commons-collections包中的常用的工具类

commons-collections包中的常用的工具类

       <dependency>
          <groupId>commons-collections</groupId>
          <artifactId>commons-collections</artifactId>
       </dependency>

1. CollectionUtils工具类用于操作集合,  isEmpty () 方法最有用   （commons-collections包中的类）

package cn.xm.exam.test;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

import org.apache.commons.collections.CollectionUtils;

public class test {
    public static void main(String[] args) {
        List<String> list = new ArrayList<String>();
        list.add("str1");
        list.add("str2");

        List<String> list1 = new ArrayList<String>();
        list1.add("str1");
        list1.add("str21");

        // 判断是否有任何一个相同的元素
        System.out.println(CollectionUtils.containsAny(list, list1));

        // 求并集(自动去重)
        List<String> list3 = (List<String>) CollectionUtils.union(list, list1);
        System.out.println(list3);

        // 求交集(两个集合中都有的元素)
        Collection intersection = CollectionUtils.intersection(list, list1);
        System.out.println("intersection->" + intersection);

        // 求差集(并集去掉交集，也就是list中有list1中没有，list1中有list中没有)
        Collection intersection1 = CollectionUtils.disjunction(list, list1);
        System.out.println("intersection1->" + intersection1);

        // 获取一个同步的集合
        Collection synchronizedCollection = CollectionUtils.synchronizedCollection(list);

        // 验证集合是否为null或者集合的大小是否为0，同理有isNouEmpty方法
        List list4 = null;
        List list5 = new ArrayList<>();
        System.out.println(CollectionUtils.isEmpty(list4));
        System.out.println(CollectionUtils.isEmpty(list5));
    }
}

结果:

true
[str2, str21, str1]
intersection->[str1]
intersection1->[str2, str21]
true
true

补充:此工具类还可以向集合中加数组元素

        List<String> list = new ArrayList<>();
        String s[] = { "1", "2" };
        CollectionUtils.addAll(list, s);
        list.add("3");
        System.out.println(list);

结果:

[1, 2, 3]

2.   MapUtils工具类,isEmpty最有用（commons-collections包中的类）

　　可以用于map判断null和size为0，也可以直接获取map中的值为指定类型，没有的返回null

package cn.xm.exam.test;

import java.util.HashMap;
import java.util.Map;

import org.apache.commons.collections.MapUtils;
import org.apache.commons.lang.NumberUtils;

import ognl.MapElementsAccessor;

public class test {
    public static void main(String[] args) {
        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));
        System.out.println(MapUtils.isEmpty(map2));
        System.out.println(MapUtils.isEmpty(map3));

        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);
    }
}

结果:

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());
    }

map.isEmpty()代码查看hashmap:

    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");

查看源码:

    /**
     *  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;
    }