CollectionUtils⼯具类的常⽤⽅法
集合判断：
  例1: 判断集合是否为空:
  CollectionUtils.isEmpty(null): true
  CollectionUtils.isEmpty(new ArrayList()): true 
  CollectionUtils.isEmpty({a,b}): false
  例2: 判断集合是否不为空:
  CollectionUtils.isNotEmpty(null): false
  CollectionUtils.isNotEmpty(new ArrayList()): false
  CollectionUtils.isNotEmpty({a,b}): true
CollectionUtils在真实项⽬中，是⼀个⾮常好⽤的⼯具类，使⽤⾮常频繁。它可以使代码更加简洁和安全。刚好在⼯作中利⽤这个⼯具类重构代码，顺便总结下分享分享:
并集
@Test
public void testUnion(){
String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };
String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
List<String> listA = Arrays.asList(arrayA);
List<String> listB = Arrays.asList(arrayB);
//2个数组取并集
System.out.String(CollectionUtils.union(listA, listB)));
//[A, B, C, D, E, F, G, H, K]
}
交集
@Test
public void testIntersection(){
String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };
String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
List<String> listA = Arrays.asList(arrayA);
List<String> listB = Arrays.asList(arrayB);
//2个数组取交集
System.out.String(CollectionUtils.intersection(listA, listB)));
//[B, D, F]
}
交集的补集（析取）
@Test
public void testDisjunction(){
String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };
String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
List<String> listA = Arrays.asList(arrayA);
List<String> listB = Arrays.asList(arrayB);
//2个数组取交集的补集
System.out.String(CollectionUtils.disjunction(listA, listB)));
//[A, C, E, G, H, K]
}
差集（扣除）
@Test
public void testSubtract(){
String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };
String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
List<String> listA = Arrays.asList(arrayA);
List<String> listB = Arrays.asList(arrayB);
//arrayA扣除arrayB
System.out.String(CollectionUtils.subtract(listA, listB)));
//[A, C, E]
}
集合是否为空
@Test
public void testIsEmpty(){
class Person{}
class Girl extends Person{}
List<Integer> first = new ArrayList<>();
List<Integer> second = null;
List<Person> boy = new ArrayList<>();
//每个男孩⼼⾥都装着⼀个⼥孩
boy.add(new Girl());
//判断集合是否为空
System.out.println(CollectionUtils.isEmpty(first));  //true
System.out.println(CollectionUtils.isEmpty(second));  //true
System.out.println(CollectionUtils.isEmpty(boy));  //false
//判断集合是否不为空
System.out.println(CollectionUtils.isNotEmpty(first));  //false
System.out.println(CollectionUtils.isNotEmpty(second));  //false
System.out.println(CollectionUtils.isNotEmpty(boy));  //true
}
集合是否相等
@Test
public void testIsEqual(){
class Person{}
class Girl extends Person{
}
List<Integer> first = new ArrayList<>();
List<Integer> second = new ArrayList<>();
first.add(1);
first.add(2);
second.add(2);
second.add(1);
Girl goldGirl = new Girl();
List<Person> boy1 = new ArrayList<>();
/
/每个男孩⼼⾥都装着⼀个⼥孩
boy1.add(new Girl());
List<Person> boy2 = new ArrayList<>();
//每个男孩⼼⾥都装着⼀个⼥孩
boy2.add(new Girl());
//⽐较两集合值
System.out.println(CollectionUtils.isEqualCollection(first,second));  //true
System.out.println(CollectionUtils.isEqualCollection(first,boy1));  //false
System.out.println(CollectionUtils.isEqualCollection(boy1,boy2));  //false
List<Person> boy3 = new ArrayList<>();
//每个男孩⼼⾥都装着⼀个⼥孩
boy3.add(goldGirl);
List<Person> boy4 = new ArrayList<>();
boy4.add(goldGirl);
System.out.println(CollectionUtils.isEqualCollection(boy3,boy4));  //true
}
不可修改的集合
我们对c进⾏操作，s也同样获得了和c相同的内容，这样就可以避免其他⼈员修改这个s对象。有时候需要对它进⾏保护，避免返回结果被⼈修改。
@Test
public void testUnmodifiableCollection(){
Collection<String> c = new ArrayList<>();
Collection<String> s = CollectionUtils.unmodifiableCollection(c);
c.add("boy");
c.add("love");
c.add("girl");
//! s.add("have a error");字符串转数组工具类的方法
System.out.println(s);
}
Collections.unmodifiableCollection可以得到⼀个集合的镜像，它的返回结果是不可直接被改变，否则会提⽰错误
java.lang.UnsupportedOperationException
at llection.UnmodifiableCollection.add(UnmodifiableCollection.java:75)

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