Java集合類工具CollectionUtils的操作方法

集合判斷： 
例1: 判斷集合是否為空:
CollectionUtils.isEmpty(null): true
CollectionUtils.isEmpty(new ArrayList()): true
CollectionUtils.isEmpty({a,b}): false

例2: 判斷集合是否不為空:
CollectionUtils.isNotEmpty(null): false
CollectionUtils.isNotEmpty(new ArrayList()): false
CollectionUtils.isNotEmpty({a,b}): true

2個集合間的操作： 
集合a: {1,2,3,3,4,5}
集合b: {3,4,4,5,6,7}
CollectionUtils.union(a, b)(並集): {1,2,3,3,4,4,5,6,7}
CollectionUtils.intersection(a, b)(交集): {3,4,5}
CollectionUtils.disjunction(a, b)(交集的補集): {1,2,3,4,6,7}
CollectionUtils.disjunction(b, a)(交集的補集): {1,2,3,4,6,7}
CollectionUtils.subtract(a, b)(A與B的差): {1,2,3}
CollectionUtils.subtract(b, a)(B與A的差): {4,6,7}