Java7下如何删除List<TestDTO>中不在另一个List<String>内的元素

Hey there! Since you're stuck with Java 7 (no fancy Stream API to lean on yet), here are two reliable, safe ways to hit your goal: keeping only the TestDTO elements whose getOne() value exists in the duplicate list, and removing the rest.

First, let's define the sample TestDTO class to match your context:

class TestDTO {
    private String one;

    public TestDTO(String one) {
        this.one = one;
    }

    public String getOne() {
        return one;
    }
}

Method 1: Use Iterator (Safe for In-Place Removal) #

This is the standard Java 7 approach to modify a list while iterating—it avoids the dreaded ConcurrentModificationException that pops up when you call list.remove() directly in a loop.

public class Main {
    public static void main(String[] args) {
        List<TestDTO> test = new ArrayList<>();
        test.add(new TestDTO("a")); // dto (keep)
        test.add(new TestDTO("b")); // dto1 (keep)
        test.add(new TestDTO("c")); // dto2 (remove)

        List<String> duplicate = Arrays.asList("a", "b");

        // Convert to HashSet for way faster lookups (optional but recommended for large lists)
        Set<String> duplicateSet = new HashSet<>(duplicate);

        Iterator<TestDTO> iterator = test.iterator();
        while (iterator.hasNext()) {
            TestDTO currentDto = iterator.next();
            if (!duplicateSet.contains(currentDto.getOne())) {
                iterator.remove(); // Safe removal via iterator's built-in method
            }
        }

        // Check the result
        for (TestDTO dto : test) {
            System.out.println(dto.getOne()); // Outputs: a, b
        }
    }
}

Quick tip: Turning duplicate into a HashSet changes contains() from an O(n) operation to O(1)—huge performance boost if your lists are large.

Method 2: Collect Keepers First, Then Replace the List #

If iterator syntax feels clunky, you can first gather all the elements you want to keep, then overwrite the original list with this filtered collection:

public class Main {
    public static void main(String[] args) {
        List<TestDTO> test = new ArrayList<>();
        test.add(new TestDTO("a"));
        test.add(new TestDTO("b"));
        test.add(new TestDTO("c"));

        List<String> duplicate = Arrays.asList("a", "b");
        Set<String> duplicateSet = new HashSet<>(duplicate);

        List<TestDTO> keepers = new ArrayList<>();
        for (TestDTO dto : test) {
            if (duplicateSet.contains(dto.getOne())) {
                keepers.add(dto);
            }
        }

        // Replace original list with only the keepers
        test.clear();
        test.addAll(keepers);

        // Verify the outcome
        for (TestDTO dto : test) {
            System.out.println(dto.getOne()); // Outputs: a, b
        }
    }
}

Both methods will give you exactly what you want: dto and dto1 stay, dto2 gets removed.

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