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.println(ArrayUtils.toString(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.println(ArrayUtils.toString(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.println(ArrayUtils.toString(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.println(ArrayUtils.toString(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");
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 org.apache.commons.collections.collection.UnmodifiableCollection.add(UnmodifiableCollection.java:75)
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