CollectionUtils select方法 详解 用途~~~

我在作一个项目时规定不让用复杂的SQL如多表联接子查询等，在作一个报表时没有办法只有生成多个List结果集再用一大堆的if,for组合成一个List。其中有很都是从一个List中查找符合一定条件的对象，当时就用for+if 查找出后再放到一个新的List中，后来发现有更好的方法实现，方便多了。 

 package mypackage;

 import java.math.BigDecimal;

 public class Student {
 private String grade;
 private int age;
 private BigDecimal money;
 public String getGrade() {
     return grade;
 }

 public void setGrade(String grade) {
     this.grade = grade;
 }

 public int getAge() {
     return age;
 }

 public void setAge(int age) {
     this.age = age;
 }

 public BigDecimal getMoney() {
     return money;
 }

 public void setMoney(BigDecimal money) {
     this.money = money;
 }
 }

 package mypackage;

 import java.math.BigDecimal;

 import org.apache.commons.beanutils.PropertyUtils;
 import org.apache.commons.collections.Predicate;

 public class MyPredicate implements Predicate {
     private String property;

     private Object value;

     public MyPredicate(String property, Object value) {
         this.property = property;
         this.value = value;
     }

     public boolean evaluate(Object object) {

         try {
             Object beanValue;
             if (property.indexOf(".") > 0) {
                 beanValue = PropertyUtils.getNestedProperty(object, property);
             } else {
                 beanValue = PropertyUtils.getProperty(object, property);
             }
             if (beanValue == null) {
                 return false;
             }
             if (!value.getClass().equals(beanValue.getClass())) {
                 throw new RuntimeException("value.class!=beanValue.class");
             }
             return myCompare(beanValue, value);

         } catch (Exception e) {
             throw new RuntimeException(e.getMessage(), e.getCause());
         }

     }

     private boolean myCompare(Object value, Object beanValue) {
         if (beanValue.getClass().equals(Integer.class)) {
             if (((Integer) beanValue).equals(value)) {
                 return true;
             }
         }
         if (beanValue.getClass().equals(BigDecimal.class)) {
             if (((BigDecimal) beanValue).compareTo((BigDecimal) value) == 0) {
                 return true;
             }
         }
         if (beanValue.getClass().equals(String.class)) {
             if (beanValue.toString().equals(value.toString())) {
                 return true;
             }
         }
         return false;
     }

 }

 package mypackage;

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

 import org.apache.commons.collections.CollectionUtils;
 import org.apache.commons.collections.Predicate;
 import org.apache.commons.collections.PredicateUtils;

 public class TestPredicate {

     /**
      * @param args
      */
     public static void main(String[] args) throws Exception {

         List stants = new ArrayList();
         Student st1=new Student();
         Student st2=new Student();
         Student st3=new Student();
         Student st4=new Student();
         Student st5=new Student();
         Student st6=new Student();
         st1.setGrade("A");
         st1.setAge(5);
         st2.setGrade("A");
         st3.setGrade("A");
         st4.setGrade("B");
         st5.setGrade("C");
         st6.setGrade("D");
         stants.add(st1);
         stants.add(st2);
         stants.add(st3);
         stants.add(st4);
         stants.add(st5);
         stants.add(st6);
         Predicate isProblem =new MyPredicate("age",new Integer(5));
         //Predicate isProblem2 =new MyPredicate("grade","A");
         //Predicate any = PredicateUtils.anyPredicate(new Predicate[]{isProblem, isProblem2});
         List ddd=(List)CollectionUtils.select(stants,isProblem);
         System.out.println(ddd.size());

     }

 }

MyPredicate实现了Predicate接口来定义你的规则 
Predicate isProblem =new MyPredicate("age",new Integer(5)); 
这里指定要找age为5的Student 
List ddd=(List)CollectionUtils.select(stants,isProblem); 
在调用CollectionUtils.select就可以查到你想要的。 
你还可以用PredicateUtils中的方法生成更复杂的条件 
这里我定义了第2个Predicate isProblem2来指定grade为A 
PredicateUtils.anyPredicate(new Predicate[]{isProblem, isProblem2}); 
就是符合isProblem或isProblem2。这样查出的就是age==5 or grade=="A"的对象。