covers that executes the lambda expression for every iteration.
With these 2 points in mind, the above code can be rewritten in the following equivalent way (for educational purposes only):
public static void main(String[] args) {
ArrayList stringList = new ArrayList<>();
stringList.add("a");
stringList.add("b");
stringList.add("c");
for(String s : stringList) {
lambdaExpressionEquivalent(s);
}
}
private static void lambdaExpressionEquivalent(String str) {
if (str.equals("b")) {
return;
}
System.out.println(str);
}
With this "less magic" code equivalent, the scope of the return statement becomes more apparent.