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一、CollectionUtils
1.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
2.并集
union()方法
@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]
}
3.交集
intersection（）方法
@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]
}
4.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]
}
5.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]
}
6.集合是否相等（相当于==号）
- 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（相当于==号）
@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
}
7.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)
二、MapUtils
isEmpty()可以用于map判断null和size为0， getString()也可以直接获取map中的值为指定类型，没有的返回null
Map map = null;
Map map2 = new HashMap();
Map map3 = new HashMap<>();
map3.put("xxx", "xxx");
// 检验为empty可以验证null和size为0的情况
System.out.println(MapUtils.isEmpty(map)); //true
System.out.println(MapUtils.isEmpty(map2)); //true
System.out.println(MapUtils.isEmpty(map3)); //false
String string = MapUtils.getString(map3, "eee");
String string2 = MapUtils.getString(map3, "xxx");
Integer integer = MapUtils.getInteger(map3, "xxx");
System.out.println("string->" + string);
System.out.println("string2->" + string2);
System.out.println("integer->" + integer);
System.out.println(integer == null);
输出结果为：
true
true
false
INFO: Exception: java.text.ParseException: Unparseable number: "xxx"
string->null
string2->xxx
integer->null
true
- MapUtils.isEmpty根踪源码:
public static boolean isEmpty(Map map) {
return (map == null || map.isEmpty());
}
其中map.isEmpry()代码为
public boolean isEmpty() {
return size == 0;
}
- 补充:MapUtils也可以获取值作为String，获取不到取默认值:
//获取字符串,如果获取不到可以返回一个默认值
String string3 = MapUtils.getString(map3, "eee","没有值");
源码分析：
/**
* Looks up the given key in the given map, converting the result into
* a string, using the default value if the the conversion fails.
*
* @param map the map whose value to look up
* @param key the key of the value to look up in that map
* @param defaultValue what to return if the value is null or if the
* conversion fails
* @return the value in the map as a string, or defaultValue if the
* original value is null, the map is null or the string conversion
* fails
*/
public static String getString( Map map, Object key, String defaultValue ) {
String answer = getString( map, key );
if ( answer == null ) {
answer = defaultValue;
}
return answer;
}
