如何用Java8替代foreach循环？嵌套列表过滤代码转换问询

Let's break this down step by step. First, let's fix the tiny syntax issue in your original code (you were missing a closing parenthesis in the if condition):

for(A aobj : branchList) {
    for(C cobj : aobj.getCList()) {
        if(!enteredBranchId.equals(cobj.getId())) {
            myList.add(aobj.getId());
        }
    }
}

Now, let's convert this to Java 8 stream style. There are two common approaches depending on whether you want to preserve the original duplicate behavior or avoid adding the same A ID multiple times.

Option 1: Match Original Behavior (Allow Duplicate IDs) #

Your original code adds the A ID once for every C object in its list that doesn't match enteredBranchId. To replicate this exactly with streams:

branchList.stream()
    // For each A, stream its CList, filter non-matching Cs, map to A's ID
    .flatMap(a -> a.getCList().stream()
        .filter(c -> !enteredBranchId.equals(c.getId()))
        .map(c -> a.getId()))
    // Add each resulting ID to myList
    .forEach(myList::add);

How this works: #

• flatMap takes each A object, streams its CList, filters out the C objects that match enteredBranchId, then maps the remaining Cs to the parent A's ID. It flattens all these individual IDs into a single stream.
• forEach(myList::add) adds every element from the stream to your list, just like the nested loops.

Option 2: Avoid Duplicate IDs (Add Each A ID Once) #

If you only want to add an A's ID once if any of its C objects don't match enteredBranchId (a more common use case), use filter with anyMatch:

branchList.stream()
    // Keep only A objects where at least one C in CList doesn't match the ID
    .filter(a -> a.getCList().stream()
        .anyMatch(c -> !enteredBranchId.equals(c.getId())))
    // Map each qualifying A to its ID
    .map(A::getId)
    // Add each unique ID to myList
    .forEach(myList::add);

How this works: #

• filter checks if the CList of an A has any element that doesn't match enteredBranchId (using anyMatch).
• map(A::getId) converts each qualifying A to its ID.
• forEach adds each ID once, even if multiple Cs in the list meet the condition.

Pick the option that aligns with your actual requirements!

内容的提问来源于stack exchange，提问作者Daniel Atlas