CollectionUtils工具類的經常使用方法

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

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

CollectionUtils在真實項目中，是一個很是好用的工具類，使用很是頻繁。它能夠使代碼更加簡潔和安全。恰好在工做中利用這個工具類重構代碼，順便總結下分享分享:

並集

@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個數組取並集 
    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個數組取交集 
    System.out.println(ArrayUtils.toString(CollectionUtils.intersection(listA, listB)));
    //[B, D, F]

}

 交集的補集（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個數組取交集 的補集
    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]

}

 集合是否爲空

@Test
public void testIsEmpty(){

    class Person{}
    class Girl extends Person{}

    List<Integer> first = new ArrayList<>();
    List<Integer> second = null;
    List<Person> boy = new ArrayList<>();
    //每一個男孩內心都裝着一個女孩
    boy.add(new Girl());
    //判斷集合是否爲空
    System.out.println(CollectionUtils.isEmpty(first));   //true
    System.out.println(CollectionUtils.isEmpty(second));   //true
    System.out.println(CollectionUtils.isEmpty(boy));   //false

    //判斷集合是否不爲空
    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<>();
    //每一個男孩內心都裝着一個女孩
    boy1.add(new Girl());
    List<Person> boy2 = new ArrayList<>();
    //每一個男孩內心都裝着一個女孩
    boy2.add(new Girl());
    //比較兩集合值
    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<>();
    //每一個男孩內心都裝着一個女孩
    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

咱們對c進行操做，s也一樣得到了和c相同的內容，這樣就能夠避免其餘人員修改這個s對象。有時候須要對它進行保護，避免返回結果被人修改。

@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能夠獲得一個集合的鏡像，它的返回結果是不可直接被改變，不然會提示錯誤

java.lang.UnsupportedOperationException
at org.apache.commons.collections.collection.UnmodifiableCollection.add(UnmodifiableCollection.java:75)