顺序很重要，因为你需要4和5是int(不是Integer)， 使用test2.removeAll (test1)。

Java 8

我们可以使用removeIf，它接受一个谓词来编写一个实用方法:

// computes the difference without modifying the sets
public static <T> Set<T> differenceJava8(final Set<T> setOne, final Set<T> setTwo) {
     Set<T> result = new HashSet<T>(setOne);
     result.removeIf(setTwo::contains);
     return result;
}

如果我们仍然在以前的版本，那么我们可以使用removeAll as:

public static <T> Set<T> difference(final Set<T> setOne, final Set<T> setTwo) {
     Set<T> result = new HashSet<T>(setOne);
     result.removeAll(setTwo);
     return result;
}

举个例子(system在existingState中，我们想要移除(不在newState中但存在于existingState中的元素)和添加(在newState中但不存在于existingState中的元素):

public class AddAndRemove {

  static Set<Integer> existingState = Set.of(1,2,3,4,5);
  static Set<Integer> newState = Set.of(0,5,2,11,3,99);

  public static void main(String[] args) {

    Set<Integer> add = new HashSet<>(newState);
    add.removeAll(existingState);

    System.out.println("Elements to add : " + add);

    Set<Integer> remove = new HashSet<>(existingState);
    remove.removeAll(newState);

    System.out.println("Elements to remove : " + remove);

  }
}

将输出如下结果:

Elements to add : [0, 99, 11]
Elements to remove : [1, 4]

如果你正在使用Java 8，你可以尝试这样做:

public Set<Number> difference(final Set<Number> set1, final Set<Number> set2){
    final Set<Number> larger = set1.size() > set2.size() ? set1 : set2;
    final Set<Number> smaller = larger.equals(set1) ? set2 : set1;
    return larger.stream().filter(n -> !smaller.contains(n)).collect(Collectors.toSet());
}

顺序很重要，因为你需要4和5是int(不是Integer)， 使用test2.removeAll (test1)。

您可以使用Apache Commons CollectionUtils。disjunction来获得所有的差异，或者CollectionUtils。相减得到第一个集合中的差值。

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.4</version>
</dependency>

下面是一个如何做到这一点的例子:

import org.apache.commons.collections4.CollectionUtils;
import java.util.List;

var collection1 = List.of(-1, 0, 1, 2, 3, 4, 5);
var collection2 = List.of(       1, 2, 3, 4, 5, 6, 7, 8, 9);

// [-1, 0, 1, 2, 3, 4, 5]
System.out.println(collection1);
// [1, 2, 3, 4, 5, 6, 7, 8, 9]
System.out.println(collection2);

// [-1, 0]
System.out.println(CollectionUtils.subtract(collection1, collection2));
// [6, 7, 8, 9]
System.out.println(CollectionUtils.subtract(collection2, collection1));

// [1, 2, 3, 4, 5]
System.out.println(CollectionUtils.retainAll(collection1, collection2));
// [1, 2, 3, 4, 5]
System.out.println(CollectionUtils.retainAll(collection2, collection1));

// [-1, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9]
System.out.println(CollectionUtils.collate(collection1, collection2));
// [-1, 0, 1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 6, 7, 8, 9]
System.out.println(CollectionUtils.collate(collection2, collection1));

// [-1, 0, 6, 7, 8, 9]
System.out.println(CollectionUtils.disjunction(collection1, collection2));
// [-1, 0, 6, 7, 8, 9]
System.out.println(CollectionUtils.disjunction(collection2, collection1));

// [1, 2, 3, 4, 5]
System.out.println(CollectionUtils.intersection(collection1, collection2));
// [1, 2, 3, 4, 5]
System.out.println(CollectionUtils.intersection(collection2, collection1));

// [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
System.out.println(CollectionUtils.union(collection1, collection2));
// [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
System.out.println(CollectionUtils.union(collection2, collection1));